On a WaterFurnace unit with the Aurora Base Control, fault code 2 is a high pressure lockout and fault code 3 is a low pressure lockout. Per WaterFurnace guide AG1562EW, a high pressure fault is recognized when the high pressure switch opens "no matter how momentarily." A low pressure fault needs the low pressure switch to stay open for 30 seconds continuously, and it is ignored during the 2-minute startup bypass. The control retries, then locks out after 3 consecutive faults without satisfying the thermostat call.
How the control decides it is a pressure fault
| Item | High pressure (code 2) | Low pressure (code 3) |
|---|---|---|
| Switch and connector | Normally closed high pressure switch, P4-9/10 | Normally closed low pressure switch, P4-7/8 |
| Recognition | Switch opens, no matter how momentarily (under 1 second) | Switch open continuously for 30 seconds; any closure restarts the 30 seconds |
| Startup bypass | None (recognized immediately) | 2 minutes (30 seconds in test mode) |
| Retries before lockout | 3 | 3 |
| Related condition | Code 2 indicates system pressure above 600 psi (R-410A) | Loss of charge: the low pressure switch is open before the compressor starts |
The high pressure switch is wired in series with the compressor contactor, so it also works as a hard-wired limit if pressure goes too high. That is why a high pressure trip can be very short: a brief spike is enough.
In lockout, the Alarm (ALM) and Lockout (L) outputs turn on, and the blower runs continuously at low speed. A running blower with no heating or cooling is a normal lockout symptom, not a separate blower problem.
High pressure causes by mode
The guide's code 2 note says the cause may be low water flow in cooling (check the coaxial heat exchanger for mineral build-up) or low airflow in heating (check filters and coil for dirt). The full list:
| Heating mode | Cooling mode |
|---|---|
| Airflow interruption: inoperative blower, dirty filters or air coil, blocked return grille, closed or blocked supply registers, restricted ducts, zone dampers | Water flow restricted or inadequate; verify against the pressure drop tables in the product install manual |
| Blocked or seized expansion valve assembly | Water-to-refrigerant heat exchanger fouled with debris; back flush with at least 20 psi of water pressure, and acid clean if mineral build-up is evident |
| Discharge pressure outside the operating range in the product install manual | Entering air too hot; equipment is designed for a maximum of 85°F DB and 71°F WB |
| Overcharge; check superheat and subcooling | Seized or blocked expansion valve assembly; overcharge |
Low pressure causes by mode
The guide's code 3 note says the low pressure switch has opened, which may indicate a loss of system charge, a system restriction or a frozen heat exchanger.
| Heating mode | Cooling mode |
|---|---|
| Unit installed in a space below 50°F; install a crankcase heater and protect the unit from the elements | Inadequate airflow; follow the same checks as a heating-mode high pressure lockout |
| Water flow restricted or inadequate | Seized or blocked expansion valve assembly |
| Seized or blocked expansion valve assembly | Low refrigerant charge |
| Return air below 50°F, which cannot be tolerated on a continuing basis | |
| Low refrigerant: check for leaks, repair and recharge using approved methods |
Board or switch? The guide's voltage checks
AG1562EW's control board troubleshooting steps separate a bad Aurora Base Control (ABC) from a bad pressure switch with two 24 VAC readings:
- High pressure, code 2. Measure from P4-9 to C. If it is not 24 VAC, the guide says to replace the ABC. Then measure from P4-10 to C. If it is not 24 VAC, the guide says to replace the high pressure switch (it calls it the high pressure sensor).
- Low pressure, code 3. Measure from P4-7 to C. If it is not 24 VAC, replace the ABC. Then measure from P4-8 to C. If it is not 24 VAC, replace the low pressure switch.
- Check the refrigeration system either way. A switch that opened because pressure really went out of range is doing its job. Work through the causes above before replacing it.
How to reset the lockout
- With DIP switch SW2-8 on, remove thermostat inputs Y1, Y2 and W for at least 3 seconds.
- With SW2-8 off, remove Y1, Y2, W and DH for at least 3 seconds.
- Or turn power off. AG1562EW says at least 5 seconds; the Versatec 300 manual (OMV3-0007WB) says at least 30 seconds. Thirty seconds satisfies both. See how to reset a WaterFurnace lockout for why the manuals differ.
- Or enable the emergency shutdown input for at least 3 seconds.
The Versatec 300 manual uses the same high and low pressure recognition rules and the same 3-fault lockout as AG1562EW.
Parts involved
| Component | Role in this issue | Where to find it |
|---|---|---|
| Aurora board | The guide's voltage checks call for replacing the ABC when 24 VAC is missing at P4-9 or P4-7. Aurora boards are listed by unit line; match yours by model. | 17X553-07 — WaterFurnace AURORA Board Commercl WTR/AIR |
| Expansion valve (TXV) | A seized or blocked expansion valve is a listed cause of both high and low pressure faults. TXVs are listed by tonnage and model. | 33P628-01 — WaterFurnace TXV TR6 3-TON ADJ W/BLD IC Switch EL24" |
| High and low pressure switches | Replace when the guide's second voltage check fails. Pressure switches are model-specific. | WaterFurnace replacement parts |
| Per-model parts | We list parts by WaterFurnace unit model. Search your model number for its collection. | Example: WaterFurnace NDV049G111CTL0AA Parts |
Scope: This article covers high pressure (code 2) and low pressure (code 3) faults on the WaterFurnace Aurora Base Control as described in AG1562EW, with reset timing also cited from the Versatec 300 operation manual, OMV3-0007WB. Other WaterFurnace controls may differ. The parts above are examples listed by live product title; match yours by model and serial number.
Before you order
Confirm your unit's exact model and serial number from its nameplate before ordering. For the full code list, see WaterFurnace Aurora Base Control fault codes. For freeze detection and condensate faults, see WaterFurnace fault 5 and fault 7. To confirm fit, contact us at help@totalhvacparts.com with the model and serial number.
Frequently Asked Questions
What is WaterFurnace fault code 2?
Fault code 2 on the Aurora Base Control is a high pressure lockout. It is recognized when the normally closed high pressure switch opens, no matter how momentarily, and AG1562EW says it indicates system pressure above 600 psi on R-410A units.
What is WaterFurnace fault code 3?
Fault code 3 is a low pressure lockout. The low pressure switch has to stay open for 30 seconds continuously, and it is ignored during the 2-minute startup bypass. If the switch is open before the compressor starts, the control treats it as loss of charge.
What causes a high pressure lockout on a geothermal heat pump in cooling?
AG1562EW lists restricted water flow, a fouled water-to-refrigerant heat exchanger, entering air above 85°F DB or 71°F WB, a seized or blocked expansion valve and overcharge.
How many times does a WaterFurnace retry before locking out?
Three. If 3 consecutive faults occur without satisfying the thermostat Y call, the control goes to lockout and turns on the ALM and L outputs.
Sources
- WaterFurnace Aurora Base Control application and troubleshooting guide, AG1562EW (safety features, fault recognition timing, causes by mode, control board troubleshooting steps, reset)
- WaterFurnace Versatec 300 Operation and Maintenance manual, OMV3-0007WB (pressure fault recognition and lockout reset timing)
- Live product titles for the parts listed above, as shown on each product page
WaterFurnace heat pumps involve line voltage, 24 V control circuits, pressurized refrigerant, standing water and rotating equipment. Service should be performed only by qualified personnel, following the manufacturer's manual for your unit and your facility's lockout/tagout procedures.