Skip the auto shop - Our mechanics make house calls

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

asked by on

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: how much would it cost to replace a 03 chevy impala fuel filter

Hi there. In order to provide you with an accurate quote, we'll need to gather more information from you. We'll need you to first fill out this request for a fuel filter replacement and one of our sales experts will...

Q: Drive shaft replacement

Hello. Replacing a driveshaft usually require replacement of the driveshaft itself and any bearings or couplings that go with it. Most driveshafts will have at least one center support bearing or flex coupler joint that should also be replaced at...

Q: Can a blown head gasket cause a map controlled thermostat not to open?

Hi there. Although a blown head gasket can cause multiple systems to fail throughout the vehicle, is less than likely that it would impact the operation of the thermostat on the radiator. However, it is quite possible that when the...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the...