Red Triangle/Thermostat Warning - Every 1-2 months after oil change

Have you already tried checking all of the fluids (engine oil, radiator, inverter)?

When the vehicle is cool, be sure to check the actual radiator and not just the overflow tank.

After checking the fluids, you might want to check for any logged codes related to those warning lights on the dash.

Does the interior fan work? (won't even ask how you drive around NH without a defroster)

When was the last time the cabin air filter was checked?

If you have access to the owners manual you can check all or some of the fuses.

Toyota Online Owners Manuals and Warranty

 

There should be a sight glass for checking the condition of the AC coolant, a little window on one of the lines, under the hood. Watch it while the AC is cycling on/off (as evidenced by the fans in the front running/stopping). While running, if the AC has sufficient coolant, you should see just a few stray bubbles whizzing by, and when it stops there'll be a swirl of bubbles.

If you're seeing a lot of bubbles, or worse: nothing, a recharge is needed. I'd be really averse to trying to DIY this, leave it to a dealership. I know our 3rd gen needs a special oil, and I'd suspect your year is similar.

OTOH, if the sigh glass appearance is ok, then maybe whatever's going on is tied into the AC as well, some computer malfunction or whatever?

 

Had the same problem, what I SFO said was the problem (quoted). I first check the overflow tank and the level was fine. The actual Radiator itself was low. You have to remove the cowl to get to the cap. I had a slow leak from the upper hose. The hose clamp failed, I replace that clamp with a new one and drained and refilled the engine coolant with the recommended stuff from Toyota and 3 1/2 years later, all is good.

 

Now would be a good time to check for thrown codes via the OBD2 port.

The heater issue might be a faulty or stuck 3-way coolant flow control valve, codes would help identify this.

Checked the fuses already?

 

This seems a bit of a no-brainer here, but is the actual thermostat working?

Does the Prius have a thermostat?

If this was my 1982 Tercel, my fix would be a new thermostat and a recharge of the A/C system. That would have fixed all the problems most likely.

Prius???

The oil thing just seems like a red herring.

BTW: I have NEVER had luck trying to recharge my own A/C. Best to take it to the mechanic. And, the Prius takes special refrigerant. If you used the wrong kind in your attempts to recharge, you might have some extra problems.

Still remember the summer day when my friends and I were swigging Budweiser and we decided to recharge the A/C on my 1978 Datsun 210 when I was about 20. Connected the recharge hose to the high pressure side. What an explosion that caused.

 

I think the AC fluid is the usual, it's the oil that is special. What with that and the trick details, amounts to the same thing though: leave it to the dealership.

 

That was a mistake.
The Prius needs a special non-conductive oil for charging.

Recharging the A/C | PriusChat

 

you definitely must pull codes and check coolant level in the radiator asap. anything else is a waste of time. once you see what your codes are, then you can move ahead with needed repairs. also, at 187k, if you haven't replaced the water pump and belt yet, there's a very good chance the water pump is leaking and the belt is cracked.

 

If you don't have the ability to troubleshoot this car, you can get it diagnosed by a mechanic and get a quote on all the things that are wrong with it. That'll probably be better than stabbing in the dark.