Cause of error code Lockout 138

Flame too low.

1. Blocked venting. 
2. Fowled or faulty flame sensor.

Solution for error code Lockout 138

1. Check for blockage of the exhaust-vent, Air-inlet, combustion blower, gas valve venturi, heat exchanger etc. 
2. Inspect flame sensor for cracks of fowling, clean or replace as necessary.