The coolant temperature sensor is by far the commonest cause of the EML appearing - without any other obvious symptoms.

The Bosch software is clever in that fuel temperature is used as a substitute value for coolant temperature (with a scaling factor) when the CTS fails. All of the original black-bodied sender units will fail as VAG, in a burst of engineering brilliance, filled them with rock hard epoxy resin.

The result is that after a few hundred thermal cycles, the brittle thermistor elements crack due to thermal stress and fail open circuit - usually the one for the engine management system which has the higher resistance.

Ensure that the replacement sender has a green plastic body and is not made in China. There are loads of good German replacements around (GSF sell them) which cost about £17. The part number of the later pattern sender is 059 919 501A. It's wise to change the "O" ring seal - which usually sticks in the head. Hook it out with a small right angled screwdriver.

659.

It's probably about time for another diagnostic check - but the replacement CTS may not be above reproach. For example, I have found some genuine VAG parts such as fuel filters are now made in China with (to put it politely) variable results. One such filter would not seal correctly around the thermostatic "T" due to the bore on the filter can being out of shape. The resulting intermittent bad starting has not endeared the vehicle owner to VAG - when I found the reason for her. As the Chinese are making CTS units for this engine (Google the part number), the VAG bean counter division may well have found them.

Access to the CTS is not good on the Passat but if you can make a slave lead or check at the engine disconnect panel you can check the resistance. The "A" thermistor has brown/blue and brown/yellow leads, and the "B" brown/white and brown/grey.

With the coolant at 25 deg C the instrument panel thermistor ("A" terminals) should measure about 1150 Ohms and the ECU thermistor about 2k55 Ohms.

The problem with these wretched things is that they fail intermittently when the thermistor element cracks and usually read OK when cold. A very brief open circuit as the coolant heats up will trip the EML.

If you're paying a garage to do the work, I would (personally) fit another new CTS first as it's an easy DIY job. If it fixes the fault, get a refund for the VAG one.

It takes a couple of runs with a good CTS to clear the EML.

659.

Edited by 659FBE on 24/07/2011 at 23:50

659. Changing the CTS as not made any difference to the malfunction warning lights,seems OK when first changed then as soon as the engine gets hot the warning lights come on,been told it could possibly be the wiring to the coil.

Colin

659. It is the temperature sensor that goes to the ECU but struggling with the part No at the moment.

Bosch have a number of Asian factories-not all their parts are German.

Thanks both