Thermostats and AC control boards

A lot of AC calls are the thermostat or control system, not the AC itself.

The short version

• Check the thermostat is getting 12V and is set to the right mode.
• Some systems use a control board on the AC. Check its fuse and any fault lights.
• Multiplex and networked systems need every module talking. Check communication before replacing parts.
• Test the AC unit directly per the manufacturer's procedure to tell a unit problem from a control problem.

On the job

1. Blank thermostat: 12V at its harness. Most run from a small fuse at the panel or 12V from the AC's control box.
2. Jumper the call at the thermostat (furnace) or check the data cable (AC) to split thermostat from appliance.
3. CCC2 codes: E1 comm cable, E2/E3 indoor sensor, E4 outdoor, E5 freeze, E7 lost 120V, E8/E9 zone DIP switches.
4. Two zones: unique DIP settings on each rooftop board, then power cycle and re-pair.

Try it on a service call

Put the lesson to work on a hands-on troubleshooting case.

• AC does nothing, thermostat is blank

Keep going on air conditioning

Learn it

• Rooftop AC
• Capacitors
• Airflow and coils
• Heat pumps and heat strips

Prove it

• AC hums and trips
• AC runs but barely cools
• AC ices up and barely blows
• AC kicks off on hot afternoons
• AC does nothing, thermostat is blank

See it in 3D

• Rooftop AC 3D wiring model

On the job

• Air conditioner
• Thermostat
• Heat pump / heat strip