OBD2 scanner picked up a code but no check engine light?

JEDI said:

Going for car inspection and decided to check codes before bringing car in.
My scanner picked up code p421 Catalyst efficiency below threshold.

But no check engine light.
I cleared the code and will drive 50miles before bringing it in for inspection.

But why was there no check engine light?

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Best answer is "It Depends". It may be a soft code that hasn't fully set itself or met software/firmware requirements. It could be an information code.

It seems that the P421 code is just during warmup and this may not set a permanent code because it works properly when it warms up.


The PCM is continuously using data, in the form of voltage readings, that it retrieves from the upstream (front) oxygen sensor and the downstream (rear) sensor to ensure that the catalytic converter is working as it should be. The upstream sensor is working as long as its readings are fluctuating, whereas the downstream sensor is designed to be more steady in its readings. If the readings from the two sensors are ever identical, then the PCM will set the P0420 code. However, if this is only happening during the engine’s warm-up period, then the PCM will trigger the P0421 code. If the catalyst system isn’t warming up properly, then it’s not cleaning up the hazardous exhaust fumes efficiently. This is why this code is critical in ensuring that your catalytic converter continues to operate as intended.

Check engine light on and I need to go to inspection. I had used Rislone - Clear P0420 code.
I used it in the past and it worked once but not the 2nd time. Replaced CAT. This was 3rd time. I used it as prescribed, and the CEL did not go out. Since it worked once before I took a chance to do the treatment again that 3rd time. The CEL stayed on but I decided to erase the P0420 code. I drove around and kept checking OBD and saw P0420. But the CEL did not come on. I checked to make sure that CAT READY was on. It was. Immediately I went to inspection and passed. 5 miles after leaving DVM light came on. Thank the powers that be. Good now for 2 years.

pcgeek11 said:

It may be a soft code that hasn't fully set itself or met software/firmware requirements

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+1`, aka pending code!

JEDI said:

drove 50 miles.
passed inspection and emissions

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That's nice. I have a friend with an F150, who failed inspection, to be advised purchase of a new cat-converter. I didn't inform him in time before he'd spend well over a grand.

This works, and it works profoundly. As much as I've been told it only needs application a week or so before the next smog test, I ran it through my exhaust about once every three months prior to running it through the emissions test. I'd kept track of my smog-test results (in CA) since 2001. For HC, CO and NOx, the HC score had been just short of the state's acceptable limit last year. The NOx had also been creeping upward over the past few years. [We're talking about a 30-year-old vehicle with the original exhaust and cat-converter).

After my preparations prior to December, 2023 -- my most recent smog test -- the HC dropped a whopping 50%, CO still showed improvements despite a low and stellar prior score, and the NOx fell by 33% (low rpm) and 50% (high rpm). And generally, the vehicle's performance also noticeably improved.

It's not going to hurt anything. For the Amazon price for two bottles, you could just continue to run it through periodically. Start with a quarter-tank of gasoline. Run it down to a point where you can still get to the next gas pump. Or you could probably add twice as much to a half tank and run it below a quarter tank before filling up again.

I'm so glad I don't have to deal with emissions testing here. Only 3 counties in WA require it. Spokane, King, and Pierce.