10 psi off the minimum isn't bad if your engine was running ok.

But it is a 2021 (un sure of miles) not a 1971. And the other cylinders are probably 140 psi.

Given the history of DFM system and lifter failures, it is a real good chance that the liter has failed and caused a slightly bent push rod which could cause slightly low compression.

I think the shop is assuming that the cam is damaged as well (not a bad assumption).

To get down to these components you have to at least remove intake manifold. To get lifters out at least one cylinder head would be removed (in your case the head with the affected cylinder).

So to fix this issue properly you are looking at removing both cylinder heads, replacing camshaft. That is well into a major engine repair. You would not want to replace only the failed lifter. You'd want to replace all of them.

Or you can do a DFM delete which replaces lifters with older style lifters. You would need to take truck to some sort of tuner or specialist to deactivate the DFM programming, etc...

By chance can you elaborate a bit more on your truck?

Miles
typical oil change interval
DFM deactivation of any sort (driving in L9 or a disabler)?

F150 here so a different engine with a different problem, but a similar fix...
2023 I got a CEL with underlying code for Cylinder 7 misfire. Local shop that has done a few smaller repairs for me diagnosed (properly) a burned exhaust valve. On the Fords, with the dual overhead cams, you have to tear the engine way down to get the head off, and of course #7 (passenger side) is the inboard of the drivers side bank, so the entire timing system on the drivers side has to come off and out of the way to remove the passenger side cams.

My mistake was letting a smaller shop do the work. They stripped everything down and sent the heads (ended up doing both) off for machining and rebuild. Keep in mind all of the parts the shop buys will come as a complete kit. So if you need ONE you're buying all of them so might as well do it all while you're in there.

Well fast forward a year and I've notice the heat wasn't working properly and shortly after the engine itself running warm. Get it in the shop immediately. Long story short they've screwed something up in the head job. They wanted to replace my engine BUT were willing the credit the warranty from the heads towards the engine replacement. They claimed deck lids were warped "because it certainly couldn't be the heads or the head gaskets they rebuilt and replaced." So NOW to go to another shop I'm still looking at a $7-8000 repair on a truck that is only worth a little more.

All that to say, I'm not against tearing down the engine and rebuilding it. It can be done and it's done all the time. Heck if you replace the engine, you're probably being quoted a rebuilt motor.

Find a shop that specializes in rebuilding engines and transmissions.

Most likely a DFM lifter failure. Delete them, don't rebuild to stock unless you are planning to trade immediately.

Wild that these engines are failing before 100K when 25 years ago a stock engine would be expected to run for 200-300K with non intrusive maintenance. You might put in a new fuel pump, water pump, alternator but the engine itself would run hundreds of thousands of miles.

Stat Monitor Repairman said:

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Wild that these engines are failing before 100K when 25 years ago a stock engine would be expected to run for 200-300K with non intrusive maintenance. You might put in a new fuel pump, water pump, alternator but the engine itself would run hundreds of thousands of miles.

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You are correct.

Picked it up from the original mechanic this afternoon and took it to the Chevy dealer where I originally purchased it from. It's currently still under the GM CPO powertrain warranty. So unless they can come up with a reason to deny the warranty claim, it should be covered…. Fingers crossed