A properly running engine is one that is running at the correct temperature. The only way you and the ECU will know its at the proper temperature is through its coolant temperature sensor. The simple sensor relays the exact temperature to the engine and coolant temperature gauge to show you when the engine is up to temperature and tell the ECU when to kick on the cooling fans.
This DIY on your Audi A4 or various VW models can be a tedious job, but it is straightforward. No special tools are needed and nothing but the temperature sensor needs to be removed depending on the path you take. The coolant temperature has a critical but straightforward job in relaying the engine's temp. The sensor needs to be replaced as soon as it becomes a fault to keep the engine running happy and cool.
Audi/VW models and years applicable:
Symptoms of a failed Audi/VW 1.8t coolant temperature sensor:
- Engine fans constantly cycling
- Coolant temperature gauge not showing any reading
- Coolant temperature gauge showing an impossibly high reading
- Coolant leaking from around the sensor
The coolant temperature sensor contains a simple four-pin thermistor to relay the temperature readings to the ECU and the coolant temp gauge. Two of the pins send information to the ECU while the other two pins send their information to the coolant temperature gauge. It is possible to have only two failed pins in the sensor that cause only the ECU or gauge to read incorrectly.
Additionally, an o-ring seals the temperature sensor to the engine block. If the o-ring gets damaged or wears too much, the coolant will leak from the base of the sensor.
How long will it take to change the coolant temperature sensor on an Audi/VW 1.8t engine?
There are two ways to get to the coolant temperature sensor. You'll need to disassemble a majority of the PCV system and its hoses at the back of the engine, or you'll have to tilt the engine forward. Neither of those options are as scary as they sound.
If you choose to remove the PCV hoses, the engine stays in place, but you'll be fiddling with, at least, 14-year-old hoses. They've seen plenty of heat cycles, and the plastic parts they connect to have become brittle. You run the risk of cracking the hoses or the nipples they clamp to, leading to more work.
Or you can tilt the engine forward. This option requires placing a jack stand under the rearmost part of the transmission and then lowering the car back down. The jack stand will push on the engine and transmission mounts, tilting it forward enough to give you the space to access the coolant temp sensor. There are no fiddly hoses to work with here, but you can flex the mounts too much if you lower the car too far.
Budget 45 minutes for this job if you take the PCV hose route, and even less time if you tilt the engine.
How much will it cost to change the coolant temperature sensor on an Audi/VW 1.8t engine?
The only part needed for this just costs a wallet-busting $5.09. If you go the PCV hose route, you'll need to replace any hoses or plastics you break.
Tools required to change the coolant temperature sensor on an Audi/VW 1.8t engine:
- Floor Jacks
- Jack Stands
- 90° Pick
- Flat Head Screwdrivers
- MAF Cleaner
- Airlift Coolant Pressure Tester & Purge Tool
Parts required to change the coolant temperature sensor on an Audi/VW 1.8t engine:
Steps required to change the coolant temperature sensor on an Audi/VW 1.8t engine:
Step 1: Remove the old coolant temperature sensor
The engine tilt method is far more straightforward and requires less disassembly. Conversely, the workspace is much tighter. The coolant temperature sensor is green and easy to find at the back of the engine.
Use a small pick or flathead screwdriver to pull out the u-shaped locking clip that holds the sensor into its housing.
Then, wiggle the sensor out of its housing and pull it up so you can see it. Disconnect the old sensor from the wiring harness and keep the harness accessible. If the sensor's o-ring doesn't come out attached to the sensor, use a pick to remove it from the sensor housing.
Step 2: Replace the coolant temperature sensor
Before anything else:
- Examine the wiring harness plug for the temp sensor.
- Use Mass Air Flow cleaner and a brush to clean the plug if there is any residue or corrosion.
- Plug the new coolant temp sensor into the wiring harness, and fit it back into the housing.
Refit the u-shaped lock when the sensor is fully seated. If you have a pressure tester, use it now to check for any leaks. Otherwise, lift the car back up and remove the jack stand from under the transmission.
Next, start the car and check for any leaks. Let the engine get to operating temperature to ensure that the coolant temp sensor is now working correctly. Then, use a scan tool to remove any coolant temperature sensor related codes.