Have a 1990 audi with 150,000 miles. I was on the highway for about 2 hours when I had to come to a stop because of an accident. After sitting still for about 25 minutes in traffic, the coolant gauge went all the way to hot and then a buzzer came on with a light which I assume meant the car overheated. I was told to get my radiator replaced with a new thermostat which I did. I really don't want this overheat condition to happen again. I do notice that the coolant gauge is a bit cooler when I am running the air conditioner. Has anyone had this happen when they were sitting still in traffic on a hot day and what exactly did they replace?

The coolant running cooler when the AC is running is because the electric fan is also running to cool the condenser sitting in front of the radiator.

For people with non working radiator fan, just turn the heat all the way up in defrost mode. The heater core will act like a mini radiator. Although it might get very warm in the cabin it is still better than blowing a head gasket.

Car running a little bit cooler then A/C is ON because A/C turns radiator fan on 1st speed all the time. I had same situation when outside temperature was around 30 degrees(C). My fan wasn't working. First (and last) that I did I replaced was temperature sensor on the radiator.

Before changing the radiator I would have tried temp sensor. I believe changing the radiator was not necessary.

Is the temperature sensor the same as the thermostat? Lets say I just run the air conditioner(it has automatic climate control), will I be safe from blowing a head gasket?

on the back, bottom of the radiator. It has wires plugged into it which operate radiator fan speeds 3 and 3.

This is probably your problem but it could also be bad bearings in the fan motor, fried fan resistor pack on frame under radiator, bad fusable link for the fan ground on the drivers side fender well or bad relays for the fan speeds 2 and three in the relay box by drivers side windshield wiper.

Search forum for lots of threads, diagnosis and solutions for this.

After I turn the car off, I notice that the fan will come on and run for about 5 minutes. Just wondering if this is part of the overheating issue. Also, if there is one part I should replace, what should it be?

There is no good answer for the one thing to fix. There are a number of things that are interrelated and all need to be functioning to have the rad fans working as they should.

You should search the forum for diagnostic solutions be fore throwing parts and money at the problem. Finding out what is the problem is much cheaper and a better use of your time.

Start by pulling the plug off the thermosensor under the lower radiator hose. Turn the key to on. Use a jumper wire to short between pairs of the three connectors. If the fan turns on to fan speed 2 and or 3 then you might start by spending $20 to replace that switch.

That's what it's supposed to do, until the temperature drops to normal. My '91 100 does not do that now so I'm taking it in to have it corrected. It may be the fan, a sensor or even a short. I'll soon know.