I have a few newer birds with Webasto heaters on them. The heaters will not fire if the temp falls below zero. You can manually start them just fine but when the timers are set they will not fire on their own if the temp drops below zero, raise the temp to +5 degrees and everything is peachy. We have other buses with Webastos on them that start just fine the problem is just the newer birds with DEF on them.
Call Webasto Tech Support. In the past I have found them very helpful.
The timers for the Webasto 2010 Schoolastic heaters were causing us a problem. I called Webasto Tech and was recommended to try the little known "White Timer". We had very good results with them for a couple of years. The last order of timers however have been fraught with faults. I am hoping just a bad batch.