Misfire p303

A couple of days ago my check engine light came on my 03 sonata (23k miles). P303 #3 misfire. A comworker changed my spark plugs and all the sudden the car is sputtering and shutting off. A new code is showing now #3 fuel injector. Anybody fimiliar with this problem? The car was driving fine before the plugs were changed.

I guess you mean co-worker . Did this person really know what they are doing ? Did they use the correct plugs ? Did they put the wires back on secure and on the right plugs ?

Use only the plugs specified by the carmaker for that engine. It will be in owners manuals and maybe on an underhood sticker. Anything else is an experiment unlikely to do anything good. But spark plugs would not affect the operation of a fuel injector. Are you sure of the code?

Your car has very few miles for its age - deposits on fuel injectors could be involved here.

Was the spark plug gap checked prior to installing the new plugs?


I assumed he knew what he was doing. We went to auto zone and those were the plugs that showed up in their system for that make and model.

Yes Gaps were checked.

Auto zone plugs came up when entering make and model. Thanks for the advice. Cars at the shop, wish me luck

Lots of spark plug makers and dealers have equivalency charts - but somehow even plugs that are mechanically similar in thread, reach, etc. don’t all work the same. The carmaker’s recommendation (brand, model, gap) is the reliable way to go - not someone’s idea of what is equal.

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Well kinda good news!.. cables were reattached backwards🤦‍♂️ running fine again but light still on #3 fuel injector. So. Either it’s just the fuel injector or mechanic thinks it could be electrical, which he doesn’t work on. Fingers crossed it’s just the fuel injectors.

I only just realized i wrote comworker lol

The battery cables were put back on backwards ? Might be a good idea to not let this co-worker near your car or even ask for advice.

Plug cables were backwards. Agreed.

It would be nice to know what the code is for the injector issue. There are two good possibilities for the injector problem. The injector itself is bad, or the connection to it has a problem. Knowing what code showed up would tell you which way to go to solve the problem.

first things first . . .

is the injector connector properly seated?

are the wires damaged?

if that’s good, check resistance . . . is it within specs?

now use a noid lite in place of the injector and start up the engine

is the noid lite flashing?

if so, we should assume the pcm is doing its thing and the injector wiring is in reasonable shape

considering the coworker hooked the ignition wires up incorrectly, maybe he also somehow caused this problem with the injector. Maybe he unplugged it, but didn’t plug it back in?

Great news! Light turned off , just needed spark plugs changed. Saved me alot of money. All worked in the end i guess :+1:

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Good deal. Thanks for the update.