Entering a highway today, I put the pedal down hard and the car jerked and the check engine light came on (steady). I pulled into an Autozone to have a code check and the meter said there were 6 codes. The guy said three looked like duplicates and printed out the following for me:

Code: PO300
Definition: Random Cylinder Misfire Detected
Explanation: Lean Air/Fuel Ratio
Probable Cause:
001. Ignition system misfire condition
002. Fuel Injector Fault
003. Engine mechanical condition

Code: PO303
Cylinder 3 Misfire Condition
Explanation: Vacuum leak on engine
001. Ignition system fault
002. Fuel injector fault
003. Engine mechanical condition

Code: PO304
Definition: Cylinder 4 Misfire Condition
Explanation: Vacuum leak on engine
001. Ignition system fault
002. Fuel injector fault
003. Engine mechanical condition

I last changed the spark plugs about a year ago and all four coil packs were also changed in the last year, 2 about 9 months ago (1&3) and 2 about six months ago (2&4).

I'd like to avoid taking this to mechanic if possible as he charged me an arm and leg last time to diagnose and change the first 2 coil packs (I changed the second two). Can anyone advise what the possible cause and next step for me would be or am I better off bringing it in?

Thanks

 

What plugs did you install during replacement?

What coil packs were used during replacement? OEM Hitachi or a third party?

Have you ever had your valves adjusted?

 

Thanks for your reply. The sparks plugs were NGK LaserIridium (IZFR6K13) and I changed those in August 2014.

In January of this year, mechanic did (from receipt):

Download code data from engine control computer, perform diagnostic test for indicated codes P0300, P0302 and P0304. Test fuel injectors, coils and spark plugs for faults. Found #2 and #4 coils resistance out of range. Replace #2 and #4 ignition coils. Clean spark plugs. Reset computer and road test vehicle.

Unfortunately, receipt doesn't say which coils were used. Mechanic did say that plugs were dirty and they were going to change them as well but when I told them I had changed them a few months before, they left them. By the way, I didn't gap the plugs as I had read they were pre-gapped.

When I changed coils 1 and 3, I used Duralast part #C1578.

The valves were adjusted at about 70k miles. I now have 120k miles. The car has been running fine though and I have not been experiencing any of the stalling that preceded the original valve job. I am probably due to have them adjusted though.

 

Just to follow up on this in case anyone has any comments, I bought my own OBDII scanner and the codes I pulled seem to be different than the ones reported at Autozone. The codes I'm seeing are P0303, P0403, P0055, and C1555.

I'm thinking of replacing the spark plug in cylinder 3 and switching the coil packs around to see if I then receive a different cylinder misfire. Does this sound like a good idea or do the rest of the codes point in a different direction?

Thanks.