How House Works

MrCool PC 03: pressure protection, what to check

By How House Works EditorialBased on MrCool manualsChecked against sources Updated

MrCool display

PC 03

“Pressure Protection (low or high pressure)”

DIY 5th Gen, DIY Select, Easy Pro, Advantage 5th Gen, DIY 5th Gen Cassette, DIY Hybrid Air Handler, Olympus Ducted · DIY 5th Gen owner's p. 39; Select owner's p. 41; Easy Pro owner's p. 41; Advantage 5th Gen owner's p. 47; DIY 5th Gen cassette owner's p. 45; Hybrid air handler owner's p. 56; Olympus ducted owner's p. 43
Owner checks, then technician

Advantage, cassette and Olympus add “(for some models)”. The outdoor board tables show high and low pressure separately as PC 30 and PC 31; the DIY 5th Gen multi-zone table has those two and no PC 03. Service manual: the outdoor pressure switch cuts off the system above 638 PSI (4.4 MPa) or below 18.85 PSI (0.13 MPa) (DIY 5th Gen Single-Zone Service Manual, p. 74).

What it means

PC 03 is pressure protection in the refrigerant circuit. The DIY 5th Gen service manual says the outdoor pressure switch cut off the system because pressure rose above 638 PSI (4.4 MPa) or fell below 18.85 PSI (0.13 MPa) DIY 5th Gen Single-Zone Service Manual, p. 74. The indoor display shows one code for both cases. The outdoor board table splits them into PC 30, “System High Pressure Protection”, and PC 31, “System Low Pressure Protection” DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 40, and the service manual sends all three codes to the same troubleshooting section DIY 5th Gen Single-Zone Service Manual, pp. 36–37.

The DIY Select, Easy Pro and hybrid air handler tables use the same wording. The Advantage 5th Gen, 5th Gen cassette and Olympus tables add “(for some models)” Advantage 5th Gen Installation & Owner’s Manual, p. 47; DIY 5th Gen Cassette Installation & Owner’s Manual, p. 45; Olympus Ducted Air Handler Installation & Owner’s Manual, p. 43.

Check these first

MrCool’s owner’s manuals give no steps for PC 03 itself. On the DIY 5th Gen, Select, Easy Pro, Advantage 5th Gen, cassette and Olympus manuals the procedure for any error code is DIY Select Installation & Owner’s Manual, p. 40; Easy Pro Installation & Owner’s Manual, p. 40; Advantage 5th Gen Installation & Owner’s Manual, p. 45; DIY 5th Gen Cassette Installation & Owner’s Manual, p. 44; Olympus Ducted Air Handler Installation & Owner’s Manual, p. 41:

  1. Wait about 10 minutes. The manual says the unit may stop or keep running safely and the problem may resolve itself DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 38.
  2. If the code is still there, disconnect the power, connect it again and turn the unit on DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 38. A buzz at power-on means the unit powered on normally DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 11, and it cannot be restarted within 3 minutes of being turned off DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 37.
  3. If PC 03 comes back, disconnect the power and contact MrCool DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 38. Give them your model number and a description of what the unit did DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 37.

The hybrid air handler manual prints no wait-and-restart step; it says to contact a local dealer or MRCOOL customer service DIY Hybrid Air Handler Installation & Owner’s Manual, p. 54. The 4th Gen cassette manual prints no steps either; it says to contact a local dealer or the nearest customer service center DIY 4th Gen Cassette Installation & Owner’s Manual, p. 53. For the 4th Gen E Star and multi-zone manuals, see the notes below.

Among the manual’s general checks before calling for service are a blocked air inlet or outlet on either unit (turn the unit off, remove the obstruction, turn it back on) and a dirty heat exchanger, for which the DIY manual’s fix is the Clean function on the remote DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 38. The same manual leaves any maintenance and cleaning of the outdoor unit to an authorized dealer or a licensed service provider DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 36.

When to call a licensed technician

Everything past the restart is technician work. The DIY 5th Gen service manual’s check for this code covers DIY 5th Gen Single-Zone Service Manual, pp. 74–75:

  • the wiring between the pressure switches and the outdoor main board, and the switches themselves;
  • the outdoor temperature: the high-pressure check asks whether it is above 50°C (122°F), the low-pressure check whether it is too low, and in both cases the chart’s answer is to stop the unit;
  • outdoor unit ventilation, the outdoor fan and a dirty heat exchanger, for high pressure;
  • whether the high pressure valve is fully open and the indoor fan runs properly in cooling, for low pressure;
  • the outdoor main board, and then whether the refrigerant system works properly.

The parts it lists to prepare are connection wires, the outdoor PCB, refrigerant, the pressure switch and the outdoor fan DIY 5th Gen Single-Zone Service Manual, p. 74.

Refrigerant is on that parts list. MrCool’s manual says anyone working on or breaking into a refrigerant circuit should hold a current valid certificate DIY 5th Gen Single-Zone Installation & Owner’s Manual, p. 4; in the US, refrigerant work is for a technician certified under EPA Section 608.

Notes for different models

  • DIY E Star 4th Gen: PC 03 is “Low pressure protection (for some models).”, with 7 operation lamp flashes and the timer lamp flashing DIY E Star 4th Gen Installation & Owner’s Manual, p. 41. Its troubleshooting section says the outdoor pressure switch cut off the system because pressure fell below 0.13 MPa DIY E Star 4th Gen Installation & Owner’s Manual, p. 55. Its owner’s procedure says “roughly 10 minutes” and adds: “DO NOT attempt these repairs yourself” DIY E Star 4th Gen Installation & Owner’s Manual, p. 41.
  • DIY 4th Gen multi-zone, 4th Gen cassette and Advantage 4th Gen: “High pressure protection or low pressure protection”, same lamp pattern DIY 4th Gen Multi-Zone Installation & Owner’s Manual, p. 49. The multi-zone guide checks high pressure (above 4.4 MPa) and low pressure (below 0.13 MPa) in separate sections DIY 4th Gen Multi-Zone Troubleshooting & Solutions Guide, Troubleshooting pp. 47, 49. The multi-zone owner’s procedure has the same “roughly 10 minutes” and “DO NOT attempt these repairs yourself” as the E Star DIY 4th Gen Multi-Zone Installation & Owner’s Manual, p. 49; the Advantage 4th Gen manual ends its steps with your nearest customer service center Advantage 4th Gen Owner’s and Installation Manual, p. 16.
  • The Advantage 4th Gen service manual’s letter-code table lists this fault as P6 Advantage 4th Gen Service Manual, Troubleshooting p. 4. On a DIY E Star 3rd Gen, P6 means something else.
  • The DIY 5th Gen multi-zone table has no PC 03. It lists PC 30 and PC 31 DIY 5th Gen Multi-Zone Installation & Owner’s Manual, p. 48.

Related MrCool codes

  • PC 30 “System High Pressure Protection”
  • PC 31 “System Low Pressure Protection”
  • PC 02 “Compressor Top (or IPM) Temperature Protection”

See also

Other ductless mini-splits: Daikin, Fujitsu, Mitsubishi Electric

All MrCool codes

MrCool is a trademark of its owner. How House Works is not affiliated with or endorsed by MrCool.

Sources

  1. MRCOOL. DIY 5th Gen Single-Zone Installation & Owner's Manual. Version Date August 5, 2026; file suffix -01; no document number printed. Cover title: DIY Series Single-Zone Mini-Split System, Installation & Owner's Manual. Accessed Oct 13, 2026.
  2. MRCOOL. DIY 5th Gen Single-Zone Service Manual. Version Date April 20, 2026; file suffix -01; no document number printed; same models as the owner's manual. Accessed Oct 13, 2026.
  3. MRCOOL. DIY Select Installation & Owner's Manual. Version Date August 5, 2026; no file suffix; no document number printed. Accessed Oct 13, 2026.
  4. MRCOOL. Easy Pro Installation & Owner's Manual. Version Date August 5, 2026; no file suffix; no document number printed. Cover title: MRCOOL DIY Easy Pro Mini-Split System. Accessed Oct 13, 2026.
  5. MRCOOL. Advantage 5th Gen Installation & Owner's Manual. Version Date September 19, 2025; file suffix -01; no document number printed. Accessed Oct 13, 2026.
  6. MRCOOL. DIY 5th Gen Multi-Zone Installation & Owner's Manual. Version Date January 8, 2026; file suffix -01; no document number printed. Its code table is titled “Error Display (Indoor Unit)” but lists outdoor unit codes.. Accessed Oct 13, 2026.
  7. MRCOOL. DIY 5th Gen Cassette Installation & Owner's Manual. Version Date December 02, 2025; file suffix -01; no document number printed. Cover title: DIY Series Outtasight One-Way Cassette. Accessed Oct 13, 2026.
  8. MRCOOL. DIY Hybrid Air Handler Installation & Owner's Manual. Version Date April 21, 2026; file suffix -01; no document number printed. Not linked from MrCool's documentation page on 2026-10-13; the file is on MrCool's own file host.. Accessed Oct 13, 2026.
  9. MRCOOL. Olympus Ducted Air Handler Installation & Owner's Manual. Version Date May 29, 2025; file suffix -01; no document number printed. Cover title: Olympus Series Mid-Static Ducted Air Handler. Accessed Oct 13, 2026.
  10. MRCOOL. DIY E Star 4th Gen Installation & Owner's Manual. Version Date 03-21-23; file suffix -01; no document number printed. Not listed on MrCool's documentation page on 2026-10-13; the file is on MrCool's own file host. It prints the same Version Date as the 3rd Gen E Star manual.. Accessed Oct 13, 2026.
  11. MRCOOL. DIY 4th Gen Multi-Zone Installation & Owner's Manual. Version Date 06-06-2023; file suffix -01; no document number printed. Not listed on MrCool's documentation page on 2026-10-13; the file is on MrCool's own file host.. Accessed Oct 13, 2026.
  12. MRCOOL. DIY 4th Gen Multi-Zone Troubleshooting & Solutions Guide. Version Date 10-22-21; file suffix -01; pages are numbered within the Troubleshooting section, cited as printed (Troubleshooting 4). A technician document. MrCool's documentation page labels it the owner's manual for 4th Gen DIY.. Accessed Oct 13, 2026.
  13. MRCOOL. DIY 4th Gen Cassette Installation & Owner's Manual. Version Date 06-28-24; file suffix -01; no document number printed. Cover title: DIY Series Outtasight Ceiling Cassette Model DIYCASSETTE**HP-230C25. Accessed Oct 13, 2026.
  14. MRCOOL. Advantage 4th Gen Owner's and Installation Manual. Version Date 09-19-23; file suffix -01; no document number printed. Cover title: Advantage Series Owner's Manual + Installation Manual. It has no code table; the codes for this series come from the service manual.. Accessed Oct 13, 2026.
  15. MRCOOL. Advantage 4th Gen Service Manual. Version Date 7/06/22; file suffix -01; cover title Advantage Series Air Handler & Condenser Service Manual; pages are numbered within sections, cited as printed (Troubleshooting 5). It prints two lamp tables: one with letter codes (E1, P4, P6 and so on) and one with EH, EL, EC and PC codes. This table uses the second.. Accessed Oct 13, 2026.