Replacing Cat - O2 Sensor, CEL

[OldAudiA4]

If I replace the CAT with a high flow CAT further down the exhaust, will it create an issue with the O2 sensors causing a CEL?

[strophius]

Not with the primary (pre-cat) O2 sensor, the secondary (post-cat) one for emissions will most likely throw CEL. You could try a spacer (may or may not work), or you can have it coded out, or just drive around with the CEL on.

[OldAudiA4]

Not with the primary (pre-cat) O2 sensor, the secondary (post-cat) one for emissions will most likely throw CEL. You could try a spacer (may or may not work), or you can have it coded out, or just drive around with the CEL on.

Forgive my ignorance, but what is a spacer?

If I just drive around with the CEL on, does this change the performance of the car?

[walky_talky20]

Nah mate, it will run just fine. The performance will be even better because of the improved exhaust flow.

If you want to ignore the Check Engine light (and you do not have emissions tests that require such lights to be off), you can drive it as-is. No worries.

[strophius]

Walky is correct, the primary O2 sensor is the only one that actually impacts engine performance and it doesn't care if you have a cat or not. A spacer is something you can install into the secondary O2 port to basically back the sensor away from the exhaust pipe to try to trick it into seeing the amount of exhaust flow it's expecting. They are kind of hit or miss if they work though, sometimes you'll still end up with a CEL regardless.

[OldAudiA4]

Walky is correct, the primary O2 sensor is the only one that actually impacts engine performance and it doesn't care if you have a cat or not. A spacer is something you can install into the secondary O2 port to basically back the sensor away from the exhaust pipe to try to trick it into seeing the amount of exhaust flow it's expecting. They are kind of hit or miss if they work though, sometimes you'll still end up with a CEL regardless.

Thanks for the info. Is there a particular spacer for the B5 or will any generic O2 spacer work?

[strophius]

Any generic one that threads into the port, I believe it's a M18x1.5mm should work. CTS Turbo has a fancy '90 degree one I used many many years ago, seemed to work fine and it did turn off my CEL.

[OldAudiA4]

After getting my exhaust done, apparently there was only one 02 sensor before the CAT and no secondary sensor. So no CEL was thrown.


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