Downstream O2 sensors

[jcaspar1]

Having trouble passing smog. New cats and front O2 sensors. No check engine light. Dealership mechanic says rear O2 sensors are reading funny and to replace them as this can effect mixture. I thought the rear O2s were for monitoring the cats only. Is there any point in changing them with no Check engine light??Thanks

 

[Doug Supercharged Vipers]

Rear 0xygen sensors can/do deteriorate in time. Remember, they are placed after the cats which produce heat and can make them subject to slow response time and possible failure.

There are tests you can perform that can determine response time and efficiency, although the question is what year is your car and how many miles?

 

[jcaspar1]

1997 39,000 miles. Makes since they can go bad. My question is, do they have any effect on air/fuel ratio? Or only throw a CEL when cat not functioning.

 

[Jack B]

They do not have an affect on the a/f. However, a bad rear O2 could make the cat look bad. The 97's typically do not set a code even if they show a bad cat.

We need to know the defintive reason the car is failing the test?

1997 39,000 miles. Makes since they can go bad. My question is, do they have any effect on air/fuel ratio? Or only throw a CEL when cat not functioning.

 

[Hoosier Daddy]

Are you sure there are rear O2 sensors. I thought they first showed up in the 1998 Vipers

 

[speedracervr4]

They added them in 96' to be OBD2/emissions compliant.

 

[REDSLED]

Might be helpful to post the readout fro the test showing the parameters of the results. Replacing the rear O2s is a cheap fix and is another possibility to check off the list, although I don't remember seeing rear O2s on my 96. Hoosier Daddy might be correct. Sent you a PM.

 

[jcaspar1]

Thanks to all. Was just curious. Was able to pass smog barely with a fuel injector flush (? poor fuel atomization). Will keep the rear O2's for now as cats are new and should be working well.

 

[1.8t]

I went catless for 2000 miles on my 99 and never threw a CEL due to "catalyst inefficiency". Simply put, my PCM didn't give a crap about what my rear O2's had to say about anything. I doubt your older model cares as well.

 

[Jack B]

On my 97 the rear O2's are about 18" to the rear of where the exhaust leaves the sill. As someone already commented, the 96-99 PCM does not care about bad rear O2's. I have run the 97 catless for ten years. In most cases later cars (post 99) will set an intermittant code when you go to a high flow cat. I could be wrong, but, I beleive there may have been a PCM change to late 99 models and they may also be more sensitive to cat issues.

Might be helpful to post the readout fro the test showing the parameters of the results. Replacing the rear O2s is a cheap fix and is another possibility to check off the list, although I don't remember seeing rear O2s on my 96. Hoosier Daddy might be correct. Sent you a PM.