Replace Which Oxygen Sensor First?

Car is a 2006 Subaru Outback XT, turbo with automatic transmission with 103K miles. Code is P2097 $10, post cat fuel trim system too rich; bank 1. I've already replaced the air filter and cleaned the mass flow sensor. I think my next step is replacing oxygen sensors. [Other possible causes seem to be leaking injector or a fuel pressure regulator fault.]

So which oxygen sensor do I replace first, the upstream ($95 at Rockauto) or downstream ($51) sensor? I plan to use an OEM Denso replacement. Or should I try a different fix first?

Thanks for your thoughts!

David

Based upon the code saying "post cat" I would think its the downstream sensor.

The code is reading that the rear fuel trim is too rich, so the voltage is spiking in the downstream. That sounds like it is working to me, but if you want to replace it because you think it is malfunctioning, replace the downstream. I doubt that will be a fix, though.

Down stream is after the convertor. If the upstream is happy, the convertor is not working.

Wait... my theory...

I would plug in to a scanner and see what the voltage is from the upstream. It sounds like it might be failing and commanding too much fuel which is being read by the correctly-operating downstream.

The downstream won't read rich if the converter is failing. It will read converter inop P0304. The downstream is reading rich; either because its failing, or because its actually rich. Usually (I say usually) an overpressure or leaking injector reads on the upstream first since some of the excess HC is burned/caked in the converter.

If your scanner won't do realtime data, go to AZ or Advance and have them read the codes. Then grab the scanner from their incompetent hands and read sensor voltage.

Just huck parts at it. Its way cheaper than taking it somewhere to be repaired correctly.

Downstream sensors don't affect engine management strategy decisions, they are there to make the EPA happy. They just monitor catalyst efficiency is within set parameters and set a code if the cat is not working. (or if it looks like the cat is not working)

P0304 is a misfire on cylinder 4.

Get yourself a Subaru manual. There will probably be more info in there about this code on this particular car, including troubleshooting info, than the generic code definition.

bentwrench wrote: Downstream sensors don't affect engine management strategy decisions, they are there to make the EPA happy. They just monitor catalyst efficiency is within set parameters and set a code if the cat is not working. (or if it looks like the cat is not working)

Yes, they do change the fuel management. They are used to trim the fuel a little to maximize catalyst effiency.

I'm with Curtis on this. A failed O2 sensor will be more 0 v than high. So I'd suspect they first sensor of being bad. Still, the suggestion to scan it and see it run live is the best idea right now.

Opti wrote: P0304 is a misfire on cylinder 4.

Yeah... I meant P0430 but my fingers went too fast.

I should mention that while I've gotten this code four times, it happens only occasionally; like four times over five months, and never more frequently than once a month. Does that change the diagnostic?

Also, my code reader can display live data. Can you explain what exactly I am looking for? A zero voltage from one of the oxygen sensors or something else?

Thanks a ton!

David

In reply to DWNSHFT:

The upstream sensors should fluxuate rapidly spiking up and down while the rear sensors should slowly change just slightly. If they are doing that, the sensors are working and so are the catalyitic converters. If everything appears right, your problem likely lies somewhere else.

Something simple could be making you run a little rich. Cam or throttle position sensor... a vaccum problem....