2003 vw jetta tdi temp gauge was going up to 120. Replaced thermostat now no flow to heater core

Sounds normal. If the gauge moved further over, it would indicate an overheating problem, wouldn't it? Check that engine is at normal operating temperature after it warms up. If it is running too cold, your thermostat might be stuck open.

If your radiator fans are not running at all than I would check over the wires your disconnected when rpolacing the temp sensor. Make sure they are fully connected. That would be the first thing I would check.

Good Luck

Replace the Engine Coolant Temperature (ECT) sensor. ECT sensor is placed in the right back side of the cylinder head on 2002 VW Jetta 1.9 TDI. See picture below:



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The exact locations and picture of Engine Coolant Temp (ECT) sensors on 2004 VW Jetta TDI:

1. ECT sensor 1 placed on cylinder head:




2. ECT sensor 2 placed on the left bottom side of the radiator:

Check your coolant level and condition. If you havent' replaced the coolant in a while then change it. You can drain it from the bottom of the radiator plug. You may also want to change your oil and oil fiter.

it is more than likely airlocked bleed air from system or open heater core pipe make sure heater core is full

Possible the Thermo sensor faulty.
Need more info...Any dash lights on...Is interior heater working correctly...Is coolant level ok...Is Temp gauge moving at all

John

Had similar problem on my 2000 cabrio---hit the dashboard lightly above the temp gauge and it dropped to normal reading---the temp gauge spring was oversprung from the previous overheat problem I had---as long as the engine is cool --then so are you

You've lost engine coolant somewhere, and need to fill the engine back up with the VW special G12 coolant.

Then find the leak, which will be in one or more of the following places -
Water hoses to heater core, including any other coolant carrying hoses in the engine compartment
Radiator hoses
Plastic manifold bolted to side of engine which is known to leak (have replaced on an '01 VW Beetle, and an '02 Jetta, both members of our family in the past few years.).
Overflow reservoir cracked
Radiator leak.

Reply back what you find. If the engine got too hot, you may need to have the thermostat replaced.

Never ever bring a jetta to a volkwagen dealer. They will bend you over and rape you.

All you have to do is remove the top plug with a hex wrench.