O2 Sensor questions
 

So, my daughter's '05 2.0 3 has a CEL that's been on for a week or two. Codes popping are for O2 sensor. I've noticed that there are two of them, and the code doesn't seem to indicate which of the two is bad. After researching the price, I've now noticed that the two sensors are indeed different, and that they don't cost the same.

So, my questions are:
1) is one of these more likely to be the bad one?
2) do they typically fail at the same time, or does the failure of one inevitably CAUSE the second to fail?
3) if one fails, will it KILL the other one?

I ask the last question because my plan is (potentially) this- to first replace the CHEAPER one, and see if the CEL goes out. If it doesn't, I'll order and replace the second one. I just want to make sure that replacing one won't end up costing me the other, etc.

Hope that makes sense. I just don't want to waste money on the wrong one, or replace one when I should just suck it up and replace both, since one will kill the other, etc.

Thoughts?

Unfortunately, without the actual code it is a guessing game. The sensor upstream closest to exhaust manifold, usually before the pre-cat is the main one that is responsible for air/fuel mixture. The one after the pre-cat only monitors the efficiency of the pre-cat (if it is plugged, comes on at around 95%).

Fault code will show which sensor is causing trouble.

H20S11 is the sensor closet to the manifold
H20S12 is in or behind the CAT.

Sorry it's been awhile guys- thanks for the help. The codes that the local parts store's reader came up with are: P0171, P2274 and P0304. Of course the printouts from their code reader (three different ones- one for each code) list the vehicle make as "domestic," "ford" and "mazda." For the P0171, one of the "probable causes" is dirty/defective MAF sensor. I have cleaned that, but haven't had the codes rechecked to see if that one's cleared or not. The P2274 cites either failed oxygen sensor, air leak near oxygen sensor or poor electrical connection to sensor circuit. The P0304 points toward ignition system fault (spark plugs), ignition wires or coil, a vacuum leak, injector fault or high/low fuel pressure. When these codes were pulled, the car would barely run to get home, in addition to the CEL light being on. I put new plugs in and cleaned the MAF sensor, and it runs great, but the CEL is still on. So, that points me toward the last code- P2274 and an oxygen sensor. But I don't know which one it's referring to.

Does any of this help? Thanks all.

I just found one more phrase that might help- "Oxygen sensor signal stuck lean- (HO2S13) Heated oxygen sensor- Bank 1 Sensor 3.

That's verbatim from the printout. Unfortunately, it doesn't match the codes posted by grim reaper. :(

I've never heard of HO2S13, but it will be the third sensors down from the manifold.

Try disconnecting the negative terminal of the battery, followed by some brake pedal applications. Re-hook it up 10-15 minutes later. Let it idle for 10-15 minutes.
This will clear all codes and let re-learning commence. Drive for 50 miles.

UPDATE- Had a shop troubleshoot, and they diagnosed upstream O2 sensor. I replaced that this morning (RockAuto delivers FAST!). I left battery connected as I didn't want to foul up daughter's radio, etc. CEL still on, although it hasn't been driven yet. The CEL should go out eventually on its own, correct (assuming diagnosis was correct)?

I'll keep you posted...

Thanks for the feedback. I'll try UseYourNoggin's method if CEL doesn't clear soon.