Check Engine Light Occurs only when I first start the vehicle



I have been noticing my Check engine light flashing (ON and OFF) after I run it for few miles and after few minutes it goes away. The strange issue here is it happens only when I first start vehicle in the morning.

During the day when I drive I do not see it. I took it to my mechanic and Auto zone and they checked to see if any error codes. It said all passed. I recently did a major engine head cylinder replacement.

Please advice.


Perhaps the connector for one of the sensors you removed in doing the head work is loose.


A P0304 code was detected. My mechanic said it could be something with COIL.


P0304 means that the computer has detected that cylinder 4 has misfired.

You could have a problem with your ignition coil. But did the mechanic test it? That’s not the only potential problem. A bad plug or wire could lead to this. Low compression in the cylinder could lead to it. A bad fuel injector… leaking intake manifold gasket… need to recalibrate the pcm…

Lots of potential problems. Several of which could easily be related to things not being quite right after the cylinder head replacement.


you didnt say what kind of vehicle or engine you have but if the light is flashing on and off like you say that means you not only have a trouble code set which you said is p0304 and shortyoh is correct about #4 cylinder misfire and the rest he said but let me add that the flashing of the light means the miss is bad enough to potentialy cause convertor damage. so get it fixed sooner than later and avoid having to replace a convertor