ok so for about a month my thermostat was stuck open on my car and it wouldnt heat up (december/january) during this time about end of december it started throwing me code 21 twice (12,21,21,55) and i know this means upper or lower o2 sensor fail, i also need to change my oil but the sob drain bolt is so tight that i cant get it off with my 12point socket going to have to get a 6point (something like 13 mm i cant rmember off the top of my head) (not quite 9/16 or 1/2') could the past due oil change be throwing a o2 code, or the lack of having a car that will warm up (fixed the thermostat now car warms up just fine)

what i was thinkin was if the car was sensing it was cold would it try and keep a rich mixture to warm up the motor and cause the cat to get messed up?

any ideas would be nice, ill prolly just replace the downstream and see if that fixes it seeing how its such a cheap sensor.








and sadly sometime this spring/summer im selling my neon to buy a (gunna get a black eye and a broken arm for this one) 5th gen ex/si civic.

 

remember to always troubleshoot codes in the order the are displayed.
Yeah Six point rules for dealing with tight fasteners(As long as they are six point).

 

If you have replaced the thermostat, I would reset the PCM, and see if it throws the code again. Disconnect the negative battery terminal for like a half hour and reconnect it. that will clear all the codes. Try that, if it still is throwing the code go for the sensors, they are a common problem.

 

lol now why didnt i think of that (ive dont it befor lol) guess u just miss the simple things...

 

well it worked for the first 2 times i started the car but now im getting one code 21 so i bet my downstream o2 just got messed up, going to replace it.

are there any special tools i need to take it out, or can i get it out with a regular old wrench.

 

just a deep socket I believe.

 

no problem then

 

Just a thought, not trying to be a nay sayer or anything like that but remember if the downstream O2 is bad then check the CAT out too. See that it is lighting off. If you have a IR temp gun (I know everyones got those just lying around) make sure it is about 100° hotter that the inlet temp. This shows the cataltic reaction is taking place. Also Not rotten egg smells. All else though if a new O2 does the trick, cool.

My concern is only that if it ran rich too long you can melt a cat and thus the O2 may be more right than wrong.

Also with just one remaining code if either of the sensors failed due to being messed up the primary O2 is a little more likely due to the Cat acting as a filter to the secondary. However if you unhook the primary and start it up you could check then if there is 2 codes or still one. If just one now showing still maybe just primary. If two rehookup the sensor and replace downstream and recheck. Just a thought.