What does an oxygen sensor do?

an O2 sensor is in the exhaust stream and measures the amount of oxygen present in the exhaust, sends a signal to the ECU or aka PCM and is part of what information the ECU uses to set fuel/air trim, among other things Here is the oil pressure switch. They are all important, personally I don't know which I would do. If the oil pressure switch, which is a sending unit, completely fails and you will have no oil pressure and not know

Thanks Dave. So basically I need to fix all three now?

'fraid so. None are any more important that the other. I know very well about money being tight these days, trust me on that one. You could skip the oil pressure switch, that is the least expensive one (they deleted a picture I had up of one, must have been a copyrighted image) but personally I would want to have mine functioning. Its a sending unit for the gauge but it's also a switch because if you lost all oil pressure it sends a signal to shut you engine off. It could save your engine from being toasted from no oil pressure with $15 part, and almost no labor, unscrew the old one and thread in the new one one, plug wires in, 15 minutes. The catalytic convertor will be the most expensive part...unless you need two or three 02 sensors. You will suffer greatly reduced mpg if you don't do the 02 sensors also. The only other suggestion I have is get a second opinion, if you don't know and trust this mechanic. If you do, them no need for that. Or have the OBD codes scanned for free at any large parts house and see if they agree. No harm in that and no cost either. Good luck kcon

Thanks for your help David. Think I'm getting a second opinion today.

kcon i would go with o2 sensor first,the cat reading could have been affected by the o2 readings,if your engine does not seem to rattle i would do oil sender at some other time,hope this helps

Thanks Ford. Engine hasn't been rattling -- at least I haven't noticed it the little that I drive it. Its had a bad secondary o2 sensor for a while. Replaced it about 2 - 3 years ago, then about 6 months later the other went bad. Replaced that one, then about 6 months ago it went bad again. Haven't fixed it since. Wodner if that matters?

kcon,that shouldnt be a problem,good luck

done oil change today on our stream an the oil light has come on can any body help out what might have caused this problem b4 spending big money on a cheap little car