Car stalls at startup when warm - P0420 Code

KA_FS14924

MEMBER
1999 HONDA ACCORD
4 CYL
2WD
AUTOMATIC
111,500 MILES

I have been having thi startup problem for 4 years now. Its not a show stopper but would like to fix it. At startup if my engine is warm (or was driven within last 20 minutes or so) my car dies if I don't give little bit of gas. Not giving gas leads to RPM droping to 0. But if I do give little bit of gas the rpm needle jumps and then drops down to slightly above 1000. Lately I have been getting the P0420 code. The first time I saw the Check engine light I read P0420 and turned it off. Next time the CEL came within 3 miles of filling up my gas tank I cleared it and it has not come back on yet in the last 20 miles of driving.

I have had the following done already on my car. Timing Belt, Water Pump, Distributor cap, spark plugs, wires replaced to solve other problems.

Thanks for the help

1 Reply

SQM

MECHANIC
6,383 POSTS

Hello,

The code:
P0420: Catalyst System Efficiency Below Threshold

Typically you will get that code when there is issue with the oxygen sensor.
However with the drivability issue you are describing, you could have a clogged catalytic converter.
That's why you have to keep your foot the gas while starting otherwise the engine bogs down and stalls.
The catalytic converter has honeycomb shape fins inside of it and they can get clogged or restrictive to air flow. When this happened it will impact the drivability of the vehicle.

Here is helpful video of how to test the converter:

https://youtu.be/kNXh1Lphzr4

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter
https://www.2carpros.com/articles/bad-catalytic-converter-symptoms
https://www.2carpros.com/articles/catalytic-converter-replacement

I have attached the diagnostics for the P0420 code.

Please let me know of any questions.

Thank you.

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