Rear O2 Sensors
Thread Starter
Burning Brakes
Joined: Feb 2005
Posts: 752
Likes: 1
From: Tulsa OK
Rear O2 Sensors
I have read in the forum that the rear O2 sensors only measure the efficiency of the cats for EPA and emissions pruposes. They do not impact the engine performance or cause any changes in the engine parameters (e.g. air fuel, timing, etc.) That is why O2 sims can be installed that create a current that the PCM reads like real sensor was installed behind the cats.
The O2s up stream are another matter and control all kinds of stuff.
Is this true?
I have installed high flow cats and recently the car has been setting a "low efficiency code" for one side of the exhaust from the rear O2 sensor. The dealer said one of the cats was bad but I figured the car is probably running too rich for the cat to do its job if it flowing faster than a stock cat.
What do you think?
The O2s up stream are another matter and control all kinds of stuff.
Is this true?
I have installed high flow cats and recently the car has been setting a "low efficiency code" for one side of the exhaust from the rear O2 sensor. The dealer said one of the cats was bad but I figured the car is probably running too rich for the cat to do its job if it flowing faster than a stock cat.
What do you think?
Safety Car
Joined: Jun 2004
Posts: 4,971
Likes: 538
From: Witness Protection Program Somewhere, USA
2026 Restomod of the Year Finalist
My hi-flow cats would throw a code at idle. No where else. Probably due to lower temps after the cat and more exhaust flow.
Just find a tuner and he can program out the rears. No more codes.
Just find a tuner and he can program out the rears. No more codes.
Race Director
Joined: Oct 2002
Posts: 16,477
Likes: 26
From: Gone
CI 6-7-8-9-10 Veteran
St. Jude Donor '03
Originally Posted by EStreeter
I have read in the forum that the rear O2 sensors only measure the efficiency of the cats for EPA and emissions pruposes. They do not impact the engine performance or cause any changes in the engine parameters (e.g. air fuel, timing, etc.) That is why O2 sims can be installed that create a current that the PCM reads like real sensor was installed behind the cats.
The O2s up stream are another matter and control all kinds of stuff.
Is this true?
I have installed high flow cats and recently the car has been setting a "low efficiency code" for one side of the exhaust from the rear O2 sensor. The dealer said one of the cats was bad but I figured the car is probably running too rich for the cat to do its job if it flowing faster than a stock cat.
What do you think?
The O2s up stream are another matter and control all kinds of stuff.
Is this true?
I have installed high flow cats and recently the car has been setting a "low efficiency code" for one side of the exhaust from the rear O2 sensor. The dealer said one of the cats was bad but I figured the car is probably running too rich for the cat to do its job if it flowing faster than a stock cat.
What do you think?
The P0420 and P0430 TWC efficiency codes are thrown when there is not enough difference in readings between the front and rear 02 sensors. High-flow cats are very well-known to cause these codes. There most likely isn't anything wrong with the cats, they just flow too much and thus don't change the 02 readings as much as the PCM would like to see.
If those codes are a continuous problem, your only real fix is to either install 02 sims, or have the rear 02's edited out of the PCM.
