Check engine light came on. I ran the diagnostic test and...


Check engine light came on. I ran the diagnostic test and it gave me code P0304. So I changed the spark plugs and turned the check engine light off and drove it for a while. It seemed OK but then the light came back on. The strang...

Check engine light came on. I ran the diagnostic test and it gave me code P0304. So I changed the spark plugs and turned the check engine light off and drove it for a while. It seemed OK but then the light came back on. The strangest thing though, it turned back off on its own yesterday and I don't know what is going on because of that. Mechanic's Assistant: Does the light stay on, or does it blink? It's not on anymore at all. Mechanic's Assistant: What year is your Spark? The car is a 2018 Hyundai elantra Mechanic's Assistant: Are you fixing your Spark yourself? What have you tried so far? I just changed the plugs myself.

Automotive Expert
Hi, my name is\*\*\*\*\*’ve been a technician on Just Answer since 2007 and I'll be helping you today. I'm sorry to hear you are having difficulties with your vehicle. I know how inconvenient car trouble is, but together, we will get your vehicle back up and running quickly. May I ask whom I am chatting with today?

Kayla
Automotive Expert
HI Kayla, it's nice to meet you. Did the engine run rough either time the light was on? did you get a chance to get the code before the light went out the second time?

the code was still the same the second time. The car wasn't running badly but I did notice that while it was sitting idle it would shake a lot.Like a vibrating type of shaking. I just don't get why the car would just shut the code off on its own if I didn't fix the problem the first time.I'm planning on making a long distance trip here soon so I want to be sure that the car is going to be ok.
Automotive Expert
Sorry I am not able to make a call at this time. Ok, the same code on the same cylinder is usually going to mean the ignition coil is the problem and not the spark plug. The light went out on it's own because the problem didn't exist at the particular time the computer ran the self test. I would replace the coil on that cylinder and then see if the code returns, I suspect it won't. Please let me know if you have more questions on this or any other thing I can do to help. The car will be ok, but with this code I would still go ahead and change the coil.

Since the code went off on its own, will it pass a smog check right now??
Automotive Expert
yes, it should without a problem. It would mean the self test was done by the computer and there wasn't a problem
Automotive Expert
Phone call session started

Thank you so much. I really appreciate your help. Have a great day and stay safe!!
Automotive Expert
You're very welcome and thank you, \*\*\*\*\* \*\*\*\*\* you!

What others are asking

Something banging around under the hood
Greetings and thank you for writing in with this concern! Without knowing more about the conditions that cause the noise (when, where, how, etc) it is difficult to advise on this concern. Mazda has released a number of Technical Service...
What does the engine oil pressure light mean in a 2012 Ford Explorer?
The engine oil pressure Warning Light is located on the instrument panel. This light illuminates when your engine’s oil pressure is below the minimum level. Operating your vehicle with an insufficient amount of oil is very dangerous, so you should...
Wire came off of starter
Hi, if this is a large diameter wire like a battery cable then it may be the cable to the starter terminal. If it is a smaller wire, it may be a memory wire for the engine controls and things...
Car keeps cutting off
If your car keeps cutting off, then this may or may not be related to the fuel line. If you suspect it is indeed related to a fuel line, then that means that fuel is probably leaking, and the vehicle...
My car will not start
The radio and no start condition could be two separate issues, or they could both be linked to the totally integrated power module (TIPM). This module is essentially a smart fuse box that supplies power to many of the vehicle's...
Vent hose broke off of battery
The vents are so you will not get battery fumes built up inside the passenger compartment. Hydrogen gas may build up inside the car and can cause a fire hazard. I would not recommend driving it till you fix the...
On interstate, after 15-20 seconds car loses acceleration
Hello. Thanks for writing in about your 2007 BMW 335xi. It sounds like the thermostat may be sticking. The thermostat needs to be checked and the cooling system needs to be checked to make sure that the coolant is flowing...
Stalling when stepping on the accelerator
Hello. Stalling can be caused by many things. If you believe it may be the fuel system then a few components to look at the vehicle's fuel pump, fuel pressure regulator, and injectors. An issue with any of these can...
My wife unplugged one of the fuses to my car, that supposedly went to the horn but now my car wont start.
Hi there. When pulling the fuse out at the car alarm is tripped will set the security system in the vehicle. Putting the fuse back in will only cause the security to stay on. Try disconnecting the battery and waiting...