I bought a used Volkswagen Passat Estate in Oct 2016 (124,000 miles on the clock) with a 12 month warranty. On the first motorway drive the glow plug warning light came on. I returned it to the dealer to fix it but it he can find nothing wrong. It came on again last week so I pulled into a service area and stopped it, it didn't come back on when I continued my journey. What is the reason for this?