Hi all, Just had my engine thermostat replaced via main dealer as my engine temp gauge on the dash was dropping whilst stationary, very slow moving traffic etc along with a ERG valve? following the car being put through the dealers diagnostic device. The fault (engine gauge) after three days has started to drop again whilst in slow traffic, anybody have any ideas what could be causing this fault as it seems the thermostat was not faulty? What does the ERG valve do? If i leave the thermostat as is,will it cause any damage to the engine. Can i return the car to the main dealer as they have not cured the fault despite the diagnostic check and get them to investigate further free of charge? Cheers.