I don’t like that phrase because I hear people use it all the time at shops. For starters just because you haven’t seen it doesn’t mean it won’t happen. Plenty of failures occur that I haven’t personally witnessed with my own eyes but they do happen.

In his defense though I’ve seen mechanics use that exact phrase all the time as more of an expression to say something is rare rather than mean it literally. Working with a lot of mechanics sometimes they just say random shit to convince you they are right rather than take the time to explain why

Did he start yesterday?

That is a mechanic to stay away from. If anything, they should encourage customers to replace the thermostat based on time and miles. They can slowly go bad and you really don’t want one to fail. A good time to do it is when getting some sort of service that requires the draining of the coolant. You want the engine to be at the correct operating temperature as much as possible to keep things in check. The last thing you want is to drive around with a cool engine. Not to mention what happens if it gets stuck closed because some people refuse to EVER change coolant.

He never drove an 86 Camaro. That stupid thermostat would fail at the worst possible time during the weekend or summer. They can fail closed or fail open. When they fail closed, they can cause overheating rather quickly. I used to pull the thermostat, put a pot of water to boil and use some tongs to suspend it in the water to watch the opening temp. When they fail closed they just won’t open.

I personally don’t like Motorad thermostats. They seem to fail too often. Most parts store thermostats are rebranded/reboxed Motorad. They stamp Motorad right on the thermostat. I try to get an OEM thermostat as often as possible. There’s nothing like doing a thermostat twice because the stupid thing failed in a couple months.

As a DIY type mechanic (not a real one) I’ve seen my car close up on me multiple times. When I think over heating I think no water circulation and the thermostat is the cheapest to start with. When it’s closed from my idea is that it prevents the flow to the engine and then the heat starts backing up. Ever see your coolant box piss out and bubble? If no water is on your oil then my guess it’s a clog. Blocked thermostat.

They dont usually get stuck closed, but it can happen. I’ve seen them stuck open more times.

I’d also make sure there’s no air in the system. We always use a vacuum fill on cooling systems

Stuck closed thermostats are very rare and difficult to diagnose. Any “shade tree mechanics” on here claiming otherwise probably misdiagnosed or accidentally solved the real problem which was low coolant or something that would have been taken care of during replacement.

Your mechanic is an idiot

I’ve been working in the industry 16 years. I can recall it happening once. It happens but it’s rare. Hobbyists seem to think it regularly occurs.

As an auto tech of 11 years I wanna say . I pretty much never see stuck closed thermostats . Must have fixed hundreds of overheat issues and never seen but like 2 or 3 stuck closed thermostats . Proabbly seen 2 or 3 dozen stuck open thermostats causing under heat conditions. I replace engines, head gaskets , trans mission assemblies and my current job I’m in this week is studding a 6.0 powerstroke diesel and replacing head hadkets (23 hours repair) . When someone hollers theromast the second they hear a vehicle overheated , really tells me the guy eats paint chips and knows nothing of diagnostics.

So in my 15 years of working on many cars(I went to school to become a mechanic but don’t do it as a job) anytime I had a car that was mine or that I was working on the thermostat was never it I’d say 9/10 times it was a headgasket. Now, it’s always possible that the thermostat getting stuck caused it to overheat at first and then the headgasket failed but replacing one has never actually fixed it and I’m talking like 100 times Ive worked on car for specifically overheating. Just saying.

Thermostats can fail in 2 ways.

They can fail open, which causes you to never get up to temperature. This isn’t good, but it’s not gonna risk catastrophic engine damage. Although your engine not getting up to temperature can accelerate premature wear, so it should still be fixed.

They can also fail closed. This is the one most people think of. Coolant simply can’t circulate properly and your engine overheats rather quickly. This is what causes bad head gaskets and the like if not spotted.

I’ve seen both types of failures multiple times, and had it happen to me once (failed closed in my case). It’s not implausible that he’s only seen failed opens, it’s just a bit strange.

They fail two ways. That is one of them.

Of the four cars I’ve worked on in the last month that came in with an overheating concern, one was due to blown hoses at the junction to the heater core, one was due to improper bleeding after a radiator replacement, and two were due to stuck thermostats.

If this “mechanic” has never seen a stuck thermostat cause overheating, then I seriously doubt his 10 years of experience is doing much more than oil changes and pad slaps in the AutoZone parking lot. I’d cross him off the list of people to learn things from. He’s a hack.