Audiheavy

Need some help with my wife's '02, 3.0... when the temp gauge isn't working... no heat from the heater. When it is working... normal heater function. I suspect that a switch that sends a signal to the gauge also sends some message to the heater. Thanks in advance!

SilverSchwinn

thermostat? or coolant temp sensor?

tonyj

Or air int he system, failed coolant pump ....

Merkin

what happens if you turn on HI

diagnosticator1

There is no functional association between the coolant temp gage and the heater controller.
The coexisting symptoms of no heater output and the unusual coolant low temp indication on the gage, are key to the cause. Those two symptoms when they occur together, indicate an intermittent t-stat that sticks open. This will result in a simultaneous low engine temp indication and the loss of hot air output from the heater. The sequence of events is first the engine temp would need to initially reach normal operating temp, or exceed normal temp, to open the t-stat, then when the coolent is below maximum, the t-stat hangs up and stays stuck in the open condition, resulting in the coolant temp decreasing excessivly as evidenced by the low temp indication on the gage, and the loss of hot air from the heater output. The t-stat eventually closes again after the engine cools down to near ambient temps. Another failure mode is a broken t-stat return spring with similar symptoms.

Melinko bq

Then they'd have lots of heat.... and pry wouldn't make it 2 miles b4 overheat.

Melinko bq

No correlation between water flow to the heater core and any sensors. Heater core is en-route to the radiator, goes straight from the engine to the core for dibs on the heat, then through radiator and thermostat and back to engine.

Usually when thermostats fail, they fail permanently, some fail open while some fail closed. It is sometimes desirable to have the thermostat fail open, that way you can still drive it without overheating. However, on a cool morning/day - especially - the car will never heat up if the thermostat is stuck open since no restriction to cool water from radiator. Similarly, on a hot day - the temp may climb above normal op temps since the coolant isn't staying in the radiator long enough to cool down.

Either way, I'd try and pay attention to the gauge and see if it climbs at all on a warmer day, or if outside temps play a role at all. If not, may still be thermostat, may be more than one problem with an extremely coincidental occurrence... =)

Best of luck!