PC31 Low pressure protection

What the control is telling you

The low-pressure switch opened. Outdoor board shows P2; 48K HH shows J6.

Applies to: DLCMRBH / DLCMHBH multi-zone outdoor units 18-48K (Midea, Carrier, Viessmann Vitocal 200-S)

Likely causes

• Loose switch wiring
• Bad switch
• Very low outdoor temperature
• Service valve not fully open
• Indoor fan not running
• Low charge

Test and repair steps

1. Safety kill power at the disconnect and wait for the inverter capacitors to discharge before touching boards or wiring.
2. Check the low-pressure switch wiring to the main board.
3. Unplug the switch and measure it. A good switch reads 0 ohms at rest; open = replace it.
4. Check the outdoor temperature is not below the operating range.
5. Make sure both service valves are fully open.
6. In cooling, check the indoor fans run (see the fan faults if not).
7. Check for low charge or a leak and repair. If all good, replace the outdoor main board.

Models that show this code

7 models in Carrier DLCM(R/H)B multi-zone outdoor

Other codes on the same units

• PC08 Outdoor over-current protection
• PC10 Low AC supply voltage
• PC11 AC supply voltage protection (over voltage)
• PC12 DC bus high voltage / 341 MCE error
• PC30 High pressure protection
• PC40 Main chip to compressor drive chip communication error
• PC42 Compressor start failure
• PC43 Compressor phase loss