did autolite spark plugs screw up coil or coils ??

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Originally Posted by YarisSedan

Okay so before it had a code for a coil problem but not a missfire. Now you have a code for a missfire but not a coil. 3 things can cause a missfire. mechanical, electrical, or fuel related. We can probally rule out mechanical as compression is most likely good. So you are left with 2 other options electrical or fuel. Again you probally do not have a clogged fuel injector so your missfire is now probally caused by electrical problem. Either a faulty spark plug or ignition coil.

OBD 2 systems go by trip cycles. You have to fail 2 trip cycles before a code will trip the check engine light. So within 2 trips you have had 2 consecutive missfires. But you havent had 2 trips of detected coil problems. Unlike before you had 2 trips of coil related problems but not 2 missfires detected yet.

So what you would have to do is drive the vehicle until it shows coil problem cylinder 4. I do not know the resistance specs for the coil. But i think you can test the coil windings with a ohm meter. You can purchase one for as cheap as 10 dollars. You can test the resistance of the suspected faulty one. And compare it to the readings of the other ones. If the reading is drasticly different on one you have found your culprit.

Otherwise you could try swapping the plugs and see if the missfire follows the plug. Or simply just replace them all. If the missfire dosnt follow the coil or plug. Put your ohm meter on the connector that goes to the coil. Rare but its possibly to have a open or short in the harness.

But if that were the case the car would be running like crap with a blinking check engine light severely missfiring.

hey, thanks for your reply again. im not going to explain this correctly im sure. but basically the only reason that code came on is becuase of me doing what that mechanic said must be the problem. but really, it was not until i switched it to where the problem was. then i read the p0354 and p0304 code. then turned off light and put back together as b4 and this morning right b4 my mom and i dropped car off at toyota light went back on reading of course the p0420 code which is what i knew would come back on.

[YarisSedan]

Well at this point if you only have p0420 then i would assume your coils are okay at this point. The missfire monitor on our cars are set as soon as the vehicle is started even if the battery has been disconected. So it would detect a missfire and set a code by now so i would no longer worry about the coil problem but the p0420 which is the catalist effiency below threshold code.

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Originally Posted by YarisSedan

Well at this point if you only have p0420 then i would assume your coils are okay at this point. The missfire monitor on our cars are set as soon as the vehicle is started even if the battery has been disconected. So it would detect a missfire and set a code by now so i would no longer worry about the coil problem but the p0420 which is the catalist effiency below threshold code.

Exactly. and that is why this is so exciting to me.