Mitsubishi Electric [L3] fault codewhat it means and how to fix it

L3 is the FTC protecting the system because the circulation water got too hot, and Mitsubishi’s explanation is a flow one: the flow rate may be reduced. The cylinder unit service manual gives the trigger, the flow or return water thermistor reading 85 °C or more for ten consecutive seconds. Water that is not moving fast enough through the plate heat exchanger picks up too much heat on each pass, the temperature climbs past that threshold, and the FTC stops the heat pump before the water boils or the exchanger is damaged. Their three published checks in the FTC manual are for water leakage, a strainer blockage and the water circulation pump function, and there is a fourth line in brackets that saves a lot of worry: the error code may display during filling of the primary circuit, and the remedy is to complete the filling and reset the error. That is why L3 turns up on commissioning day and after a repair that drained the system, and why an L3 the day after the installer left is usually a partly filled circuit rather than anything broken.

Mitsubishi’s UK lookup adds the other possibility, that the flow pipe thermistor is reading the wrong value, and sends the engineer to compare its reading in the service menu; the service manual’s longer list runs from insufficient system head and reduced flow, through valve and actuator faults and a stuck booster heater relay, to a detached or faulty thermistor and the board.

Several other codes point back here. L6, circulation water freeze protection, L9, low flow detected by the flow sensor, and P6, anti-freeze protection of the plate heat exchanger, all carry the instruction "See Action for L3", because the same reduced flow produces overheating in one direction and freezing in the other depending on what the heat pump was doing. What reduces flow in a house is mostly ordinary: too many radiator valves shut, an underfloor manifold with every loop closed, a zone valve that has stuck, air in the pump, or a strainer that has collected the debris of an old system. The strainer and the pump are installer work on a pressurised sealed circuit; the valves and the pressure gauge are yours.

1. 1

   Read the pressure gauge on the cylinder unit or hydrobox. If it is below the band marked or below what the installer told you, the circuit has lost water or was never fully filled, which is the case Mitsubishi single out.

2. 2

   Open more of the system. Turn a few radiator valves fully up, check the underfloor manifold has loops open, and make sure any zone valve you can see is not stuck shut.

3. 3

   Look for water where there should not be any: under the cylinder unit, at radiator valves, at the outdoor pipework. A leak is Mitsubishi’s first check.

   Do not open the strainer or bleed the pump yourself. The circuit is pressurised and the water can be very hot.
4. 4

   Reset the code once from the controller. If it returns with the pressure normal and the system open, call your installer to check the strainer and the pump.