MIL/CEL is on - P0300 - hel;p!
 

My 2016 C7 with 43K miles has the CEL/MIL on solid. It happned once about a month ago but was not permanent. It came back this time with a vengeance. First diagnosis was fuel injection service since I've been running on regular. No joy. It came back within 20 miles and is solid again, even after filling with 91. Unfortunately, there are a lot of things that Google says can cause this.... Anone have ideas? Tuneup due to age? Fuel filter? What else has anyone seen for this issue?

I'll be starting a separate thread for those in PHX area or general comment on where to go for service also......

Welcome to the forum. As you've probably already found out, misfires can be caused by many things. But the first thing I would check would be plug wires, coils, and spark plugs.

Can I ask you a follow-up question? I'm thinking the same as you, but... The misfires seem to be almost all cylinders, albeit some much more than others. Does that eliminate the plugs, coils and plug wires as culprits? I'm thinking not because of age, but....

I'm guessing the scan codes aren't telling you anything definite. You might try running a couple of bottles of Techron through it to see if that has any effect.

Thanks for the reply. It's better now, but not perfect. I took it to the dealer and plugs were fouled, prolly from me running 87. Occurrence is down quite a bit, but not quite gone, still comes in clumps and still indicating 3,5,8. I am running Chevron premium now and have heard a lot of good things about Techron, thanks. I'll probably check a few bottles out in my Yukon.

If the C7 doesn't require premium gas then you're wasting your money. And I don't think regular would foul the plugs even if it did require premium. You would probably get reduced power due to the computer retarding the spark on regular if premium was required.

Since the problem is staying with the same three cylinders then I would suspect either injectors, plug wires, or coils. But I would try the Techron first.

I would also check compression and leak down on all eight cylinders. Could you tell if the file plugs or oil fouled or something else?

Everything seems fine according to the dealer, but still getting the error on those three. A few here and there on all, but nothing in the range of the 3/5/8 pattern. Putting a bottle of techron in today, but I was admittedly disappointed in the results from several YT videos I watched that showed before and after.

Premium is required in the C7, minimum of 91 octane. By running regular what would the O2 sensor be seeing and what adjustments would they be making?

From the little bit I know, it would retard the spark under load to avoid knocking. Shouldn't have any effect on the O2 sensor though.