You can test a thermostat in a pan of hot water. If you use a thermometer you can check the operating temperature

Yes you can, but I wouldn't bother. For the cost of a new one it isn't worth it. Besides, if it's lost some wax it'll open too late and close too early. This'll mean the engine will always tend to run hot. By all means test the old one in a saucepan if you are curious, but as I said earlier you should always renew the thermostat if the engine has ever overheated.

You can test a thermostat in a pan of hot water. If you use a thermometer you can check the operating temperature

Yes you can, but I wouldn't bother. For the cost of a new one it isn't worth it. Besides, if it's lost some wax it'll open too late and close too early. This'll mean the engine will always tend to run hot. By all means test the old one in a saucepan if you are curious, but as I said earlier you should always renew the thermostat if the engine has ever overheated.

If I test something as easy to replace as a thermostat, and it works to spec, I'm going to be reluctant to replace it, cheap or not.

I suppose you don't reproduce the operating pressure in the cooling system, so there is a possibilty that a bench test is innacurate, though its a bit of a stretch.

You can (sometimes) get different spec thermostats, so if replacing it might be possible to get one that opens at a slightly lower temperature.

I'm not with you on that one. I was an AA Patrol for 12 years, and I honestly couldn't tell you how many vehicles I've attended at the roadside for engine overheating other than it's a big number. Seeing broken down vehicles first hand gives you an insight that garage technicians don't get. They only see the after effect and they make their diagnosis on that basis. Working as a Roadside Patrol is a totally different job to a garage technician, and you see everything differently and more clearly.

I used to find that the thermostat was the most common cause of overheating by far. As a temporary repair I used to jam them open (essential with a two stage thermostat). I would then pressure test the system again whilst it's warming up. On a cold day I could rarely get the engine warm enough to bring in the cooling fan, especially if the radiator was in good condition. On a hot day it was a different matter, but as long as the fan did cut it and was drawing heat away from the radiator I knew it was going to be OK.

Blocked, corroded, damaged and leaking radiators were another common cause of overheating. But even then if I could get it safe for the member to continue I would always write on the form advising the thermostat be replaced.