Mmmn... good question. Unfortunately, it was a while ago, and the unit is the hardwired one, not the plug-in, so I can't easily test it. I don't recall the nature of the malfunction - I am familiar with the diagnostic flashes on other things (OBD1 on older Volvos), and don't ~recall~ seeing flashes, so it MIGHT have been the trouble light - but it's been a while and I really didn't allocate that to long-term memory - sorry!