Mitsubishi Electric [L4] fault codewhat it means and how to fix it

L4 is domestic hot water tank overheat protection. The cylinder unit service manual gives the trigger: the tank thermistor, THW5B, reading 75 °C or more for ten consecutive seconds. Mitsubishi’s UK lookup gives the cause as the immersion heater increasing the cylinder temperature too high, or a cylinder thermistor error, and their check as the voltage to the immersion heater and a comparison of the actual cylinder temperature against the thermistor reading in the service menu.

The service manual lists four causes in order: a three-way valve actuator fault, an immersion heater relay fault, a THW5B thermistor fault, and the FTC board; the FTC installation manual condenses that to "check the immersion heater and its contactor". It is worth more attention than a protective cut-out normally gets, because of what the immersion heater is doing in a heat pump system. It is there as a backup and for the periodic legionella cycle, and it heats water the expensive way — direct electric resistance, at roughly three times the running cost of the heat pump doing the same job.

When it runs more than it should, because a relay or contactor has stuck on or because a fault has handed the work to it, the house stays perfectly warm and the electricity bill climbs with nothing obviously broken. Daikin publish the same phenomenon in their own words for the equivalent situation: when the backup or booster heater takes over the heat load, electricity consumption will be considerably higher. So L4 is one of the few heat pump codes where the symptom that should worry you is financial rather than physical. If it has appeared alongside a bill that has jumped, tell the engineer both facts, because together they point somewhere specific.

1. 1

   Look at your recent electricity usage. A step change with no change in habits is the strongest single clue that the immersion has been doing the work.

2. 2

   Check whether anyone has switched the immersion heater on manually — there is often a separate switch near the cylinder, and it gets used during a cold snap and forgotten.

3. 3

   Note whether the hot water is unusually hot at the tap as well. That distinguishes a real overheat from a sensor reading it wrongly.

   Run taps carefully if you suspect the water is over temperature. A cylinder that has genuinely overheated can deliver scalding water for some time afterwards.
4. 4

   Book an engineer and give them both facts: the code, and what has happened to your electricity use. Together they narrow it considerably.