Just replace the O2 sensor?

March 5, 2014 11:01 AM

Have an 05 Subaru Legacy. ~70k miles. The check engine light comes on reliably with wide open throttle (and considering I'm in Pittsburgh, the land of bridges and merge points...). The diagnostic code says low voltage on bank 1 sensor 1. Should I just swap the O2 sensor? I ask because I don't really understand why I'd only be getting the low voltage code with WOT.
posted by bfranklin to Grab Bag (3 answers total)

Best answer: Because WOT is the only time you get max airflow, so only under that condition does the reading from the lambda sensor fall outside expected parameters (which is what the light is telling you).

Change both sensors if there is a downstream one (it implies there is) - no point just changing one unless you have done so recently. The upstream one always goes first, but the second one will likely follow.
posted by Brockles at 11:22 AM on March 5, 2014 [2 favorites]

Yeah, what Brockles said. I have found that Just changing the O2 sensors on used cars is a good idea, unless they are crazy expensive. I have gotten a little better mileage and the car runs smoother (especially when cold it seems) with new 02 sensors. I then try to replace them every 50k or so after that.
posted by bartonlong at 1:31 PM on March 5, 2014

'07 Legacy here, 127k miles, original owner.

I just had both O2 sensors replaced (due to a P0420 code thrown and thus CEL constantly on, which *conveniently* started right before my emissions inspection came due) and it was about $750. My mechanic said both sensors had failed. If only one has failed for you, it will be cheaper to only replace the failed one. I think the downstream one was more expensive for some reason, but I don't have the receipt in front of me.

CEL has been on and off since then, but my mechanic did warn me that I may need a new catalytic converter as well.

This is an expensive repair. I'm sorry. The silver lining is I got a 4 month extension on the state inspection, and I've spent enough on repairs at this point to get a repair waiver if I do fail it.
posted by tckma at 3:12 PM on March 5, 2014