Engine cranks but won't start

Error codes are P0421, P0455, and P0443.

Not car savvy but my understanding is it’s a cat/air flow issue. Just need advice and maybe a general idea for repair cost.

Well the good news is we know your car is at least a 1996…

What would be really nice if you included the year make model sub model engine transmission type and FWD RWD 4WD/AWD so we have a clue as to what you are driving…

The repair cost may vary a little bit depending on if you are driving a Ferrari or a Corolla…


The codes indicate there’s a problem the the catalytic converter and an EVAP leak.

But none of this would prevent the engine from starting



The p0421 is saying the pre-cat and post-cat O2 sensor signals are not matching up as expected as the engine warms up from a cold start. That wouldn’t normally cause a cranks but won’t start, unless the cat is completely clogged. Your shop might be able to temporaily remove the pre-cat O2 sensor as an experiment. If the engine cranks and starts immediately , pretty good chance you’ll need to replace the cat.

The other two codes are indicating some sort of problem or problems in the evap system, and unrelated to the first code. The evap system is the part of the fuel system that contains the gasoline fumes so they don’t escape & pollute the air. That problem could cause a cranks but won’t start, if gas fumes were entering the engine during cranking making the mixture too rich. There’s a valve (purge valve) that’s supposed to be closed during cranking to prevent this, but maybe yours is stuck open. You might try holding the accel pedal all the way to the floor while cranking. If it starts then, pretty good chance your purge valve is stuck open. That could cause both of the last two codes. Evap problems are usually diagnosed by a visual inspection, then if nothing obvious is found the shop will do a smoke test, where they inject visible smoke into the evap system then look around for any smoke escaping.

For best help here btw, suggest to also include make/model/year/engine & trans configuration.

Best of luck.

To the basics anyway, spark or fuel. When you turn the key, do you hear the fuel pump start? It should run for a few seconds and then shut off unless the car starts. Checking for spark would be the next thing. If you can determine one or the other, that would help narrow the problem. After that more diffficult issues like timing or compression. An explanation of the circumstances of it not starting would be helpful.

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Cost—-In addition to above, cost will vary by location. Cats in California are more expensive. Independent exhaust/muffler shops usually less expensive than dealerships.
First cost is diagnosis ~$150-$200. If it will not start and is not drivable and you do not have towing covered by insurance, add cost of towing. Avoid chain stores such as Midas, Pep Boys, etc.
The good news, if applicable:
The Environmental Protection Agency mandates that automakers provide special warranties for emissions components, including catalytic converters. Under these warranties, major emissions components are covered for eight years or 80,000 miles, whichever happens first.

None of these fault codes are for a system that would keep the engine from starting. You’ll need to find a shop or friend with a little more mechanical ability.

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Sorry for the lack of info. Mazda 626 2001 don’t know much else. A pretty recent purchase. Got the same response from a mechanic that stopped by while we were waiting on a ride. He’ll be running diagnostics to figure out why it won’t start. Thank y’all for responding!

The engine may not start due to a bad crank sensor.

And if the mechanic is going to run diagnostics on the car, inform them the a bad crank sensor may not turn on the Check Engine light and set a code.


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I will just emphasize that twice I have been left high and dry from a bad crank sensor. Car running fine one minute and stalled on the interstate the next and no code. ‘‘Twas why I just pre-emptively had a new one put in this summer.

Crankshaft position sensor? Maybe, but I didn’t hear any mention of whether or not the engine even has proper compression. These cars were offered with a 4-cylinder and 6-cylinder engine, both of which use a timing belt. If it was never changed, I’d expect the timing belt to fail at around this age.

I’m pretty damn sure a mechanic can tell if the timing belt broke by the way the engine sounds while cranking over.



Exactly. I’ve only had one chain fail but the sound while cranking is evident. If you have a second person and you can see if the lifters are moving is another quick test.