O2 sensors causing misfire?

[thrice1825]

Hey All - I've been having some intermittent misfire issues that seem to be worse at low rpm. I went to get the codes read today and had:

P0300 - multiple cylinder misfire
P0305 - cylinder 5 misfire
P0306 - cylinder 6 misfire
P0135 - bank 1 sensor 1 malfunction (o2 sensor)
P0141 - bank 1 sensor 2 malfunction (o2 sensor)

What do you all think? 02 sensor causing misfire or misfire causing bad o2 sensors?

Car has 212k. New plugs, fuel filter, and valve cover gaskets have been installed to fix the issue but no luck...

Thanks!
-Matt

 

[WallyWest]

I would think it's the other way around. Your misfires are causing the O2 sensors to not see what they expect, which causes the O2 codes. I had a faulty O2 sensor for a while, it didn't throw any other codes. Either that or you've flushed enough unburnt fuel through the engine that it's killed the O2 sensors. Have you ever seen a blinking CEL? If so, possibly injector issues, although you should see some other codes if that's the case. In any case, if you've actually damaged the O2 sensors your CAT isn't far behind.

You say new plugs, how about the wires? If they're old replace them too.

 

[thrice1825]

I would think it's the other way around. Your misfires are causing the O2 sensors to not see what they expect, which causes the O2 codes. I had a faulty O2 sensor for a while, it didn't throw any other codes. Either that or you've flushed enough unburnt fuel through the engine that it's killed the O2 sensors. Have you ever seen a blinking CEL? If so, possibly injector issues, although you should see some other codes if that's the case. In any case, if you've actually damaged the O2 sensors your CAT isn't far behind.

You say new plugs, how about the wires? If they're old replace them too.

Thanks, Wally. I did really lace the plugs about 3 years ago. But perhaps as a last resort it may be worth doing again.

 

[TheDurk]

I don't like to guess; I like to troubleshoot. They are trying to kick me off the internet but I ain't gonna change.

That's two circuit malfunction codes, basically the ECM is seeing open or short on both sensors. If those are original, at 212k, they are well beyond end-of-life, and I'd just change them as required PM. If more recent, then do as the FSM suggests and check circuit integrity.

On a misfire like that (more than one cyl., not on the same coil), the first thing to do is inspect the plugs and wires involved. That can tell you a lot. I wouldn't venture a guess until that was done. Feel free to post pics of those plugs. Are you sure you got the right plugs? What were they?

I'm thinking the engine can't even go into closed loop with those sensor codes, so the running rough could definitely just be a function of that.

 

[thrice1825]

I don't like to guess; I like to troubleshoot. They are trying to kick me off the internet but I ain't gonna change.

That's two circuit malfunction codes, basically the ECM is seeing open or short on both sensors. If those are original, at 212k, they are well beyond end-of-life, and I'd just change them as required PM. If more recent, then do as the FSM suggests and check circuit integrity.

On a misfire like that (more than one cyl., not on the same coil), the first thing to do is inspect the plugs and wires involved. That can tell you a lot. I wouldn't venture a guess until that was done. Feel free to post pics of those plugs. Are you sure you got the right plugs? What were they?

I'm thinking the engine can't even go into closed loop with those sensor codes, so the running rough could definitely just be a function of that.

I like the way you think, TheDurk, and I appreciate your help. Here are the plugs I took out of it. They are about 3 years old with probably 20k-ish miles.

 

[thrice1825]

And another

 

[TheDurk]

OK, no red flags there. I'd change my upstream O2 sensor unless it's recent. The downstream may throw a code but has no effect on engine operation.

 

[thrice1825]

OK, no red flags there. I'd change my upstream O2 sensor unless it's recent. The downstream may throw a code but has no effect on engine operation.

I'll give that a shot. Thanks for the help!