Replacing the Blue Coolant Temperature Sensor in the Golf VR6 Mk3

Problem: My VR6 was having trouble starting and idling when cold. Many forums suggest this is due to a faulty “Blue” temperature sensor.

Solution – Testing the Sensor: To test the sensor while it’s fitted to the car, remove the latched plastic connector from the sensor and measure the resistance (Ω) between the 2 pins.
If the sensor is working correctly it should measure about:

• 5500Ω at 0°c
• 2500Ω at 20°c
• 1250Ω at 40°c
• 600Ω at 60°c
• 325Ω at 80°c

I’m my case, the resistance was infinite, I.e. open circuit. So needed replacing.

Solution – Replacing the Sensor: The parts you will need are:

• 1 x 2-pin Blue Sensor (VW: 025 906 041 A, GSF: 929vg014)
• 1 x O-ring / Seal (know the part number? Please tell us in comments below)
• Metal or plastic retaining clip (know the part number? Please tell us in comments below)

Step 1 – Clear access to the sensors.
Above the row of 3 sensors is a metal bracket providing cable support. For access to the sensor, especially the retaining clip, this bracket should be removed by removing the two bolts holding it to the block. The cable clips can be levered-off with a screwdriver or pliers. I decided to do away with the bracket altogether so I cut mine off with end cutters.
In the image below you’ll see that I’ve loosened the bracket but the wrapped loom is still still attached:

Step 2 – Remove the retaining clip.
Once the bracket is removed, unplug all three sensors and tuck them out the way. With a small flat screwdriver and/or needle-nose pliers, remove the sensor retaining clip. DO NOT pull the sensor out yet! Coolant will flood out.

Step 3 – Prepare the new sensor.
When you remove the old sensor, the old O-ring will be perished and crappy, so DO use a new one. Place the new O-ring on the new sensor and take note of the orientation of the existing sensor, I.e which side the connector ridge is on – this will be useful when acting quickly in the next step.

Step 4 – Swapping-out the sensor.
You have a choice here:
A) Drain the coolant system of a few litres so that it’s below the level of the sensor.
B) Leave the cap on the Expansion Tank to create a partial vacuum and work quick! I did the latter.

Have a watering can or a hose near-by. Have the new sensor in one hand and hold it near to the position it’ll need to be placed in the next step. With the other hand, pull the old sensor out. It should come out very easily, coolant will gush out but work fast and press the new sensor in place. This process should take about 3 seconds and you will lose about half a litre of coolant.

Step 5 – Clean-up and re-attach things.
As soon as the new sensor is in place, douse the area with water to wash off the spilled coolant. Try not to wet the area more than necessary, and consider taking the car for a drive when you’re done to dry things off. Now secure the new sensor in place with the new retaining clip, probably a plastic one if you sourced the part from GSF or Euro Car Parts. As you can see below, I decided to attach the wiring to the engine loosely with a small zip-tie, but you might want to re-attach the original bracket and use the original wire clips.

Step 6 – ECU/ Faults…
As soon as the sensor is replaced and the engine is run again, any stuttering or odd idling should be cured, but the Diagnostic Trouble Code (DTC) will remain in the ECU memory. This can be removed with Vag-COM or a fault reader like the Vgate VS450 (about £25 from eBay).

Tips and Reminders:

1. Work with the engine cold.
2. Invest (pence) in a new O-ring and Retaining Clip, the old ones are very likely to break.
3. Do have some water handy, coolant will do horrible things with your paintwork.
4. Don’t forget to re-attach the sensor retaining clip!

If you have any questions or corrections, please post in the comments below. I’d also love to know if you found this how-to useful? just one click!
- +11 thumb, 13 overall.
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