Originally Posted by kappa4201
Since I got the car they have been P0421 and P0431. Warm Up Catalyst Efficiency Below Threshold (Bank 1)/ (Bank 2). Clearing the codes removes the CEL for about a day until they come back on.
My thought was the system sees the cats are not heating up properly and dumps more fuel to raise their temp. But by adding more all the time a lot goes unburnt, making the situation worse.
And moisture in the case should get burnt off as the car reaches temp. But the foam comes back quickly and smells very gassy so that is why I believe the foam is from fuel in the oil versus water.
Also I have not done a pressure test at the pre-cat 02 to measure back pressure to indicate restricted flow. I can't find a gauge to rent in my area, I may just have to buy one.
Can anyone explain to me how the car computes the P0421/P0431 code?
Sure I can explain how the codes are determined, the post O2 sensor signal value behind the cat is compared to the pre O2 sensor so in detail if the post signal is the same the cat is not efficient.
In the exhaust system the only component that regulates fuel trim is the pre O2 sensor before the cats the CTS is the main one in the engine, gas in oil does not foam.
Your best way to diagnose is to use vagcom to see real time values and pending codes that are not latching but resident.