1st post, hope I am in the right place and correct protocol. If not, please adjust accordingly and excuse me.
I have a 00 Z71 with 5.3, 165K miles. Stock except flo exhaust and k&N replacment filter.
I bought this truck with check eng light on about 6 weeks ago. Truck is super clean and runs great, obvious that it was always garaged and cared for.
Truck belonged to PO's dad, no service info available.
I checked it with my scanner and read P0420 catylist system efficency below threshold (bank 1)
I have cleared the code and turned off the light several times, always the same code. Sometimes light comes back on in 10 miles, sometimes 100.
With no service records, How should I approach the repair?

 

Take it to the muffler shop and have them remove the cats!

 

More than likely the left side cat isn't doing it's job...it could be the rear O2 sensor also, but from what I've seen that code usually equals bad cats. You could try swapping the rear O2's side to side and see if the code moves with the sensor. If not, the cat is probably at fault.

If you decide to stay stock, I still have the stock y-pipe off my truck that I've been wanting to sell, it had less than 50K on it when I removed it IIRC (I can check my records when I get home)...

 

Thanks Guys. I live in an area that we have emission check (atlanta). The tech that does the emission test looks under the vehicle to 'see' the cats are there.....what happens if they are gutted? How does that effect the computer? Will it set a code and turn on the C/E light? Billy, I will send a pm regarding your y-pipe. Thanks! Chuck

 

get a custom tune and get the p0420 and p0430 codes deleted. along with a few other improvements.

 

PM returned

 

In Calif guys are installing hollow cat($39 each) to pass inspection. You can just gut yours with a plumber snake chucked up in a 3/8" drill. You need to get those DTCs deleted or buy some 02 Simms if they still sell those things. The 02 simm outputs a square wave that transitions back and forth from about 300mVdc to 700mVdc a few times every second(RC network). The ECM doesn't know the difference so those two DTC never goes set. Neat! You could have a def cat or a def 02 sensor or even a voltage or wiring harness problem. Better to gut the cats and delete the DTCs.

 

! You could have a def cat or a def 02 sensor or even a voltage or wiring harness problem. Better to gut the cats and delete the DTCs.[/QUOTE]

ding! ding! ding!

Other issues that this un' has that may be related-
It all sounds like a related electrial issue to me- heres the list:

The a/c compressor circuit is screwed up somehow. When the a/c compressor cycles off, it wont re engage. I have to turn off the a/c and wait a few minutes and then it will enengage. When it has done this, I have arrived home and as was suggested on another forum, (the air gap may be too wide on the clutch) tap the clutch face while its trying to engage and see if that makes it click on. It doesnt, but the clutch face is magnetic at that point and pulls the tool I'm using to it. I havent been able to verify the voltage at the compressor when this happens yet. I have replaces the a/c cycle switch and the a/c relay. If I drive with the selector in the 'defrost only' position, the compressor doesnt cycle off and its 'kool' relatively speaking. I am suspecting that there is a voltage drop at the compressor when its signaled to 'come back on' When compressor works, air is ice cold and compressor is quiet.

The fuel gauge works intermedently. I had thought this was the common sender problem, but havent had the $270 for the part from auto zone or time to change it out yet.

And the real screwy part is that I have noticed weird coincidinces where the C/E light comes on and the fuel gauge stops working at the same time, or engaging the cruise control will cause the fuel gauge to work or not. The same with the a/c compressor.

I have been just trying to put some miles on this truck to gather data to be able to ask for help, and not just throw $$ at it (that i dont have). I've never had a late model truck, so all the computer stuff is new to me. Thanks for thr help, guys.

 

ttt anyone?

 

I don't know of any common failure that causes all of those symptoms, but if you feel that there's a fault that's common to all of those circuits, I'd suggest getting a wiring schematic and looking for a common ground or something along those lines.