L6 and P6 on a Mitsubishi Electric Ecodan both mean the system has stepped in to stop water freezing inside it. L6 protects the circulating water, and P6 protects the plate heat exchanger, where heat passes from the refrigerant into your heating water.
A single code on a bitter night can clear once flow and temperatures recover. If it comes back, book a heat pump repair so an engineer can find the cause before anything freezes for real.
This guide covers the two freeze protection codes in detail. For every other Ecodan code, see our full Mitsubishi Ecodan heat pump error codes guide.
Please read first: this guide is here to help you understand what your Mitsubishi Electric Ecodan heat pump is telling you. Every effort has been made to keep the information accurate, but manufacturers update their manuals and codes can differ between models and software versions, so always check the official Mitsubishi Electric Ecodan manual for changes before acting on anything here. Do not attempt anything that could be dangerous, and avoid any work that could void your warranty. If in doubt, call a qualified heat pump engineer.
What does L6 or P6 mean on a Mitsubishi Ecodan heat pump?
L6 means circulation water freeze protection has been triggered: the water moving around the primary circuit has become cold enough that the FTC controller has acted to protect it. The Ecodan FTC6 installation manual gives the same checks as for L3, which are water leakage, a blocked strainer and whether the circulation pump is working.
P6 means anti-freeze protection of the plate heat exchanger has been triggered. The manual's checks are the same water-side checks as L3, plus the refrigerant charge.
In plain terms, both codes say the water is not moving heat through the system well enough on a cold day. P6 adds the possibility of a refrigerant problem, which only an F-Gas certified engineer can check.
Is ice on a heat pump normal?
Yes, some frost on the outdoor unit's coil is normal in cold, damp weather, and the heat pump clears it with a defrost cycle. During a defrost the fan may stop and you may see steam rising from the unit for a few minutes.
It is not normal for ice to build into a solid block, for the defrost never to finish or for L6 or P6 to appear. Our guide to the heat pump defrost cycle explains what a healthy defrost looks like, and our page on a heat pump frozen outside unit covers heavier icing.
Why does L6 or P6 appear in cold weather?
L6 and P6 appear in cold weather because low outdoor temperatures push the water and heat exchanger closer to freezing, so any weakness in flow shows up fast. The manual lists the checks, and our engineers see several underlying causes behind them.
Causes from the Ecodan FTC6 manual:
- Water leakage from the primary circuit (L6, P6)
- A blocked strainer (L6, P6)
- A circulation pump that is not working properly (L6, P6)
- Low refrigerant charge (P6)
- L6 repeating after cooling mode because the freeze stat temperature is set too low (L6)
Other causes (general engineering knowledge, not from the manual):
- Low flow from sludge, magnetite or air locks, which may also raise L9 (low primary circuit flow)
- Low system pressure, often after a leak or a failing expansion vessel
- A frozen or blocked condensate drain, so meltwater refreezes under the unit
- Snow, leaves or debris blocking airflow through the outdoor coil
- The unit sitting too low, so snow and meltwater build up around its base
- A failing defrost sensor or valve in the outdoor unit
If the pressure gauge keeps dropping as well, read our guide to a heat pump losing pressure.
What can you safely check yourself?
You can safely make a few outside checks without opening either unit. Work through them in order.
- Keep the power on. Freeze protection only works with power, so do not switch the heat pump off at the isolator in freezing weather.
- Clear around the unit. From outside, brush snow, leaves and debris away from the sides, top and base of the outdoor unit so air can flow and meltwater can drain.
- Check the condensate drain. Make sure the drain path under the unit is clear and not frozen. If it is iced, thaw it only with lukewarm water, never boiling.
- Check pressure and valves. Read the pressure gauge and make sure any heating valves you closed are open again, as low pressure and closed valves both cut flow.
- Check settings, not the switch. If you are going away, use holiday or frost settings rather than turning the system off. Our guide to heat pump holiday settings explains how.
- Reset once. Note the code, then press F4 (RESET) and F3 (Yes) on the main remote controller. Leave the power on throughout.
What not to do:
- Never chip, scrape or prise ice off the coil, as the fins and pipes damage easily.
- Never pour boiling water or use a heat gun on the coil.
- Never remove the panels on the indoor or outdoor unit.
- Never touch the refrigerant circuit. That work is restricted to F-Gas certified engineers.
When should you call an engineer?
Call an engineer if L6 or P6 returns after one reset, or appears alongside L9 or an outdoor unit code. Book a visit straight away if:
- The house is losing heat or hot water has stopped
- P6 appears, as the manual links it to the refrigerant charge
- L6 keeps coming back after the system has run in cooling mode
- Water is leaking, or the pressure gauge keeps falling
- Ice stays on the outdoor coil for hours
Checking the strainer, pump and freeze stat settings is an engineer's job. Anything involving the refrigerant circuit needs an F-Gas certified engineer by law.
How can you stop it happening again?
The best way to stop L6 and P6 coming back is to keep water flowing freely through the primary circuit and keep the outdoor unit clear. These steps are general engineering knowledge rather than Mitsubishi instructions.
- Annual service. An engineer cleans the strainer, checks the pump, pressure and freeze settings, and inspects the outdoor coil. Our heat pump annual service checklist shows what is covered.
- Clean filters. A magnetic filter or strainer that is cleaned every year keeps sludge and magnetite from cutting flow. Badly sludged systems may need a heat pump powerflush.
- Freeze protection. Where fitted, antifreeze or freeze protection valves should be checked, and the inhibitor level kept correct.
- Insulate pipes. Insulate the outdoor flow and return pipes fully.
- Raise and drain. Keep the unit on raised feet with a clear path for meltwater to drain away.
- Location. Keep the unit clear of plants, fences and drifting snow, and away from dripping gutters.
Our heat pump winter tips cover more ways to prepare before the first frost.
Other Ecodan codes you might see in a cold snap
Cold weather can trigger other Ecodan codes alongside or instead of L6 and P6. Each one is explained in our Ecodan error code guide.
| Code | What it means | Cold weather link |
|---|---|---|
| L9 | Low primary circuit flow rate | Low flow is a common trigger for freeze protection |
| LP | Flow rate outside the outdoor unit's range | Restricted flow on the water side |
| LB | High pressure protection | Reduced flow or a clogged plate heat exchanger |
| E6 to EF | FTC to outdoor unit communication failure | Outdoor unit switched off at the isolator |
| U, F or A codes | Outdoor unit failure | Faults in the outdoor unit itself |
Need help with a frozen Mitsubishi Ecodan heat pump?
We fix frozen and iced-up Mitsubishi Electric heat pumps across the UK, including Ecodan systems of every age. Our F-Gas certified engineers can check flow, the strainer, the pump and the refrigerant side in one visit, and our repairs carry a 12-month guarantee.
If you have lost heating or hot water in a cold snap, our emergency heat pump repair team aims for same-day visits where possible. For a code that keeps returning, book a standard heat pump repair.
Regular heat pump servicing catches blocked strainers and low pressure before winter, and our cover plans spread the cost. Call 0333 041 5130 or contact us online.
Frequently asked questions
Will my Ecodan still heat the house with L6 showing?
It may run in a limited way or stop until the fault clears, depending on the cause. Freeze protection is there to protect the system first, so heating can drop while it is active. If the code keeps returning, get the cause fixed rather than living with it.
Can I change the Ecodan freeze stat setting myself?
The manual's fix for L6 repeating after cooling mode is to adjust the temperature at which the freeze stat function starts. This is a service setting, so ask an engineer to check and change it rather than guessing a new value.
Is it normal for my Ecodan to go straight to hot water after a power cut?
Yes, the manual lists returning to hot water mode after a power failure as normal operation. The display may also show "Please Wait" for a few minutes while it restarts.
Why does my heat pump struggle more on damp days than dry, frosty ones?
Damp air near freezing carries more moisture, so frost builds on the coil faster and the unit has to defrost more often. Our guide to running a heat pump in cold weather explains how performance changes as temperatures drop.
What if the heat pump shows no code but still is not heating?
Not every problem raises a code. Our guide to common air source heat pump problems covers other causes and checks.
