Help with Emission Codes

Hello, Wanted to see what people know about the following codes in terms of how to correct them: PO131 - Oxygen sensor ahead of catalytic converter PO157 - Oxygen sensor after catalytic converter PO137 - Oxygen sensor after catalytic converter I went for an emissions test last weekend and got fault codes P1124 and P1126, so I checked the forum and cleaned the MAF, sprayed contact cleaner on each of the O2 sensor connectors, cleaned the air intake (which was remarkably clean already) and changed the air filter. Reset the fault codes and went for a drive per the recommendations from people on this site. Now I'm getting these three codes. So I guess the question is: can the sensors be cleaned effectively or do I need new sensors? My car is a 1999 996 with 39K and I'm the original owner. Many thanks!

You can't clean 02 sensors. Or, at least, it isn't generally recommended to try. (I've seen folks claim you can, but given my thoughts below, it seems very likely to do more harm than good). I'm very skeptical that you had three bad ones pop up at once at 39k miles. I'd check the connections. I'd also check the resistance with a DMM when I opened them up. But my money is on something else causing an air fuel ratio problem, given your other code history. While 02 sensors isn't going to hurt anything, the same (or less) money will get you a Foxwell or Durametric. And unless somebody smarter says otherwise (I'm pretty new to 996s), I bet some live sensor data will get you closer to a solution than three new 02 sensors. Paul

What year? What model? How many miles? Past and present maintenance done? What are your current air/fuel ratios? Changed your coils? Spark Plugs? Air filter? Fuel injectors? Do the codes pop up cold or hot? See...you can't just throw a bunch of codes at us and then expect a solution. Need a better and more precise picture of where things are.

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Some of the information you're asking about is in my thread: 1999 996 39K miles I'm the original owner Originally had P1124 and P1126 codes, then reset the codes. cleaned the MAF, the air intake, the electrical connections on the O2 sensors and changed the air filter. Drove the car per the recommendations to reset things. Now I'm getting the new codes but not P1124 and P1125. I don't remember the last time the spark plugs were changed as I don't drive the car that often but certainly less than 10K miles. Everything else on the car is original (coils, injectors, etc.). I'm just trying to get an idea of what I should be looking at next. I don't have the air/fuel ratios you're asking about and no idea how to get them.

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Wow... you being the original owner of 21-year-old '99 model means you only drive it around 1800 per year. I would get the fuel injectors and catalytic converters tested for performance as well as a test for any vacuum leaks. In the meantime, run the car until it's near empty on fuel and then put two cans of Seafoam in the tank. Clear the CEL. Drive it around for an hour to let the seafoam do its job and complete a drive cycle with the ECU, then fill up with ONLY Shell V Power. Drive again for an hour or so. Report back your findings.

Since u pulled the connectors for the o2 sensors apart, cleaned and reset them i would look at that connection first.

Thanks for the advice, that's very helpful and I appreciate it. I'll let you know what happens.

Will do, thanks for your help.

If you have a scan tool you should be able to pull the freeze frame data.This data is a snap shot of a predetermined set of data that can show what happened to generate the codes.You should be able to see fuel control data to let you know where to start the diagnostics.With the very low mileage it could be the cat and ego sensors could be coated in unburned fuel,known as coking. I would use fuel supplement and then take it on a good fwy run to clear out the exhaust .Then retest after.If codes return get a shop manual and do the diagnostic routines

Get the Durametric scanning tool and start looking at what the O2 sensors are doing while the engine is running. If you don't know how to interpret this info then get someone that does. Whatever you do, stop throwing parts at it until you 100% what the actual problem is. Right now you are just guessing. You could visually check the wiring for the sensors to see if the are broken or chewed.

Any 02 sensors that are over twenty years old are suspect. If you work on your own car remove the offending sensor and check continuity across heating element. If no resistance replace. If too high a resistance replace.

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With the low mileage your cat converter can be coated with soot or coking because it doesn't get to operating temps where it can burn the coking off and make cat functional .This can and will affect any temp related sensors .I would make sure the fuel is between 3/4 qnd half tank then perform the OBD drive cycle ,you will have to do this so the pcm can test its components and pass the monitors.Take it on a spirited road test on fwy keeping rpm's in upper range ,then see if your monitors have passed. Also make sure your oil is not diluted with fuel ,with low miles it might not be able to get up to operating temps to burn any fuel in oil . If you pull the dipstick rub some oil between your index finger and thumb, if it feels gritty theres fuel in your oil. You might have to change oil also .After any repair to emissions system you will have to do the drive cycle to let it test itself and pass the monitors.

Wanted to give everyone an update. Per advice, ran two cans of Seafoam through the car after resetting the codes and did the drive cycle; still got the codes. Took it to my mechanic who's got about 45 years experience working almost exclusively on Porsches and he hooked it up to his diagnostic tool. He replaced all four O2 sensors and told me to drive it for 100 miles. Drove it for 44 and the "Check Engine" light came on; codes were showing P1124 and P1126. Reset them just for giggles and took it for another drive; this time the check engine light came on at 53 miles. Identical codes are still showing. MAF sensor has been cleaned, new air filter installed and I cleaned the air intake thoroughly before taking it to the mechanic's. I'm taking it back out to him tomorrow but any thoughts on what else could be going on would be appreciated and I'll pass it onto him. The car is a 1999 996 and has 39,906 miles; I'm the original owner.

Replace the maf with a factory unit instead if just cleaning it? Although they should be able to read the maf voltage and determine if it is operating correctly.