have an '05 Legacy Sedan with a 2.5I that throws p0131 and p0031 consistently. eats the a/f sensors...haven't had one last for more than 9 months or 15,000 miles for three years...CEL will come on for p0131...replace a/f sensor...CEL will come on for p0031...replace a/f sensor and so on and so on...if it comes in for one code you can bet money the next time the other code sets...have replaced numerous sensor and the a/c relay but cannot find the gremlin...anybody else ever come across this type of thing?

How many miles? Automagic or manual transmission?

 

Recurring issues generally mean the sensor is probably not at fault.

How long does the code stay away after replacing the sensor? Days, weeks, months? If you just reset the code how long does it take for it to come back?

Have you done any diagnosis or had any diagnosis done by a dealer or mechanic?

i actually work at a dealership...code reccurs no longer than 9 months or 15,000 miles(varies)...have gone through the whole troubleshooting tree from checking connectors, resistance,harnesses,replaced a/c relay per our hotline, and it always leads to a/f replacement...if we just clear the code it almost immediately comes back on...out new sensor in and(again) lasts from anywhere between 4-9 months or at the most up to 15,000 miles

The A/ F relies on a reference voltage [battery or a 5 volt reference from the PCM]. If This voltage is not very close to battery voltage, with in .2 volts or the 5 volts is off say 4.6 or lower this will be enough to set a code. The same is true for ground. If you put the black on the batt. neg run the car and probe the ground wire with the red pos. lead there should be no more then .05volts better less .02. If ground is bad this will set a code also. This sensor works on heat generator-ed by the sensor as air passes over the heated element the element cools and the resistance changes this is what the cars PCM calculates fuel demand for the engine. I hop this helps Kevin

I actually had one of these in the shop ('05 Outback 2.5i) a couple days ago with the exact same issue but I also had a P0132 and a P0134 as well as the P0031 and P0131. Only thing I can think of is a bad ECU.