1) There is a bad connection.
2) Incorrect off-set setting in software.
3) No or low airflow through Pre-Heater.
4) Bad heater.
5) Bad solid state relay.
Possible Solutions (corresponding to the Causes #):
1) Check connections to all heaters, both solid state relays, and the control PCB.
2) Check all software calibration settings.
3) Check to make sure airflow is within specifications.
4) Replace Pre-Heater assembly (APR-XLPHK). Replacement instructions: APR-XLPHK Replace.pdf
5) Replace solid state relay (APR-SSR). Replacement instructions: APR-SSR Replace.pdf
If you need instructions on how to replace or check any of these parts, please contact our APR Support Team.