2007 Cayman S code P0421
 

My 2007 Cayman has 41,000 miles on it. The headers and cats are stock. I have a borla catback exhaust on it. I put the exhaust on at about 32,000 miles. Lately its been throwing a P0421 which relates to the drivers side cat or 02 sensor.

For a while it was only throwing the code in cold weather. Now its all the time. The car runs fine.

Could the cats actually be bad? No other symptoms other than the code.

I am thinking of replacing the secondary o2 sensors. Or possibly swap the drivers side to the passenger and see if it moves over to bank 2.

If it moves over to bank 2 then its simply a bad sensor right? Should I still replace both sides if it is?

Yes, the cat could be bad. Before replacing anything, you need to scan the car with a tool capable of graphing the O2 sensor voltages over time. The two sensors should have totally different curves, but if they are following each other, the cat is gone.

I just swapped the sensors left to right to see if the code moves to bank 2.


If the cats are bad I may consider buying headers and going catless...

After swapping the secondary O2 sensors the car through the P0421 code again.

Is it safe to say the drivers side cat is not functioning properly?

What options would others suggest to replace the cat? OEM will be very expensive. The car is a weekend driver. I have a borla cat back exhaust. What about going catless with headers?

Will my wife ever want to ride in the car with headers?

I was considering catless headers but have been told I need to flash the ECU for it to run properly. You could try contacting Porsche to see if they'll do anything even though the emissions warranty is only 8 years.

If you do need to replace the cat it shouldn't be that costly. I've seen people sell their used OEM headers/cats for $300/set.