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1998 Mazda 626 thermostat housing

I had no heat in the car so my husband went to replace the thermostat. We live in New England and it was apparently too cold for the steel bolt that attaches to the thermostat housing (which is aluminum). My husband was told that you can not get it out now because steel and aluminum have a tendency to bond together when under pressure. The only remedy is to replace to entire housing. Is this true? And how much do you think a garage will charge to do this as I can only assume is VERY time consuming

I’m no tech but I don’t believe that for a second.

Drain some coolant into a clean container down to the t-stat level.
Pull the two t-stat housing bolts and replace the t-stat. Make sure to clean off the old gasket before applying the new one.

It’s not temperature and pressure that causes the bolt and housing to bond together. It’s a combination of age, moisture, and dissimilar metals.

Bolts (along with many other items made of steel) can certainly corrode together and getting them out or apart can be a hit and miss thing. With enough wrestling the bolt may come loose but there is also the chance that it could snap off.
Maybe the shop is just preparing your husband for the 50% chance that the bolt could break off in there.

No idea on any charges as it’s impossible to gauge the situation.

This is a good example of why I replace T-stats every 3 years or so. It not only keeps a 5 dollar T-stat from ruining an engine it’s also an excuse to remove those bolts before they seize.
Bolts are always reinstalled with anti-seize on them. And you’ll also see water pump bolts seize in the same way.