Q: How do I know if I have a bad coil or a bad fuel injector?

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I had a tune up done and it still runs really rough and bad on gas, the check engine light will blink while misfiring. P0303 cylinder three misfire is the code.

My car has 145000 miles.
My car has an automatic transmission.

The easiest way to test for spark is to pull the spark plug boot off of the number 3 cylinder spark plug and install a spark tester. Most auto parts stores have them for a few bucks. The spark plug boot plugs into the top of the tester, and the other end of the tester has a boot that gets installed onto the spark plug. When the engine is started, there is a section of the tester that actually will light up, or spark, if there is an ignition pulse going to it. As far as the injector goes, you can also purchase what is called a "noid light", that will check to see if the injector in question is getting the electrical signal necessary to fire the injector. You would simply unplug the injector, plug the noid light into the harness for the injector, and when the engine is started, will show a light when the pulse to the injector is present. If this is something that you feel you could use a hand to diagnose and/or repair, contact a certified mechanic, like the mobile mechanics available at YourMechanic.com.

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