Forget the thermostat. The person who said that about 70 degrees is wrong. Nothing you have said up to this point suggests there is anything wrong with it. If the thermostat was stuck closed, as it appears he thinks it is, the engine would overheat regardless if it was 90 degrees or 30 below. The fact it is not overheating at any outside temperature proves beyond any doubt the coolant is flowing through the radiator, and that proves the thermostat has opened up as it was designed to do.
Last summer I had to replace the leaking radiator in my '94 Grand Voyager daily driver, and until I'm sure all leaks have been handled, I filled the system with mostly water, and just enough antifreeze to have a little of those additives it includes. Being the procrastinator I am, I forgot to add more antifreeze before winter. The first day it got down to below 0 degrees, the coolant turned to slush that restricted the flow through the radiator. The engine overheated exactly the same way one would if the thermostat hadn't opened. It was 0 degrees and the engine ran hot. Kind of shoots an arrow through your mechanic's theory that yours is not overheating only because it's cold outside. When an engine is overheating, it has no idea what the outside temperature is. It's overheating because the coolant isn't carrying the heat to the radiator.
To address the Check Engine light, Chrysler made reading the diagnostic fault codes yourself much easier than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds, without cranking the engine, leave it in "run", then watch the code numbers appear in the odometer display. You can go here:
to see their definitions, or I can interpret them for you.
This method only works for diagnostic fault codes in the Engine Computer. For that, and all of the dozens of other computers on the vehicle, repair shops use scanners to read the fault codes. Those also show the definitions at the same time. Scanners are very expensive, so most repair shops do have a charge for using it on the vehicle, but is that worth $307.00? I have a Chrysler DRB3 scanner for all of my older vehicles. At $307.00, I would have had it paid for with just 20 cars and 20 over-charged customers. At the very nice family-owned Chrysler dealership I used to work at, they charged $50.00 for using their scanner if it was connected for more than just reading fault codes. Typically that charge applied to more-involved diagnostics that often took more than a few hours.
Be aware the people at most auto parts stores will also read fault codes for you that are in the Engine Computer. They usually use less-expensive code readers that only do a small fraction of what scanners can do, but they do that for you for free. You can't top that for value.
I should point out the people at the auto parts stores are there to sell parts. Diagnostic fault codes never ever say to replace a part or that one is bad on any car model. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a part is referenced in a fault code, it is actually the cause of that code about half of the time. First we have to rule out wiring and connector terminal problems, and mechanical problems associated with that part before we spend your money on a replacement.
Thursday, January 30th, 2020 AT 3:33 PM