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Time Bomb left or caused by shop?

03 trail blazer–check engine light was on computer says replace thermostat. The work was done I left shop got about 8 miles and I thought engine blew up as it was missing out soooo bad didn’t think I would make it back to the shop. They said 2 coils went out. Was this a $400 “time bomb” caused by something the mechanic did when they removed the front of my engine to replace the thermostat? Car ran great but with with the check engine light on I could not pass inspection. They told me that they had to take apart the front of the motor to get to the part. On guy thought maybe it was going to be the timing chain? Turned out each spark plug has a coil and two were bad.

Assuming that changing the coils fixed the problem, then the only way that changing the thermostat could have caused it is if they spilled coolant into the plug wells.
Alternately, if the Blazer overheated before the work was done, that may be what damaged the coils and caused their failure.

The car never got hot or over heated that darn light was just on. They checked the computer and it said bad thermostat. They told us we needed to drive it some so that the computer would reset or something along that line. Drove off from the shop and within 8 miles the motor started missing out and driving very poorly. I had a hard time getting back to the shop. Why wouldn’t the computer say coils before I drove off or even before they started taking the motor apart? They said that they drove it around and everything was fine. They know I was only gone for less than a half hour when I brought the car back into their shop to fix the motor problem. Again the truck NEVER got hot.

It’s certainly possible that they spilled coolant into some of the plug wells. It might take it several miles before it leaked into the boots and shorted out the output of the coils. If this is the case, it could probably have been fixed just by drying out the wells and the boots. The shop would know if this happened, but they might not be honest about it.

You will never know because you have no proof. It’s possible that the coil failure was unrelated to anything they did and just happened when it did.

I think that your suspicion is possible, but you’ll never prove it. You need to move on and next time find a shop that you have more trust in.