H42 on a Dimplex LA MI inverter outdoor air-to-water: Compressor low pressure abnormality

What it means

The manual says: Compressor low pressure abnormality.

Part involved: Outdoor pipe temperature sensor / refrigerant circuit.

When it shows: During heating, 5 min after compressor ON, outdoor pipe temperature below -29C or above 26C.

Likely causes:

• Dust accumulation on outdoor heat exchanger.
• Air short circuit at outdoor unit.
• 2-way valve partially closed.
• Faulty outdoor fan motor.
• Refrigerant shortage (leak).
• Clogged expansion valve or strainer.
• Faulty outdoor pipe temperature sensor.
• Faulty outdoor main PCB.

Is it safe?

The heat pump stops until the fault clears or is reset.

Resetting itself: unknown.

Manual reset: Clear via control panel per Dimplex article (method shown as image).

What you can check

• Make sure nothing obstructs or restricts air movement around the outdoor unit (official end-user action) and clear debris/snow.
• Until an engineer attends, switch on the electric backup per the Dimplex guide 'How do I switch on the backup on the LA-MI heat pump?' (GDHV help centre).
• Note the code and contact the installer or Dimplex service.

What the engineer will check

• Isolate the heat pump from mains before any work.
• Follow Dimplex flow chart.
• Leak check and refrigerant charge (F-Gas).
• Check fan motor, expansion valve, sensor, PCB.

Who to ask for: refrigeration / heat pump service engineer.

Certification: F-Gas (refrigerant handling) certification for any refrigerant-circuit work.

What you might notice

• Error code on LA-MI display
• No heating

Sources

• GDHV Help Centre article: i have h42 error on my la mi heat pump what does this mean ↗ · GDHV article (modified 2023-02) · page n/a (web article) · Reviewed 23 September 2026

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026