Don’t buy a cheap VW temp sensor is the general advice on the internet. I hadn’t read this until after I’d bought a £6 sensor from eBay because 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 the one on the right is from 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|
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 in to 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…