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03 Suburban misfiring

I have a 03 Suburban LT with the 150,000 miles. and it misses out at 50mph or higher. I thought it was the transmission, and the tranny guy confirmed a new transmission was needed, but said he thought the missing was maybe the coils. the local Chevy dealer did the diagnostic, and determined the coils, plugs, and plug wires were all good. I 've replaced the oxygen sensor as well. The service manager said to keep driving it until the Check Engine light comes on. That concerns me, but hey what do I know. The mileage hasn’t dropped off, and it runs fine the rest of the time.

Any suggestions?

has all the filters been changed lately and when where the pluges last replaced?

Could be a plethora of things.

One of the cheaper things to try is clean the MAF sensor and your EGR valve. Usually they unbolt easily and you can clean them with some MAF sensor clearer and carburetor cleaner, respectively.
If you know which way to turn a wrench, it could be an DIY job.

They are waiting for the check light to come on becasue that will tell them which cylinder is having an issue if any… They will then swap coil packs with another cyl, and see if the missfire follows the pack or stays with the Cyl.

Wouldn’t the ECM record the misfire and cylinder number? On the ones I’ve seen, it records the misfire and, depending on how many times it misfired, turn the MIL light on. Or is that different for all cars?

If a check light was thrown it would store the info, but if it did not throw a light it probably does not have a record of the misfire… I think that is why the dealership said wait for the light to trun on.