Found pilot out upon arrival. Unit was just lit to heat up from cold about 15 min prior to re light and monitored temp. Started out about 300 deg , set thermostat @ 350. Heated up and cycled between 345-365 . ok. High limit not tripped. Drained some oil and found a gap between high limit and burner tube. No debris build up. will return tomorrow hopefully around 12:00 PM with high limit and thermostat. Test unit and heating from cold -- Returned and monitored MV DC readings from Hi limit and temperatures during cycling. Also checked air vent on gas valve and found obstruction. This could have caused the problem. Replaced vent tube and monitored readings for 1 1 /2 hrs. Pilot was lit upon arrival and remained lit the whole time. MV readings never went over 4.1 on initial heat up or any cycles temperatures cycled consistently. If problem re occurs call and we will return