YHM - Low Pressure Fault (Heating)

York Heat Pump: YHM

Re: Low Pressure Fault - Heating

The YHM heat pump has a 5-stage Mitsubishi compressor a DC Inverter board.

A small control board below the Inverter Board has the diagnostic and status indications.

A Low Pressure Fault is indicated by the following LED pattern on the outdoor unit control board:

Soft Lockout:

Hard Lockout:

Low Pressure Switch

If the low-pressure switch opens for more than five seconds, the control will cause a soft lockout condition and display the appropriate fault codes.
If four LPS openings occur within a twelve-hour period, it causes
a hard lockout which can be reset by cycling system power, or by shorting
the TEST pins for more than 2 seconds.
The control will ignore the low pressure switch input and not cause a soft lockout condition if it opens during the following conditions.
- Defrost operation
- First two minutes of compressor operation
- Two minutes following the completion of a defrost cycle
- TEST input shorted with Y1 input energized

Qty	Possible Causes:
1	Bad Heating Expansion Valve
	Low Refrigerant Charge
	Bad Pressure Switch
	Refrigerant Flow Restriction
	No Condenser Fan Operation

CHECKOUT:

1. Check Unit Operation

Make sure the thermostat is set correctly and calling for Heat.
Allow up to 5 minutes for compressor to start.
- Outdoor Unit Operating → Step 2. Check Refrigerant Pressures
Check the Diagnostic Indication Lights.
- Unit in Lockout ⇒ Reset the control by removing 24VAC power or by jumping the Test pins for more than 2 seconds.

2. Check Refrigerant Pressures

Hook-up refrigerant gauges to the pressure ports on the unit.
- Blue --> "True Suction" Low Pressure
- Red --> High Pressure (either refrigerant line)

Ensure that the unit is running at full capacity (H4).
- It is possible to force the unit into High Speed by moving the dip switch on the small control board to "Hi".
Monitor pressures as the system operates.

Low Pressure Issues