Aaon Rooftop Unit Troubleshooting & Alarm Codes

Problem

Solution 

Unit won’t turn on

• Check the power at the contactor(s) line side
• The thermostat is not set to heating

Evaporator motor won’t turn on

• The overload relay has tripped 
• The heater relay is not energized 
• The blower contactor is not energized 
• There is an open or short circuit in the capacitor (PSC motors only)

Blower not working

• The blower’s direction is reversed 
• The air filters are dirty 
• The condenser/evaporator coils are dirty 
• The ducts are dirty or obstructed

Blower turns on but no heat

• The heater contactor(s) are open 
• The limit switches are open 
• The heater relay is open 
• The heat strips are burnt out

Problem

Solution 

Unit won’t turn on

• Ensure there is power and gas 
• Double Check the thermostat is set to the right temperature and mode 
• Check there is 24 volt power to the ignition control

The burner won’t turn on

• Ensure the main gas valve has power
• The gas valve is broken 
• The gas valve’s connections are loose 
• Check if the limit controls are open 
• Replace the the differential pressure switch if it’s open
• The ignition control is faulty

Combustion air blower not working

• The thermostat is not set to heating mode 
• The relay won’t close (no power to motor)
• The motor is faulty 
• The internal motor overload is open

Burner goes “off” on high limit

• If the blower is not turning on, check the fan control
• The blower motor’s direction is reversed
• The filters are dirty 
• The ducts are obstructed or the dampers are closed 
• The manifold gas pressure is too high

Ignition is on but burner won’t ignite

• If the hand valve is in the “off” position, turn it on
• The gas is off or the pressure is too low 
• Ensure that the main gas valve has power
• Replace the the differential pressure switch if it’s open
• Sparker sensor is out of alignment
• The main orifice is blocked

Supply fan blower isn’t working

The ignition control is faulty

Problem

Solution 

Unit won’t turn on

• Check the power at the contactor(s) line side
• The thermostat is not set to heating

Blower won’t start

• The overload relay has tripped 
• The heater relay is not energized 
• The blower contactor is not energized

Blower won’t deliver air

• The blower’s direction is reversed 
• The air filters are dirty 
• The condenser/evaporator coils are dirty 
• The ducts are dirty or obstructed

Blower turns on but no heat

• Inspect the steam traps, valves and steam or hot water to ensure they adhere to the manufacturer’s guidelines
• The thermostat is defective

Problem

Solution 

Unit won’t turn on

• Check the power at the contactor(s) line side
• The thermostat is not set to heating
• The high pressure control has tripped 
• The low pressure switch is open (loss of charge)

Condenser fan not running

• The motor’s overload thermal protector is open 
• There is a short or an open in a the motor’s run capacitor
• The motor is faulty 
• Something is obstructing the fan or its shaft

Evaporator blower not running

• The motor’s overload thermal protector is open 
• The relay won’t close 
• The motor is faulty 
• There is a short or an open in the capacitor (PSC motors only)
• Something is obstructing the shaft or blower wheel

Compressor short cycling

• The refrigeration charge might be too low 
• Check the compressor overload setting 
• The ambient temperature is too low 
• The filters are dirty
• The evaporator blower isn’t running

Fan motor runs hot then stops

• The line voltage is too high

Compressor not turning on

• The line voltage is too low 
• There is an open in the limit switches 
• There is an overload or the pressure control has tripped

Blower won’t deliver air

• The blower’s direction is reversed 
• The filters are dirty 
• The duct is obstructed

Problem

Cause 

Unit won’t turn on

• Check the power at the contactor(s) line side
• The thermostat is not set to heating

Evaporator blower not working

• The motor’s overload thermal protector is open
• The contactor is not closed 
• The motor is faulty 

Fan motor runs hot then stops

• The line voltage is too high

Blower won’t deliver air

• The blower’s direction is reversed 
• The air filters are dirty 
• The duct is obstructed

Blower turns on but won’t cool

• Verify there is a water supply and it’s at the right temperature 
• Ensure the water control valves are working properly 
• Examine the water temperature rise as it enters and exits the unit to see if enough water is flowing

Problem 

Cause 

FLT: LAT Probe A

There is an open or short in the LATA probe

FLT: LAT Probe B

There is an open or short in the LATB probe

FLT:  OAT Probe

There is an open or short in the OAT probe

FLT:  LAT Hi Limit

Exiting air temperature is too high

Hi LAT Cooldown

The unit is still cooling down after a high limit event

LATbelow Lo Limit

LAT is below the low limit

Mech Lmt Recovery

Stage 1 only recovery after a mechanical limit

Mech Lmt Shutdown

FLT: SCR 0-10

There is an overload or short on the 0-10VDC analog signal output

BB (Base Block)

The 24V enable is not being received

Motor Overload oL1 or Drive Overload oL2

The load is too large 

The cycle times are too short for the accel and decel