1997 chevy lumina DTC: P0420

[cfrank2]

About a week ago i took my lumina to the shop to have the lower intake manifold gaskets replaced. After getting the car back a few days later in a store parking lot the car would not start and it backfired a few times and i heard a hissing noise, long story short i had it towed back to the shop that did the work and when the did the LIM job they left a nut in the engine and it made its way into the water jacket in the back middle cylinder and cracked the head and flooded the piston with coolant. So i just got it back today after they replaced the head/and most valvetrain components on that cylinder bank and i get a DTC P0420. What is this and what could have casued this? I just replaced both oxygen sensors before it went in the shop to have the LIM gaskets done.

Thanks

[jeffcoslacker]

I didn't look it up, but if that's a o2 sensor code, coolant burn residue is poison to o2 sensors...they are probably ruined...

[jeffcoslacker]

P0420 Catalyst System Efficiency Below Threshold
Might just be burning off coolant fouling...

[cfrank2]

Could it be the oxygen sensor? i just replaced both of them about 3 weeks or a month before it went into the shop but it was leaking coolant for all this time and i didnt have a problem. I cleared the code using a scanner and a few miles down the road it came back on, could it still be burning stuff off?

[jeffcoslacker]

It could be the o2 sensor(s)...they will usually code for a sluggish response or not enough variance in the signal, but not always. If they are hogged, but not bad enough the ECM still believes the signal to be valid, it could show as a cat efficiency problem, I guess...it compares the pre cat and post cat o2 signals to determine if the cat is working properly...

[sad-lumina-owner]

Originally Posted by jeffcoslacker

It could be the o2 sensor(s)...they will usually code for a sluggish response or not enough variance in the signal, but not always. If they are hogged, but not bad enough the ECM still believes the signal to be valid, it could show as a cat efficiency problem, I guess...it compares the pre cat and post cat o2 signals to determine if the cat is working properly...

If this is the case, then only ONE of the sensors (or one pair) is actually fouled, and may need replacing (or cleaning?).

This would make sense: The sensor in the exhaust manifold BEFORE the Catalytic converter which got fouled would not register enough oxygen.

Then the good sensors would maybe show nearly the same Oxygen after the converter, when in fact there is more oxygen before the converter, and the converter (hopefully) isn't fouled.

First just replace the one (or two) sensors BEFORE the converter. It is unlikely the ones AFTER the converter were fouled.

Maybe they can be cleaned, saving a few dollars. Here's a great thread explaining how they work:

http://www.aa1car.com/library/o2sensor.htm

[cfrank2]

That makes sense, the nut that was left in the engine from the LIM job destroyed a water jacked on the back middle cylinder so that cylinder got flooded with coolant/water and locked up the engine so the 02 sensor in the exhuast manifold is most likely the cause right? how do you clean them? i just replaced them and i would really like to clean them instead of replacing them if i can.

[jeffcoslacker]

There is no way of cleaning them that I am aware of...cooked on coolant residue is very tough stuff, the o2 sensor's construction is a vented shield shell around the actual sensing element. The sensing element is extremely prone to damage or fouling, methylated cleaners of the kind you'd normally use to remove deposits like that are sure death to one...and there's no direct access to the element anyway, exhaust gases follow a kind of convoluted path to, to protect from direct heat that would otherwise torch it...

Now if it were a matter of the vents simply being clogged up, that could be dealt with, I suppose...but I've never run into that...

[cfrank2]

I didnt think they could be cleaned either, but i thought i would ask. So based on what what has happend to my car would the 02 sensor in the exhaust manifold be a good place to start trouble shooting? If this sensor was bad and reading too close to the sensor after the converter could it set the DTC code that i have?

[jeffcoslacker]

In theory, yes. I would look at the pre cat sensor first...if it's dusted off-white and has a fairly good coating, it's probably bad. Normally they are slightly tan/brown to near black. being that new, if all is well, there should be very little residue on it.

[jeffcoslacker]

Of course I'd probably take it out on the highway and run the bejeezus out of it for 20-30 miles at 80+ mph and see what happens before doing anything...I've put out many a CEL before with this simple act...

If you get pulled over, tell them I said it's ok...they won't mess with ya...

[cfrank2]

I replaced the pre-cat sensor and it looked terrible(not to mention it looked like the idiots in the shop used a pair of pliars on the sensor body!), after that i took it for about a 15 minute drive and i got the same code again. Could it be anything else besides the converter or the post-cat oxygen sensor since i just replaced the pre-cat?