Don’t buy cheap VW temp sensors. I bought a £6 sensor from eBay and I got this fault appear in VAG-COM along with “Emissions Workshop” on the dash.
17664 – Engine Coolant Temp Sensor (G62): Open or Short to Plus
P1256 – 35-10 – – – Intermittent
The black sensor on the left is the original VAG item, the one in the middle is the £6 from eBay and the one on the right is from Darkside Developments, made by Beru and cost about £16. All replacements are now green so I’ve read. The cheap one didn’t work properly on my car (ARL engine code) and wasn’t the best fit. The O-ring that comes with it seems to be a bit small and a poor fit although it didn’t actually leak. The other problem was that the coolant temp reading is wrong. Below you can see the 4 pins labelled A, B, C and D. Two are for the ECU and the other two supply the temp gauge on the dashboard.
I took some resistance readings for all three sensors at the same ambient temperature. This is what I found:
| Sensor | KΩ: Pins A-B | KΩ: Pins C-D |
| Original VAG | 1.2 | 2.6 |
| Cheap aftermarket | 2.2 | 3.5 |
| Beru aftermarket | 1.1 | 2.7 |
So the cheap sensor, despite being advertised as having the same part number (079 919 501 C), doesn’t fit very well and doesn’t actually work. When the car was fully warmed up, the highest temperature I saw in VAG-COM was about 70°C. The Beru replacement works perfectly and fits better. The coolant temp shows 90°C like it should do now.
The problem originally started with difficulties starting when the engine is cold which I thought was just down to the temp sensor but I think there is more to it than just that. The cold starting problem hasn’t gone away and is worse the longer the car is left. I think that the tandem pump may be leaking and it leaked on to the temp sensor causing the intermittent fault code. This would also explain why it is difficult to start when cold if the car has been sitting and air has slowly made its way into the fuel supply. I have to crank the engine 2 or 3 times on a cold morning before it starts and then it’s fine once warmed up.
So the next item on the Golf is to rebuild the tandem pump or replace it if necessary. The battle continues…
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I just replaced mine, on 2001 audi allroad 2.5TDI. The sensor for motor ECU was open, and my idle speed increased til about 12-1300. With automatic transmission this is a problem. After a while it started spinning up when trying to change to 2nd gear. I ordered two cheap sensors from Ebay. I fitted the first one i received, and got leakage. The Oring was to slim. Changed Oring, leak stopped, but temperatur in dashboard goes up to 115C. This worried me. I changed to the second sensor from Ebay, and temp reading is fine. However, now I am worrying about the reading for the engine control. I will take a reading with ohmmeter to check it. I do not know if the two sides of the sensor are identical. Have checked Elsa (audi workshop manual) and it explains the sensor for the dashboard. (I am shure you can get a faulty sensor from audi dealer as well). Thanks for sharing. The manual says:
60C – ca 259 Ohm
90C – ca 107 Ohm
120C – ca 40 Ohm