Temp gauge the temp gauge will peak then drop then peak then drop several times in a row then it will not do it for a while then does it agian.... we have changed out the thermostat.... HELP please, at wits end with this can not figure it out, we have also pulled the casing off to see if there is water coming off the belts and there is nothing... is there a sensor of some sort in the reserve area that can be the problem???

when a temp gauge drops to cold ,it indicates a problem in the cooling system
temp gauge sender units need water to activate the resistance that makes the gauge read so a falling gauge indicates no water around the sender unit --- air lock in the system--faulty wire connection to the gauge sender unit
the ac unit is not connected to the temp gauge so there may be another separate problem there
the temp sensor is an entirely different unit to the temp gauge sender unit and the short term working would be from an altered coolant level from changing that sensor --air lock in system
the temp sensor connects to the ECM to activate the fans and monitor engine operating temp so as to keep the air/fuel mixture exact for the engine conditions
the temp sender unit just makes the temp gauge read
a faulty thermostat will either be stuck open and the engine temp will take a long time to rise or it will be stuck closed and that will cause an overheat very quickly

use a termo laser to check temp of coolant at filler cap ,with cap off run vehicle till you have hot heat coming out of your heater then check with laser. if it reads above 140 then you know 2 things . either the therm sensor is going bad or the temp gauge is bad , make sure the radiator fans come on ,if they do and you have good heat then you have bad gauge.

Sound like either the Thermometer Housing, Thermometer or water pump may be in trouble.

If no leaks you can rule out the Thermometer Housing.
But regardless you will need to replace all three if any one is bad.
And the timing Belt while you are in there,
Only because it a 5 hour Minimum Labor job to replace either of them.
And there all right near one another,

A real pain to get to those. Thank you Audi...

Selvol

I would not have replaced the sensor, but replaced the temp gauge assembly itself..........normally a temp sensor does not "drop out" but is either full defelction or off........., or alternatively can sometimes misread......but the dropping out is unfortunately due to the gauge being faulty.........!!

more likeley the thermostat(when faulty it opens to max flow...)hence drop in temp/increase in fuel consumption..

one gauge is tripping it on,
but you have to be fast enough to check gauges when it happens,it could be the fuel gauge going to empty,or the charging one going below 12 volts,or the oil gauge falling off at an idle,the temp gauge you should be able to catch that on if it was peaking

replace temp sensor and thermostat.
sounds like your engine has been over heating while the gauge was not working.

try changing the temp sending unit

I had the same problem or more so that my temp would run cold for miles and miles then go to normal and drop. It was my glow plugs. They needed to be changed. My boyfriend changed them himself in 10 minutes after I had been quoted 2 HOURS for labour!!

bad altenater