1 hour ago, linni said:

Bought this one for 61 euros:

https://spareto.com/products/metzger-relay-glow-plug-system/0884010

It had number 457 on the back instead of 527 as original, but works just fine, no more faults anymore.

Thought it would, so much cheaper than new glow plugs or pressure sensor.

For reference, the part reported as working above is Metzger 0884010. The other number discussed is Bosch 0 281 003 085. Those links help identify the controllers; the large number printed on the casing is not enough on its own to choose one.

For another car with P0684 or P064C, check the exact code description, controller power, earth and connector before ordering. Also match the engine and OE reference. A controller fault should not automatically turn into replacing the pressure-sensing glow plug.

Metzger 0884010 reference

Bosch 0 281 003 085 reference