check engine light came on 2 days after service (timing belt water pump replaced on 2004 Volkswagen Passat

My son said to remove neg battery wire to reset computer... could be water temp sensor/air in coolant. Light went out. 2 weeks later came on again. Repeated same, light out operation normal. Also since water pump/timing belt was replaced the coolant temp has been running lower by 10-20 degrees.

Asked by for the 2004 Volkswagen Passat