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Flashing P0420

W

WingBender

CEG'er
Joined
May 28, 2006
Messages
59
Location
Seattle
The CEL goes on for a while, then goes off for a while, then goes back on, etc. I took it to AutoZone today and downloaded the code: P0420

"The PCM has determined that the catalyst system efficiency for bank 1 is below threshold for the current engine operating conditions. (Bank 1 identifies the location of cylinder #1....)"

"Definition: Catalyst efficiency low bank 1"

"Explanation: The oxygen sensors monitor the catalytic converter's ability to store oxygen."

"Probable causes:
1) Catalytic converter defective (failure possibly due to #2, 3 or 4)
2) Engine misfire or running condition
3) Large vacuum leak
4) Engine oil leakage into exhaust-valve guide seals, piston rings

Well, let's see here. The catalyst and the O2 sensors have been replaced within the last couple of years. If there were a large vacuum leak, it would have affected how the car runs, but it is running fine (I can't tell any difference in how the car is running between when the CEL is on or off). When the CEL goes on, it is usually when I have been driving for several miles. My understanding of valve guide leaks is that they occur when the engine is shut off, and the oil gets blown out during a restart. Based on that, I would eliminate leaky valve guides.

Which leaves me with #2, engine misfire. Any thoughts, or am I all wet here?
 
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A miss would give you other codes long before it threw a P420. It's either your cats or you downstream O2 sensors. As long as you're not having any kind of exhaust restriction problem (you would feel the poor performance), just get MIL eliminators. Way cheaper than replacing the cats or sensors.
 
Thanks for the tip. I live in an area where I have to get a smog test every two years. Would the MIL eliminators affect the results? Can they be left on for the smog test?

(BTW, what does MIL stand for? Other than mother-in-law? Pardon my ignorance.)
 
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MIL = Malfunction Indicator Lamp.

Yes, the MIL can be left on for the smog test. They'll probably not even notice it's there. You can tape it up so it "blends" in with the wiring.

We have an emission test here and I pass with MIL eliminators. Although, I don't know your emission laws, I would think you should pass.
 
In California, you cannot pass a smog test with the check engine light on.

Sometimes a 420 code can be cured by replacing the UPSTREAM oxygen sensor.

OFTEN, the 420 code on our cars is a phantom code with nothing really being wrong with the cat.
 
In the state of Washington where I live, the first thing they do when you go for your smog test is to look for a CEL. If it's on, they send you packing. They flunk you without even putting the probe in the exhaust pipe.

And now that I've had time to think about it, a product called a Mother-In-Law Eliminator has a lot of potential. Anyone here work in marketing?
 
correct me if I'm wrong, but doesn't a flashing CEL mean misfire? I know that's what my brother SVT_22's car did when it has a misfire...
 
Intermittent CEL

Intermittent CEL

CSVT_2004 said:
correct me if I'm wrong, but doesn't a flashing CEL mean misfire? I know that's what my brother SVT_22's car did when it has a misfire...
Click to expand...

OP does not have a flashing CEL. He has an intermittent CEL.

WingBender said:
The CEL goes on for a while, then goes off for a while, then goes back on, etc. I took it to AutoZone today and downloaded the code: P0420
Click to expand...
 
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