Be Careful Not to Dismiss Knocking as 'Bad Gas'

Dear Car Talk:

I love your column, and thanks for helping us all out with the poltergeists in our cars!

I have a 2018 Chevy 2500HD Silverado with a 6.0 V8 and the trailering package. I pull a 6,000-pound trailer with it.

After I get gas, hitch up my trailer, and hit the open road, within about 30-40 miles, the poltergeist in my truck turns on the "check engine" light.

Since I have OnStar, I have them run a diagnostic, and it's always the same code: "P0324 ECM Engine System." This has happened three times. After I unhitch the trailer and get more gas, the "check engine" light turns off.

Since my truck is under warranty, I took it to the dealer (twice), and they told me it is "bad gas." How can that be? I know I am a young chick, but I feel that they just don't want to fix it. Could it be a bad knock sensor? They did say they checked everything, and it's fine.

Can you do a paranormal investigation and tell me what my issue is? -- Gena

When I have bad gas, it doesn't cause my "check engine" light to come on, Gena. But it does cause my houseguests to say "goodnight" early.

It could be bad gas, but if it's happened three times and, I'm assuming, with gas from different gas stations, it sounds like they're just trying to get rid of you. Or they don't know how to fix this.

You can humor them by trying different gas if you haven't already, but when that doesn't help -- and before your warranty runs out -- go back to the dealer and tell them they need to look harder.

"P0324" is the code for a knock sensor malfunction. Bad gas (with too low an octane rating) could cause knocking, which might turn on the "check engine" light. But it's also possible the knock sensor is faulty. And it may malfunction only under "load," or high-stress conditions.

For example, if they tested the knock sensor at the dealership, it may have tested fine. But when you're driving uphill at 65 mph, in 80-degree heat, pulling a 6,000-pound trailer, the sensor misbehaves.

So they should start by swapping out your knock sensor. It's also possible your whole engine control module is bad. The knock sensor "reports to" the ECM. So that'd be next on my list.

Either way, this truck shouldn't be knocking when you pull 6,000 pounds, since its towing capacity is more than twice that. So they're going to have to make a more serious effort to fix this for you.

I'm going to guess that you're pulling horses with this trailer, Gena. That would be perfect. Next time you go to the dealer about this, feed the horses first, then take them into the showroom with you, sit down and tell them "you'll wait" while they figure it out. Good luck.

Todays Car-o-Scope

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