Hello all, I'm new to the site!

Recently I had my 1994 850 Turbo checked out at a garage because there were 02 sensor codes triggering the CEL. I was informed that they inspected the sensor and a few things related to the sensor and it seemed to be just fine, that maybe the light was stuck on by mistake from earlier codes or something, so they cleared the CEL for me. 2 days later, the CEL comes back on so I checked the codes and got the same 2 again:

5-2-2: Rear H02S preheating
1-5-3: Rear H02S signal

Should I have someone else check out the sensor again/replace it, or is there anything else that could trigger it by mistake (like they said before), such as the wiring to the CEL?

P.S., forgot to add that my average mpg was about 20.2 up until a week ago when it's dropped slightly to about 19.6, don't know if that has anything to do with it.

Thanks!

 

Have the sensor replaced.

 

You definitely need a rear O2 sensor; I would replace the pair and use only OEM sensors. Not aftermkt, not OE equivalent, etc.

The reason I say replace both is:

1. If one went bad, the other is usually not far behind.
2. The rear is only there for OBD2 to test the cat convertor efficiency; not generally used to control the fuel mixture. Be surprised if bad rear sensor would effect gas mileage dramatically.

 

I was getting a code P0422 and replaced the rear o2 sensor. The bad one is stuck in the exhaust so I had a muffler shop put another sensor socket behind the cat. I installed a used o2 sensor back there and now my gas mileage has dropped 15%. I wish I could afford a new sensor or two, I'll see what happens. There is an off chance that my mpg drop has something to do with my ECU not getting a speed signal, well see soon.