Modine Heater

I have a how dawg heater 45,000 btu unit.
Only works sometimes. Appears that when it gets below 20 degrees outside it does not function properly. Exhaust blower kicks on, about 10 seconds later the unit fires up. It stays fired but then the high limit must kick on and shut it down. Tha fan does not come on. I then unplug the unit let it sit for a hour or so and then it will work for a few cycles and then it’s the same problem. The reset buttons near the flame do not kick off. I’ve checked the fan and it’s okay. I’ve cleaned the flame rollout. Is there anything else I need to check? Exchast is clean not bird nests.

Thanks
Any chance it could be the time delay relay? And is there a way to test it? Now it seems to be working. Flame is clean and sensor clean. Every now and then we get off blue flame on our gas stove, possibly this also happens in the heats and kicks on the flame rollout. Can I put a jumper wire of the sensor as a test?

Thanks