check engine light
 

I have a 2000 s with 52,000 miles. About 200 miles ago the light came on. The car was running and idling well and the gas mileage was good. I checked the code and it read P1128 O2S Ran 9e 2 cyl 1-3. I assume this meant and O2 sensor on the odd cylinder bank.
Since the car was running well, I cleared the code and it came back the other day after about 200 plus miles. I cleaned the MAF and it has not come back on in the last 50 miles. I assume it will come back again.
My question is: Which sensor should I replace? I would like to replace all 4, but we are talking $500 plus for the parts. Any suggestions as how to figure out which one is bad? Does the code give an indication of which one?
Thanks for any help anyone can give.

Should be the pre-cat O2 sensor on the passenger side. If it's P1128

That is not a typical code for a failed sensor. Sounds more like an intake air leak/bad maf.

engine light
 

If it was the O2 sensor causing the light to go on would it stay off for 200 miles after I reset the light? Could an O2 sensor be inetrmittant or would it be bad all the time? The oil cap comes off easily while the engine is running and there a a slight change in idle when I remove the cap which i think would indicate the AOS is ok.
After the first time the light came on I changed the air filter, but then it came back on again. Could the plugs be a problem?
The car runs great.

Causes for P1128
 

– Incorrect signal from MAF sensor
– Intake air system leaking
– Fuel pressure too low
– Volume supply of fuel pump too low
– Fuel injectors fouled
– Exhaust system leaking

Of those, fouled fuel injectors may be the cheapest/easiest fix. Sometimes adding Techron to your gas may solve the problem. Chevron gas comes with this additive already.

Otherwise a mechanic will have to clean (or replace- last resort) the injectors.

Good luck.

engine light
 

Thanks for the comments. I cleaned the MAF and maybe cleaning the MAF will help. I will also try some Techron and see what happens. Maybe that will work.

Cleaning the injectors is an interesting operation. It comes down to running the car on something like pure techron for an hour or so. I had to have this done for my Saab Viggen a couple of months ago when it had cold start problems. Most likely one of the injector solenoids was a bit fouled up and sticking. I think the operation is also referred to as 'engine vacuuming'.
If you have this done make really really sure that the oil gets changed afterwards. The cleaning solution is nasty stuff that even eats rubber and some of it can find its way into your oil during the cleaning procedure.