Help with Fault Code (please...)

Hi all. Had a CEL pop up yesterday Using Torque I can see Fault Code P0420. Car runs great - no issues. I have a 2003 MY C4S. I ran the O2 sensor test (result below). It seems one of the O2 sensors is failing. My question is, can anybody assist with which one may be the problem? The software said Bank 1 but I have to confess I don't know which side of the engine this is! (my car is right hand drive, UK spec). Wiring looks good to all four sensors........ Any advice greatly appreciated. Thanks in advance.. [B]Test report: ------------------ [B][B]TID:$01 CID:$05 - Rich to Lean sensor threshold voltage(constant) Max: 164 Test result value: 189 FAIL ---- TID:$01 CID:$06 - Rich to Lean sensor threshold voltage(constant) Max: 164 Test result value: 62 PASS ---- TID:$05 CID:$05 - Rich to Lean sensor switch time(calculated) Min: 2,294 Test result value: 2,325 PASS ---- TID:$01 CID:$05 - Rich to Lean sensor threshold voltage(constant) Max: 164 Test result value: 189 FAIL ---- TID:$01 CID:$06 - Rich to Lean sensor threshold voltage(constant) Max: 164 Test result value: 62 PASS ---- TID:$05 CID:$05 - Rich to Lean sensor switch time(calculated) Min: 2,294 Test result value: 2,325 PASS ---- End of report.

Trying to figure out some CEL codes myself. This is what the Porsche ODB manual lists for your code

P0420 means the DME is complaining the bank 1 (left side of car) catalytic converter is losing efficiency. Bad O2 sensors have their own codes. If those test are the report you got from hitting the "Test Results" icon in Torque app, those are catalytic converter test results. The failed one also indicates bank 1. Just plot your 4 sensors. At idle engine fully warmed up, the precat ones should swing between 0.3-0.7 at about once a second. The postcat ones should remain steady at 0.6-0.7v. If they follow the precat reading, the cat is bad.

Thanks to you both for the speedy advice! I looked at all four sensors. I think it is the pre cat sensor on Bank one which is throwing up the fault - out of range. Would this make sense given the code? Thanks again

Um, why do you think the bank 1 precat O2 sensor is out of range? Again, the P0420 has nothing to do with O2 sensors. The test report you posted (tid 01/cid 05 and tid 01/cid 06) are for the cats, not the O2 sensors, despite the "rich to lean" descriptions, which are not accurate.

OK, thanks Ahsai - think I get it now - guess i was just looking for a cheaper solution :-) I did as you suggested - the second sensor on bank one pretty much follows the first - on bank two the second remains constant. Looks like I will have to replace bank one Cat? Any other tests I should do before shelling out on a new Cat? Great advice on here- great forum....

Nope, you nailed it. Very common for cats to go on these cars

Quotes source post: 11597742

While it does read like the converter is bad, read the info posted about diagnosing converter problems. One very important part deals with sensors. If there are any sensor errors, and these may not be active but pending, these should be dealt with first. Only then if the converter error comes back then the converter should be dealt with. I know of no tests one can do to confirm the converter is bad. The DME is pretty good at catching a bad converter. Emission rules/regulations require proper converter operation and automakers strive to monitor converter operation in order to keep them operating efficiently and to also catch any problems that might damage the converter, since these are covered by an emissions warranty over and above the new car warranty. My '02 Boxster was generating a P0430 code every once in awhile. I had the car in a while back for some other work and asked the tech to deal with the noisy heat shields. He reported the shields were not noisy the buzzing I heard was coming from a converter. The brick was loose. I found a pair of used converters are a salvage business for $475/each and after shaking them to ensure the bricks were quiet had the tech install these. Buzzing noise gone. And a deep knocking noise at cold start that I had attributed to crankshaft bearing (thrust bearing) wear was gone too. I decided to replace both converters even though only one was bad. The converters were original and had over 280K miles on them. I do not know the miles on the used ones but they looked fairly "fresh" and were quiet. So far they have remained quiet and the CEL dark. You can maybe replace just one converter though afterwards you might want to check for any differences in say the long term fuel trims between the two banks. My concern is if one converter is old and the other newer the DME will of course adjust fueling of each bank to get the performance from each converter it wants and this can result in less than optimum fueling for the engine. Generally, my rule is to keep things as close to the same on each bank. Replace sensors in pairs. Replace all plugs at the same time. And if a converter needs replacing replace both. I would amend this to say if a new plug was found to be bad to replace just the new plug and not the other 5.

Thanks Macster. I am looking at second hand cats in the UK - there seem to be plenty around, mainly from 'written off' cars, but in the UK they will write off a 911 for relatively minor damage, due to repair costs! I only have the one code and the sensors seem to be operating OK, so guess I will replace both cats as you advise...