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1999 Volkswagen GTI - Temp reading

My temperature gauge was registering cold even though I had heat in the cabin. After a google search and a couple of days with a mechanic, who couldn’t solve the problem I bought a temp sensor and installed it. Now I do get a read on the gauge but it stays relatively low unless the car is in idle mode. Any ideas?

First off, you may have already figured this out, your mechanic is incompetent. This is a simple problem

The first thing I’d do is check the coolant temp with a thermometer (or IR temp reader) to determine if the coolant is hot, and how hot. It should be about 185 degrees F or 90 C. Next check the gauge - no reading? Probe the disconnected coolant temp sensor with an ohm-meter and check resistance to ground if it is a single wire type and across the terminals if it is a 2 wire type. Open or short, 0 ohms or no ohms, failed sensor. But you already figured that out and replaced it.

Next, replace sensor, check coolant temp readings again. If the gauge is in the middle, the coolant temp should be about 185-190 F.

From your post I think you are saying, the car is warm at idle but gets cool when driving, right? If that is the case, your thermostat has failed and is open all the time so the engine won’t stay warm when air passes over the radiator.


Thank you, although not the news I hoped to hear. The mechanic said that since I was getting heat inside the car it was likely not the thermostat. Live and learn…I had never used him before but after my hours (literally) of time with him I came to the same conclusion. He told me he was googling my car’s problem!

Thanks again.

Cars of that era sometimes used a separate sensor for the dash temperature gauge, and the computer used its own coolant temp sensor. Some designs instead multi-purposed the computer’s coolant temp sensor. I’d guess yours is the latter. You can sometimes somewhat judge the coolant temperature just by feeling the top hose going into the radiator, and by feeling the two hoses going through the firewall, which are for the heater. When the engine is at normal operating temperature the top hose of the radiator usually feels quite hot to the touch, and one of the hoses through the firewall will be nearly the same temperature, and the other pretty warm, but not quite as warm as the hotter one (with the heater on max flow).

If the hoses don’t feel warm enough, probably a thermostat problem. Wouldn’t be unexpected in 19 year old car at all. Usually replacing a thermostat isn’t an expensive proposition. I’m seeing $40 for the part and 1.5 hours labor to replace it. That’s not gonna cost that much, but more labor than is usually required for most econoboxes, maybe something about the engine design. That’s for the 2 L AEG engine. Think of it as a “GTI” extra expense … lol …

Before replacing the thermostat, make sure the shop is sure the thermostat is the actual problem, and not just a sensor or gauge problem. The sensor has a calibration chart showing temperature vs resistance, so that’s easy for a shop to test. A scan tool can usually measure the coolant temperature, and it should match the dash gauge, so that’s a way to see if the dash gauge is the problem.

The sensor for my car is a four pin sensor, so I am told the same sensor is used for dash temp gauge as well as coolant temp. Thanks for your comments…will check out suggestions today.