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Another P0420 code.... Technical

Old 04-26-2015
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Another P0420 code.... Technical

130k miles.
Serviced Regularly.

CEL came on a few weeks ago (P0420) which prevented car from passing smog (California).
Turned CEL off and drove 100mi before heading back to get smogged.
Drove an additional 100mi with at least 6 warmup/cooldown (after smog) before CEL came back on again with same code.

"Freeze Frame" data from CEL doesn't show either O2 sensor values at time of CEL so I decided to drive to work today (30mi) with scanner plugged in the whole way to monitor the O2 sensors along the way. 02s1 fluctuated rapidly within its limits while 02s2 fluctuated little within its limits also.

I was hoping to see one of the O2 sensors reporting out of its limits to pinpoint a fix but now I'm lost at what I should do next.

Could one of my sensors (most likely O2s2) intermittently be becoming "lazy" and be the suspect of the CEL?
If the converters were bad, wouldn't they immediately throw a CEL after warm up? No way for them to be intermittent right? Either they're good or they're bad right?

What should I check next or plan on replacing first?
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Old 04-26-2015
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Re: Another P0420 code.... Technical

Could one of my sensors (most likely O2s2) intermittently be becoming "lazy" and be the suspect of the CEL?
The sensors have to pass a good ten or a dozen tests or more. If any of those O2 sensor tests fail, the cat monitor is suspended.

If the converters were bad, wouldn't they immediately throw a CEL after warm up?
Nope.

Quote from some service info (random guessed at 2003): "The PCM calculates the moving average of six drive cycles and compares it to the predetermined threshold in the PCM", so it would seem that after clearing your codes it takes at least 6 drive cycles for the computer to gather enough data to judge the cat during a self test..

Once it has gathered enough data about the cat:
The cat code is a 2 trip monitor (meaning it takes two consecutive self test failures to turn on the check engine light), and a rather specific set of conditions must be met before a test is enabled.
That means the cat monitor doesn't necessarily run each time you drive the car.

No way for them to be intermittent right? Either they're good or they're bad right?
A cat that is "borderline" may pass or fail a test at any time.
So in a way, when combined with the 6 drive cycles info noted above, it may seem to appear like an intermittent failure (as evidenced by how long it took for your code to return).


As the oxygen storage capacity of the catalytic converter continues to degrade, it will fail the self tests more consistently.



What should I check next or plan on replacing first?
About the only thing that commonly causes false cat codes is exhaust leaks between the engine and the cat. Cracked manifold? Broken studs and leaking around the gasket?

If there are no exhaust leaks, it will need a cat.
You will need a cat that is CARB approved or an OEM cat.

You also need to figure out if this cat died a 'natural death' or if some poor running conditions caused 'premature death', and correct that problem so another cat doesn't fail prematurely.

HTH
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