To check the thermo switch run-on time:
Turn heater on and regulate to full heat output. Let heater run for five minutes.
Push heater flap lever forward until warning light goes out. At the same time start stop watch.
The heater air and combustion air blowers, controlled by the thermo switch, should run-on 120-240 seconds. This time is for a nomoinal voltage of 12 Volts and an outside temperature of about 68 F/20 C. The run-on time is shorter at lower outside temperatures.
If the run-on time is too short or the heater does not switch off, install a new thermo switch. The thermo switch is not adjustable.