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Cel p0421

I know that any cleaning additive in the gasoline, when going through the pre-cat, sensor 1 and sensor 2 will be in a gaseous state while the engine is running.

But if I rev the engine to 5500 rpm and then shut off the ignition wouldn’t a great deal of the incoming fuel, since there will be no spark to ignite the gas, pass through the pre-cat sensor, the pre-cat and the post-cat sensor in a liquid form and perhaps have a cleansing effect on the system?

Any mechanical engineers here?


No. Shutting the ignition off will stop both fuel delivery and spark at the same time.

Since we don’t know what year, make, model, motor this is, it’s a guess, but most likely no. The fuel injection system (if fitted) is shut off at the same time. If you have an older, carburetor model, maybe.

If you happen to have an intercooled turbo fitted to this one, and you shut down at 5500 rpm, expect to replace the turbo very quickly.

I have to assume that you’re not just curious and that instead you have a problem. Furthermore, I assume that you have a clue - which is a P0421 error code.

You’ll likely get more mileage here by just describing exactly what is going on and asking about how to address it. You have a P0421 - what has led you to believe that you should try to clean your O2 sensors and pre-cat?

P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)

Anyway, if you have a modern multiport injection fuel system each injector opens specfically for its associated cylinder only, and the moment you shut the engine off the injector circuits are disabled and none open. There will be no fuel coming in.

If you have a very old carburated system there might be. In the early '70s, a few of the first emissions reduction efforts the manufacturers made was to increase engine operating temps (T-stats went from typical 165F to typical 195F) and to “lean out” the burn mixture. That resulted in cylinder temps so high that if there were any fuel in them it would self-combust when heated by compression. This process is called “dieseling”, because using compression generated heat to ignite the fuel is the way a diesel works. And since fuel was drawn in and metered by the air being pulled in every time a piston went down, for many engines that meant that when the key was turned off the engine would continue running until the fuel level in the float bowl dropped to a level insufficient to continue providing fuel. Engineers had to solve this tendency with “idle stop solenoids”. Thay totally shut off the air passage when the key was OFF via a circuit that got deenergized, and the engine choked.

To answer your question directly, the only way to possibly clean the oxygen sensors with raw fuel is to remove them and use a toothbrush. They still might not work but they’d be clean. Since it’s impossble to clean the catalyst, it probably wouldn’t matter anyway.

I turned the CEL off Wednesday morning (P0421) and so far it has stayed off. It must be the Chevron with Techron I’ve used for the last 230 miles, or the fuel additive suggested by Auto Zone, or, perhaps, it is the Berryman’s B12 fuel injector cleaner I added.
Whatever caused it, the light has been off for the last three days. Let’s hope it stays off. I’ll continue using Chevron for another 500 miles.

I read somewhere that you should change brands occasionally to remove the build up from the additive. Each of the Top Tier gasolines have their own special cleansing additives but, over time, they too can build up deposits and changing brands will remove them.

Anyone know for sure if this is something for Myth Busters?

Oops!! The CEL came back on yesterday, Saturday, after staying off since Wednesday. Quite a long time to go through all the resets necessary to activate the P0421 error.

I bought a downstream 02 sensor and had my mechanic install it
yesterday. So far the light has stayed off. I’ll report back in a week
and let you know if it stayed off or not.

Three days and still no CEL.

CEL came back on last Monday. While waiting for a new upstream sensor I turned off the CEL. Now, 6 days later still no CEL.

My Mazda passed smog with one “Not Ready”. Happy, happy!!!