1GTR9CED3KZ198419
2019 gmc sierra 1500 new body 5.3 double cab.
I have been getting some help from justanwsers

Known Issues and what the gm tech told me to check.

• Fault Codes that were always the issue before and not now after trouble shoting:
  • Active: P2535 U0100 as of today
  • Pending: none as of today
  • New codes that are the only ones active after troubleshooting are : p2535,U0100.
  • Now after disturbed the entire ecm harness, cleaned all the pins of the ecm, replaced K77 (ECM IGN) and K70 (starter). There still is No CEL but there are still 2 different active faults that I was not getting before and now am consistently getting (p2535 and U0100). The pending faults from 2 days ago are gone.
  • Triggered Conditions: Fault occurs only below -20°C and starts with a remote start attempt or just a manual start attempt.
  • Past Troubleshooting: Extensive testing of ECM connections, voltage, continuity, harness inspections, and replacing relays. I also suspect issues related to aftermarket modifications, though these seem unlikely.
  • Suggested Steps by the GM TECH
  • 1. Focus on Key Wires During Fault ConditionsSince the ECM is comparing its battery feed (B+) to multiple ignition feed voltages, you'll need to monitor:
  • B+ Wire at ECM: Confirm consistent voltage at all times.
  • Ignition Wires (IGN):
    • IGN1, IGN2 (common in GM vehicles).
    • Trace specific ignition wires referenced in the service manual or circuit diagram.
    • Use AllDataDIY or a factory wiring diagram to identify which IGN wires the ECM monitors.
  • Monitor these with back probes while duplicating the fault (cold start conditions, -20°C or below).
  • 2. Address New Fault Codes
  • P2535: Likely related to the ignition switch RUN/START circuit. Check if the ignition voltage fluctuates under colder conditions. This aligns with potential ECM comparisons.
  • U0100: Loss of communication with ECM, which may indicate harness issues or power supply interruptions.
  • P0685: ECM/PCM Power Relay Control Circuit.
  • P0700: Generic transmission fault indicator but might stem from the same communication issues.
  • P25A2: Secondary fault, possibly tied to voltage discrepancies affecting modules.
  • 3. Inspect and Test Specific Components
  • ECM Harness and Connections:
    • Focus on the 6 inches of ECM harness you haven't inspected.
    • Inspect for micro-chafing or cold-related expansion/contraction causing intermittent faults.
  • Battery and Grounding:
    • Test and clean grounds (G114 and others relevant to ECM).
    • Use a thermal imaging camera (if possible) to identify weak connections under load.
  • Light Bar Wiring:
    • Disconnect completely, even if unlikely. Aftermarket accessories can introduce unexpected behaviors.
  • Relays and Modules:
    • Test the BCM and ECM modules for updated software.
    • Check BCM software versions, as per TSB PIT5735B.
  • 4. Advanced Testing
  • ECM Live Data:
    • Monitor ECM ignition voltages and compare them with expected values during cold cranking.
    • Use a diagnostic scanner capable of live data during fault replication.
  • Voltage Oscilloscope Testing:
    • Use an oscilloscope to observe voltage stability on IGN and B+ lines during remote start attempts in cold conditions.
  • 5. Evaluate Software Updates
  • Even if the BCM and ECM updates seem unrelated, apply them if available. Some updates address intermittent faults that aren't explicitly documented.
  • Plan for New Codes TroubleshootingWhen you return:
  • Disconnect the light bar.
  • Back probe IGN wires during cold cranking attempts.
  • Test the new fault codes systematically:
    • P2535: Ignition switch testing and voltage at ECM RUN/START circuit.
    • U0100: Harness inspections near ECM and modules.
  • Replicate the fault with ECM harness slightly disturbed to rule out latent mechanical wiring issues.
  • Next StepsIf these steps don't isolate the issue:
  • Consider ECM replacement under the powertrain warranty.
  • Perform a dealer-level diagnosis to rule out any advanced firmware or hidden issues.
  • 
    
    Today, 7:12 AM
    
    I was looking for a ground on the right front when I was under there. I will check that out first thing when I get back home in a few days. A ground issue would explain all these random codes. I have only confirmed G114 has no issues underload in a warm garage. But maybe when its cold its having contact issues. There is a ground strap behind the right front wheel liner that I haven't found and checked yet. Okay so Its pretty rare in the last 2 months that my truck starts with out any issues when its colder than -20C.(like maybe 1 of every 5 start attempts it will start with out any issues).So thats 2 cold starts in a row at -37 that it fires right up with no issue.So I'm starting to think I may have done something to improve this problem
    
    I did noticed when running the truck this morning that The ECM (42) Voltage was at 15.53V and the Obd adapter Voltage was all over and would go down to 15.1V. Looking at the ecm Print again I noticed I missed one ignition feed (accessory wake up serial data circuit wake up voltage). Which is pin 33 of X2 at ECM. The BCM sends power to wake up the ECM and TCM when the Ignition switch is put in ACC or RUN position or fob start is requested. I don't think its relevant because if their was a an open or short it would throw a code for that circuit. I did once get U0101 and U00146 in the past. I didn't look them up yet.But I will.
  • Which exact IGN pins do you want me to back probe, their are many (10) on X2,X1. Or back probe and monitor all of them. I wasn't completly certain which IGN wires the ecm are actually comparing to B+ to determin and trigger the fault. I would assume (hate doing that) its the wires stated in the DTC guide I attached which are 14,15& 16 on X2 at ECU and 48 and 49 of X1 at ECU.
    Today, 7:23 AM
    
    I will do everything the tech stated. I have not actually looked at the troubleshooting guide or troubleshot the new active codes yet. p2535 and U0100. Those are the only 2 active codes I have now. .I am sourcing out a GM MDI2 and GDS2 software as I will likely need it if I want to be able to keep and fix this truck.
    
    
    So you want me to test the ignition switch when its cold and when the codes are active.There is a section of ECM harness I have not inspected yet. I must removed the airbox to do the complete inspection properly. I will bring it to the dealer asking for the Latest ECM and BCM update. I know they updated it probally 2 years ago so I'm sure there is new versions. I will completly disconnect the light bar from everywhere. I want to buy a scanner if there is one that allows me to see all ECM inputs and output of every wire. Autel Maxisys Ultra or Lunch 432 Pro or elite may allow me to do this but I am currently looking into this. I have a Oscilloscope on my 24 hour data logger blutooth multimeter that I will use to to backprobe the ecm. I was quite rough with the 6 inches of harness from the ecm while continuity testing. but beyond that it was to well secured to pull and bend it or even see it. I must remove the airbox next to shake test/ inspect it properly. I think I must readjust my troubleshooting guide towards the 2 new active codes. Your response was very helpfull.
    
    to perform a Perform a dealer-level diagnosis to rule out any advanced firmware or hidden issues. I must have GM diagnostic software and hardware to really do this step correct? If so I may have to bring them the truck.
    
    I looked up all of those active codes That i did have in the past, all loss of communication of each modules. U0284 Airshutter Module, U0101 TCM, U0146 Serial Data Gateway Module. They probabbly all have a a common ground that could be bad. I have not cleaned any grounds yet. Just sprayed 2 with fluid film.
    
    
    
    
    
    
    

The code I was getting for 3 years and no longer get after troubleshooting was basically the ecm comparing its battery feed to its multiple ignition feed voltage, If there is to big of a voltage gap it will trigger a code and I get a crank no start condition. This usually starts when I try a remote start which triggers the code, then I try manually starting it which I then get a crank no start and a Cel that appearsafter the remote start attempt.

 

The condition is: for the last 3 years, any time it gets colder than -20c. I get a ingition related code which causes a crank no start condition. The cracking will always be fast ( like there is a good battery reserve), and when I press the start button, it will get stuck cranking for as long as possible, till I repeatedly press the start button multiple times to get it to stop cranking. Its not due to a low battery as I have a charger on the battery the whole night charging the battery and the problem will still occur. And every time its warmer than -20c, I never have any trouble or any codes. Meaning in the last 3 years, every winter i have this problem and same codes and every spring summer fall (3 years) I have no codes and no problems. Every winter when it gets colder than -20C, I would get these same DTCs: p16a7 would always be active and these 2( P1682,P16BC) always pending when the fault occurs.

Recently the problem has been more frequent, and a new condition has occured: no crank no start after hitting start ignition button after multiple attempts. To resolved this new condition that only happen once at-30c, I disconnected the battery and reconnect it and it fired right up no starting issues with the cel then going on.

Other times it ooccured, if I would just let the truck sit for 5 mins (after a failed attempt to manually start it), it would fire right up with no issues and CEL would then go on with those same 3 codes (active: p16a7, pending:P1682,P16BC)



I have used alldatadiy D16a7 dtc troubleshooting guide, and 3 relevant technical service bullitins to extensivly troublehoot this problem. I have run out of things to check for litterature for troubleshooting the active code p16a7. I Inspected and done all of the requirements in the bullitins relevant to my 5.3 cover codes I have, which may have been caused by bad crimp at H6 terminal of X3 connector under the fuse box, and 3 know locations of ecm harness chafing points which I have all rulled out. I have done everything but replace the ecm and bring it to the dealer to check if I need an ecm firmware update. All I have left is a a powertrain warranty.

What I have done in 10 hours of troubleshooting: load tested battery, checked charging system, checked all the fuses underhood, removed and took apart the fuse box to check for any terminal thats corroded, poorly crimped or unseated from its connector.All appeared perfect. tested kr35 relay, its coil circuit from the ecm when the falt actually occurs. When it wont start i rulled out the the fuse panel as the problem. I did all inspection and testing requirements in the attached technical service bulletins attached. I inspected only 6 inches of harness from the ecm for damage (non visible as it was properly secured to spec). With all 3 ecm connectors removed, inspected all male and female pins, did a pin draw test to see if there were any worn female terminals of the k20 3 ecm connectors. with ecm disconnected, while shaking squizing, bending 10 inches of ecm harness from the plugs, I was continuity testing each ignition, batt wire from the ecm plugs to the Ignition relay terminal 3 whicj was was relays output. All were 0.0hms. Since that is useless static test, I did a static voltage with no load then with a introduced load on each wire (ecm not connected). With key on, ignition in service mode with a charger on the battery, with the 3 K20 ecm connectors disconnected.

I did a static voltage test at the ecm plug, first compared the B+ feed to the ecm plug to each ignition feed wires to each of the 3 ecm connectors. No voltage difference. Then I did the same test again, but this time I put a 8 amp load on each wire (to rule out corroded wires). I compared the loaded voltage drop of the B+ ecm feed to the loaded voltage drop of each IGN wires feeding the ecm. All had an identical voltage drop under load.
For that 8amp loaded test, I used one of the 2 ecm grounds one at a time, those ground fed to ecm are from I think it was G114. All of this voltage testing is done when the problem was not present because its in a warm garage.At this point I was no longer going to remove the airbox to futher inspect the ecm harness behind the left front strut mount. l Replaced the ignition ecm relay kr35 (k77) and the starter relay with gm parts. I brake cleaned and blew dry the ecm and its connector pins and closely inspected each pin a second time.

The next thing I was going to do was install back probes in the ecm connectors to monitor the ign and b+ voltage at ecm connectors while the fault actually occurs. I need you to tell me which IGN wire to monitor as there are many going to the 3 ecm connectors. I dont exactly know which ignition wires the ecm is comparing to its battery feed voltage to cause the DTC I get.

So after doing all this. I turned off the ignition and immediately torqued the left front tire. Which may have triggered this new set of codes according to this bullitin (#PIT5735B). The bellitin says if the the tires slightly rotate shortly after igntion is turned off it could generate these codes I got, which I never had ever before. A body control module software update may correct it but I think a latest update for the bcm and ecm my not be relevant to fix any of my problems.

I currently do not have a CEL on, but 2 of my code scanners see these codes, which I cannot clear any of them manually, before during and after a 6 hour drive.So I did disturb the ecm harness the night before, The New active codes are p2535, p25a2,U0100.Pending codes are p0685, p0700 (that was the case 3 days ago) .I will be back home in 5 days to continue troubleshooting but till then I need a new plan to troubleshoot these new codes.

I will be disconnecting my lightbar completly to reference bulletin #PIT5743 which states aftermarket mods tied into the ignition circuit of the fuse box or IP causes a crank no start condition. I tied my light bar relay coil 85 power feed from ignition source in the instrument panel RAP ign source I think it was. I doubt this is the issue, if it was it would cause me crank nonstart issues year round and not just when its -20c or colder.

 

 

For one thats this big of a pain in the *** I wouldn't waste tons of time on diagnostics that may or may not lead you anywhere I'd throw three parts at it right off the rip. It's one of those PITA electrical gremlin trucks.

Replace Ign switch cheap, replace the underhood power center and Put a new ECM in it probably has a solder joint problem. Unless you have the software it'll have to go to a dealer or a shop for ecm programming. Use all new GM parts

After changing those parts if that don't fix it I'd trade the turd off and never look back.
This is pro non DIY advice not a key board mechanic. Can you fix it after maybe but at what expense and incovenience due to a non operational truck.

Old trucks used to wear out the new ones like you are describing well they don't wear out they have electrical gremlins set in that render them intermittently unreliable and may as well be wore out.

 

Heres a tech bulletin I found after the fact in our shop system. Id say replace terminal first if that don't fix it then change the three items listed above.

#PIT5687A: SES MIL / No Start / Stall / DTCs P16A7 P16AF - (Jan 2, 2024) Subject: SES MIL / No Start / Stall / DTCs P16A7 P16AF Brand:

Model:

Model Year:

VIN:

Engine:

Transmission:

from

to

from

to

Chevrolet

Silverado 1500 (New Model)

2019

All

All

LV3 L82 L84 L87

All

Chevrolet

Silverado 1500

2020-2024

All

All

LV3 L82 L84 L87

All

GMC

Sierra 1500 (New Model)

2019

All

All

LV3 L82 L84 L87

All

GMC

Sierra 1500

2020-2024

All

All

LV3 L82 L84 L87

All

 

Involved Region or Country

North America

Additional Options (RPO)

With Engine RPO LV3, L82, L84 or L87

Condition

Some customers may comment on a SES MIL, Crank No Start, and/or stalling concern. When checking for DTC's, the ECM will have P16A7 and/or P16AF set.

Cause

These concerns could be caused by no/low voltage at the ECM power circuit 5290, connector X1 terminal 49. In most cases, this is due to poor terminal tension at the ECM X1 connector pin 49.
CorrectionThe following information provides some diagnostic tips and does not replace SI diagnostics. As always, follow SI diagnostics.

1. Inspect ECM Fuse #86 (30 amp), in the UBEC, for being open. If the fuse is open, perform normal diagnostics by inspecting the harness for a short to ground.

2. If the fuse is not open, install a battery tender (GR8) and turn the ignition on. Verify that the battery voltage at the ECM Fuse #86 in the UBEC is greater than 12 volts. If no/low voltage is present, inspect for proper operation of the KR75 Engine Control Ignition Relay in the UBEC.

3. If battery voltage is present, turn the ignition off and gain access to the ECM X1 connector. Using the correct test probe (J-35616-35 VT) perform a terminal drag test at pin 49 of the X1 ECM connector. If the terminal is found to have poor drag, replace it with the terminated lead listed in the SI connector end view document.
 

Version

2

Modified

05/17/2019 - Created on

01/02/2024 - Update to the model years
Warranty InformationFor wiring repairs covered under warranty, please refer to latest version of bulletin 10-00-89-005.

 

 

Thanks, I have meticulasly looked threw all the requirments in that bulletin already as well as 2 others relevant to my issue. I see lots of people on other forums have this issue and the dealer cannot figure it out.

There is no longer any active codes even tho I have had a failed remote start attempt. In those links are videos of the IGN switch voltage being monitored. I have seen it Drop to 4V while cranking.
ignition switch has gone down to 4 volts while cranking.
https://drive.google.com/file/d/1hKH...ew?usp=sharingScanner Data looking at the ecm while trying un unsucessful remote start

https://drive.google.com/file/d/1XdM...ew?usp=sharing

In the past there was a report from the TCU that it could no communicate with the ECU. My thinking was because the wake up signal from the BCM to the ECM didnt happen because the BCM didn't see the signal from the keyless entry module/bad ignition switch. Its odd that the IGN switch voltage drops to 4 volts while cranking and no code is logged. Its sitting arround 8 to 10 volts while the truck is running while the keyless entry module battery supply voltage is always normal . That must be the issue? There is a bulletin for crank no start extended crank. I never had relevant codes that match those in the bulletin, but the solution is a ECM update. Going to load the parts Canon with a New battery, new igntion switch and ecm/bcm update. I was closely inspecting the ecm ignition / batt pins. So your saying there could be a bad solder of one of the pins to the board?

Something tells me that because I have issues with starting it via the button and the fob, that the ignition switch is not the issue but I will replace it. The fob might bypass the bad ignition switch. I have never have had a time where I can start it from the fob and not from the push button or vice versa. its always both that act up together.

I need an active code anyway so I'm going to quite troubleshooting till I have one active. I have been plugging my truck in with a trickle charger and block heater and its been starting remotely or via push button with no issue. So I will no longer do that and see if the problem returns. Maybe the voltage is dropping to low somewhere due to a weak battery. Doubt it to be honest because I had a full size charger on it all night (before cleaning ecm pins/disturbing harness) and it still produced the crank no start fault before.

My thinking is if the ecm is not receiving one of its IGN Signals, that would mean the ecm isn't at fault for not receiving an Ign feed or wake up signal ect.

https://drive.google.com/file/d/1uUk...ew?usp=sharing

ECM IGN circuits graphs no issues while starting (link above)

Compare that to the other video of the failed start attempt. Another failed start attempt i didn record. the scanner actually told me in the ECM datastream screen that voltage was too low on one of the ign circuits (forgot which one) while i was cranking.

 

Replace the terminal first thats first stop dont go off of well we checked it their drag tests are **** some dumbass engineer writing. They way they shoulda phrased it is replace the damm terminal don't bother going anywhere else first. IF that has actually been replaced then change other 3 mentioned items all at same time. If that don't fix it then for your sanity and wallet dump it trade it off.

I cant even imagine -20 jesus Im cold when it's 40 outside that severe of cold can make for all kinds of terminal problems.

 

Your talking about replacing the prone female ignition terminal in the ECM connector. The terminal (I think it was 49 of X1 IGN) that is mentionned in one of the bulletins that matches my past codes (not returned yet as its not cold enought for it to return) and fault. I had bought the female large terminal/pigtail from gm, I will just change it. I just wanted to make sure I wasn't misunderstanding you. Your not talking about a bad male pin thats poorly soldered to the ecm circuit board right? Looking at the live ECM data, there are some circuits tests that malfunctioned and some Referance voltages out of range (5.01V) while its running. Even tho there is no CEL currently. I will do what you recommend when I get home from work. It was -34C and it fired up remotely. But I think its because I got into the vehicle first (woke everything) then got out and locked it and tried it. Someone on another forum was saying it will always successfully start remotely if you enter,exit, lock and remote start. So I will have to make sure not to enter it when I try the remote start next time its ignorantly cold out. I wouldnt have any of these problems if I just moved to Texas with this truck. My truck only works if its warm out, just like me.

 

Replace the below pin. You've got to have gm crimp tool to properly crimp it though. You may be better off finding a pcm connector remove that lead and 6-8 inches of wire then you just have to deal with solder and heat shrinkin the wire only. Find a set of connectors that have been cut off from a boneyard EBAY etc.

"Perform a terminal drag test at pin 49 of the X1 ECM connector. If the terminal is found to have poor drag, replace it with the terminated lead listed in the SI connector end view document."
 

 

I have so many specialty crimper tools at work and home. I got what I need to crimp. I cannot find a part number for the 2 different size female terminals in the X1 K20 ecm connector. I just want to order new female terminals and replace every terminal in all 3 connectors. I do not want one splice on the ecm harness, ever.

found the big terminal K20 Large ECM connector Female Terminal https://www.te.com/en/product-1326030-3.html