View Full Version : Misfire PC0300 Code

04-13-09, 01:16 AM
Car started misfiring tonight along with SES light flashing. Pulled codes, the only current one is PC 0300. Been reading up on past threads and trying to get a game plan for tomorrow. I will start and look at the air filter, plug wires and spark plugs, they have approx 50K on them and then move on to the coils. What is the best way to check them out? I was going to hook my timing light to each plug wire and watch for any inconsistencies. Any other suggestions?!?

04-13-09, 09:20 AM
Plugs (ACDelco ?? #41-950) with only 50K should be good. Remove the engine cover and idle the engine. Listen for a spark tick around the coils and ICM pack (under the coils). If the wires are original, they're prime suspects after 13 years. We had another fix earlier this year for a P0300, and that involved removing the coils and cleaning them and the contacts. If you feel you need wires, either use a set of ACDelco wires or maybe a set from NAPA. I don't think MSD or ACCEL make a dedicated Cadillac set. Check all the injector connectors while you're under the hood. I think your car has the FPR (Fuel Pressure Regulator) inside the intake manifold, so that's a bear to check for a diaphragm leak. Make very sure that the plug wires are connected according to the diagrams.

Anyone else ???

04-13-09, 09:41 AM
Thanks Sub,

Yes, the correct plugs are installed. The wires were changed at that same time. The FPR internals were replaced sometime last year - it's on the fuel rail. I will check the coils and injector connections. This just happened out of the blue, no recent work has been done under the hood.

Mark C
04-13-09, 02:13 PM
You can't tell which cylinder is misfiring from the DIC, you need some kind of auxilliary scanner. the P0300 code only tells you its one of your 8 cylinders misfiring. Could be one cylinder or more than one, Just happened to me on my 00 Deville yesterday, car started idling rough, and the P0300 code popped up with the SES light. Had to get out my PC with OBDII software reader on it to find out its the #1 cylinder that is misfiring. If I had wires I would just put a timing light on each wire until I found the one that is not firing (assuming its a coil thats gone bad). I don't have wires on mine so I can't do any kind of checking without pulling the coils off the engine.

I think I'm screwed and my rear ignition coil module is gone belly up and now I need to replace the whole thing.

04-13-09, 04:43 PM
Mark, RockAuto has a few of the original equipment DELPHI coil cassettes left. About $185 with ICM instead of over $225 for others, about $8 shipping for both.

Mark C
04-13-09, 05:15 PM
Thanks, I saw them but I picked up a new in the box (allegedly) 1104075 GM unit with the ICM for 180 with $10 shipping this morning off Ebay. It's not the money that worries me, its the fact you need hands and arms like ET to reach those three 10MM bolts along the back of the coil pack up against the firewall.

04-13-09, 07:46 PM
I did use a timing light to check all the wires. Looks like the #3/8 coil is not working correctly, #3 wire was steady, but flash was much slower than all the others, #8 was not steady at all. Pulled it off, cleaned the two contacts, but it didn't change anything. I will order one from RockAuto and have it waiting when I get back home.

Mark C
04-13-09, 10:18 PM
Should swap the 3/8 coil with another one to make sure its not the ICM under the coils. If it still doesnt work correctly in its new position its that coil, if the new coil in the 3/8 position doesnt work correctly then its the ICM that has a problem.

04-13-09, 10:47 PM
That is a good point. What are the chances that it is the ICM and not an individual coil that has gone bad? Another question; what is the correct replacement coil? I checked RockAuto and they list Standard, Delphi, Airtex and two ACDelcos, #D555 and #E530C. The only info listed is what part #'s the D555 replaces and the E530C has "4 door" included in the description. I cannot check the one that is on the car for a part # since I traveled out of town today. I was hoping to order a replacement and have it waiting for me when I return so I could fix it right away.

04-13-09, 10:58 PM
The coil is more likely to go bad than the ICM.

04-13-09, 11:46 PM
Any recommendation or suggestion on which replacement coil to use or do I need the part number from the original part?

04-14-09, 01:49 AM
I'm having the same problem. I have a 97 Seville STS and the code reads P0300. I went to Autozone and they hooked their scan tool and said cylinder number 6 misfire. I replaced the coil pack, spark plug, and the spark plug wire. Still having problems. What else could be wrong with my baby? Please, any suggestions would be helpful.

04-14-09, 12:23 PM
Racinrod, not sure what to tell you. Maybe call a dealer parts dept. and get a number.

Diamond, might be the ICM or an injector.

04-22-09, 02:04 PM
Just a follow up: Ordered AC Delco coil #D555 from Rockauto which is a replacement for the original GM part#10472401. Fired her up, runs smooth, no SES light. Just to double check, hooked up the timing light and new coil has the same flash frequency as the remaining three originals. Wondering how long until the others go bad?!?!?