Didn't know if anyone on here has heard of this but you probably have. But I wanted to share my new found knowledge on this.

O.K. have a Y2K ECSB Z71 5.3 and had the "Catalyst Efficiency Low, left bank" code come up. The first time, about two years ago, I just pulled the left rear O2 sensor and cleaned the crap out of it. Code went away!

This time it came back, I found out thru a friend that if I take the two front O2 sensors and put them in the back, and the back O2 sensors and put them in the front, reconnect all of them the code will clear. Well at first it didn't. I had Autozone clear the code and it came back. I thought, crap I gotta replace the cats now. Well after a day of driving the truck with the code coming back after the swap. It cleared itself and hasn't come back!!!!


Yay, I found something that could be useful on here!!!!

Laters.

 

that makes me think that one of the rear sensors was mis-reading....and you just put it in the front which will cause your fueling to be off.

 

From what my friend told me was that the computer in the truck could care less if the rear sensors are even working. It "blinks" a look at the rear sensors and kind of "expects" it to be bad????

I don't know that is what he told me. And he is a really big Fbody LS1 guy.

 

It uses the rear O2 just to check if the CATs or bad thats it.

By new front O2's you'll yours are worn out it stock and could hurt mileage. If the code comes back you can just have someone remove it from the PCM using HPtuners.

 

when i changed my front o2s there was a noticable increase in power and smoothness and i didnt even have a o2 code they just had 110k miles.

 

I believe it is the front sensor that works with the computer on adjusting fuel trims. the rear just confirms that its burnning properly after the cat.

If I am wrong some please correct me on that.

The only way to be sure is to Look at what the sensor is doing while the engine is running. I have seen some have intermetin problems. They are refered to as a Lazy sensor.

More often though ( I work for a smog guy ) we replace the front O2 sensors to correct emmision issues and if it does not make a drastic enough change then the cat is usually Bad and also has to be replaced. The sensors are usually around $50-$60 unless it is a Toyota then you could be in the $300- $500 range for the stupid sensor. They use alot of Platinum in there O2 sensors and cats. Thats usuually why the theifs go for the Toyota's when the steal cats...

any way hope that helps a bit...

 

Well an update... To my dismay, I don't think I fixed it. After swapping the sensors and the code being cleared, coming back and going away on its own. I have had the code come back and another code on top of it. This time I am getting Catalyst efficiency low and left bank running lean???

I get the code and then have it cleared, it takes two days for the code to come back??? So now I was thinking of doing some troubleshooting.... Taking the sensors and swapping them from left to right in their current configuration. That way if I get the codes coming up saying "right" side, that way I will no my sensors are bad, and if it still comes up with "left" side, I will know that my Cat converter are bad on the left side?

Does this sound like something you guys would do? Reason I am asking is I have no way of getting to a "tune" for reprogramming of my truck at all!!! I do really need to buy a laptop and get the cable for tuneage of my truck!!!

Laters.

 

It will come back on for sure and you probally need to replace the 02 sensors.

As far as the cat, I would guess its also bad too at this point sense the code keeps coming on for that too... usually you have to drive it 2 or 3 drive cycles before it will come back on.

I would also guess you might need the intake gaskets replaced. If you have a leak there it will cause the lean code. This is actually a fairly common cause for a lean code.

Easy way to check is spray break clean around the intake to head with a constant/ steady stream and see if the idle changes. if it does you have a leak for sure...

When you pull the O2's and swap them back clean them up with a wire wheel or something.

Let us know if this helps...

 

All this O2 sensor swapping is making me dizzy. Doesn't anyone use a scan tool before they begin swapping or changing parts? You can NOT rely on the post- cat sensor reading to determine if your cat is defective. It should normally be in the range of 600mVdc, but that's not a hard and fast rule. You could actually see 600mVdc on your scan tool at the post-cat O2 and still have a clogged cat. I have seen this condition myself. You need an Exhaust Gas Back Pressure Tester to PROVE that the cat is acually defective. You have to use this tester to COMPARE the pre-cat pressure to the post-cat pressure to determine whether the cat is clogged or not. I won't bore you with a detailed explaination of what the different readings you can possibly get mean except to say you should normally see about a 0.5lb increase at the post-cat bung over the pre-cat bung on a properly functioning cat. The pre-cat O2s monitor the oxygen left in the exhaust to determine combustion efficiency. The post-cat 02s monitor the cat efficiency. How good of a job is your cat doing at reducing emissions. This is what the post-cat 02 tells the PCM or ECM. It doesn't have to do a very good job according to the EPA. It just has to reduce emissions a small amount from what went into the cat to pass the test. You really need a scan tool to get much use from parts swapping. If you don't have one it might take days to determine if the SES is going to come back "ON" again. If your going to do your own maintenance these days a scan tool is a must. All you need is a laptop, scan tool software on CD and on interface to plug into the vehicle DLC and your well on your way to becoming a real OBDII powertrain technician.