On hot days it won't start.
My car has 97000 miles.
My car has an automatic transmission.
A couple of things come to mind. I would first ask these questions: Is this a no crank or a no start? I will address both scenarios below.
A no crank is when the motor doesn’t turn when you turn the key. This is a starting issue. Based on your description, I suspect this is what you mean when you say your Volvo 850 won’t start. It is very common for heat to cause the starter motor and bad connection problems. Electricity doesn’t like heat. Heat creates high resistance in an electrical circuit. Most commonly this is a starter issue, but other parts of the primary circuit can have bad connections.
Another possibility is similar to your no crank in that it involves and electrical module. Heat plagues all electrical components. The diagnosis of this, however, will first require you or a mechanic to figure out what is missing when and if there is a crank no start. An internal combustion motor needs spark, fuel pressure, cam timing and injector pulse to run. If any of these are missing, you will need to pinpoint why. Once it is known what your car is lacking, only experience will guide you to determine why or what to check for next.
Simply put, if you don’t have fuel pressure, you will likely need a fuel pump. If you are missing spark, but have injector pulse, you likely need an ignition module. And that module could be part of the PCM. If you are missing only injector pulse, it could be the PCM. Keep in mind, the first thing I do before diagnosis any of these systems is check all the fuses on the car. If you have all of these, then it is likely a problem with camshaft timing. Checking camshaft timing is an extensive process.
Keep in mind, your failure is only occurring when it is hot. For a mechanic to diagnose your car, you will need to reproduce the problem. Reproducing the problem could be the most challenging part of this diagnosis. If the failure isn’t occurring, there is no way to diagnose the intermittent failure, and you should have it checked when the problem occurs. If you need help with all of this, I recommend having a certified technician, such as one from YourMechanic, diagnose your starting issue in order to have the right repairs done.