1998 A-6 Quatro running very warm. Replaced fan clutch. Car still was running very warm but not completely overheating. Fan was working until yesterday. The fan is now not engaging. What causes the fan not to function? Thermostat? water pump? etc? Malfunction indicator light also stays on. Are these related?

missing a little and running hot are indicators that you could have a head gasket problem. have it tested for a bad head gasket. the missing could be caused by coolant leaking into the cylinder causing a misfire, because of a bad head gasket. running hot is another indication of head gasket. if the rad cooling fan working properly. I think these have what is called a clutch fan which means, when engine is cold the fan turns slower than it does when its warm. if its turning slow when warmed up like when its cold it could be the fan causing the hot running.

Replace you fan clutch! It sounds like it is dying. And is a simple fix.

Check to see if your truck is overheating. Also check to see whether you purchased the same size clutch fan.

Hello,Be sure the compressor clutch is cycling. Also, check the orifice tube, evaporator and condensor for blockages. Freon level should be checked with manifold gauges and compared to specs.
Thanks

is your fan clutch kicking in? It could be bad. crab the fan blade and try to move it back and forth. If it moves more than minimal you will need to replace your fan clutch.

The fan clutch is probably worn out. With the engine warmed up but NOT RUNNING...see if you can sin the fan. Grab a blade and try to spin it.
If it freewheels, it's shot. Should be a slight resistance.....
Replace if nec.

Yes, if the fan clutch is bad, the car may overheat, especially when going uphill, check for a replacement or repair the esisting one filling it back with new silicona.

Sounds like Heater matrix blocked, check pipes going from engine bay into bulk head both should be getting hot if one is not then may be worth removing pipes and trying to flush it through as would be expensive to renew it as its a large job.

change the vicous on the fan. ( fan clutch)

cloged radiator or head gasket blown
clogged radiator let cool then run car without radiator cap when warms up the water should be running fast in direction without bubbles
if slow the there is a clog in radiator

the other things of over heating tranny low on fluid
oil fluid low
drive line fluid or grease low
binding brakes
warped fan blades or improper fan clutch or defective
hope this helped its off the top of my head since im not there