Mitsubishi Electric [U1] fault codewhat it means and how to fix it

U1 on Mitsubishi equipment is a high-pressure fault: the high-pressure switch, 63H, has operated while the compressor was running. Mitsubishi's air-conditioner installation manuals list it as abnormal high pressure, 63H worked, and their public code lookup returns two rows for it, one headed Ecodan high pressure fault and one headed simply high pressure fault, both describing the same event. This page exists because of what the two rows do next. The Ecodan row says the fault is usually caused by abnormal water flow on the primary circuit and sends you to the flow setter, the pumps, the zone valves and air in the water circuit. The second row, despite its plainer heading, does the same: its checks are the flow setter, pump failure, air locks and blockages in the water circulation, and its technical note is about reading the water temperature. Both rows were written for the heating products. There is no split air conditioning row for U1 in the lookup at all, and Mitsubishi's Ecodan service manual opens its U1 cause list the same way, with decreased water flow, a clogged water filter, a dirty plate heat exchanger and a locked pump. A split air conditioner has no water circuit. On that appliance the same switch trips for reasons on the air and refrigerant side, and the checks are in the split's own outdoor unit service manual.

For the Mr Slim PUZ outdoor units Mitsubishi list them in order: short-cycling of the indoor unit, a clogged indoor filter, reduced airflow from a dirty indoor fan, a dirty indoor coil, a locked or faulty indoor fan motor, a stop valve not fully open, a clogged or broken pipe, a locked or faulty outdoor fan motor, short-cycling or a dirty coil at the outdoor unit, an outside temperature thermistor reading low, the high-pressure switch connector and its connection, the outdoor controller board, the expansion valve and the fan drive circuit. Notice that the first four are the indoor unit's airflow in heating mode — in heating the indoor coil is the hot side, so a filthy filter raises the pressure the switch is watching — which is why a clean filter and clear grilles are the one thing an owner can do about this code before anyone attends. The switch itself operates at 4.15 MPa on those units, which is 41.5 bar. So the code did not change meaning between the two products; the machine changed, and the checks with it. Third-party code lists tend to copy one of Mitsubishi's rows and drop its heading, which is how an owner with a wall-mounted split ends up told to look for air in a heating circuit they do not have.

The first thing to establish is therefore not the code but the appliance. An Ecodan is a heating system — it makes hot water for radiators or underfloor heating and usually a cylinder, and it lives alongside a hydrobox indoors. An air conditioner is a split system with an indoor unit on the wall that blows air. They share a manufacturer, a code format, the fault behind this code and a good deal of engineering, and they do not share a check procedure. Daikin's tables behave the same way across their Altherma heat pumps and their air conditioning, with the same strings carrying consistent meanings, and the same rule applies there: the fault travels, the checks do not. There is a second layer to this on Mitsubishi. Two schemes run in parallel: two-character codes for A-Control, Mr Slim and M-Series splits, and four-digit codes for M-NET on City Multi, Lossnay and central controllers. Mitsubishi even publish bridge codes that translate a split's two-character fault onto the M-NET side — there are four-digit codes whose published meaning is, in effect, "the split is reporting E6 or E8". So a four-digit code can be a wrapper around a two-character one, and reading it as a fault in its own right will mislead you.

1. 1

   Establish which product you have before looking anything up. An air conditioner blows air from a wall unit; an Ecodan heats water for radiators and a cylinder. The fault is the same on both; the checks are not.

2. 2

   Find the model number on the indoor and outdoor units and write both down. This is what tells an engineer which code table applies.

3. 3

   Note whether your code is two characters or four digits. Two-character codes belong to the splits, four-digit codes to the larger M-NET systems.

4. 4

   When you use Mitsubishi’s lookup, read the heading and the check procedure on each row as well as the code. For U1 both rows are written for the heating products — if the checks mention flow setters, pumps or air locks, they are not for your air conditioner.

5. 5

   If you have a split, switch it off, clean the indoor filters and make sure nothing is blocking the indoor or outdoor unit. Airflow at the indoor unit heads Mitsubishi’s own cause list for this code on their splits, and it is the only cause you can deal with.

6. 6

   Give whoever attends the code, both model numbers and the product type together. That combination is what makes the code mean something, and a stop valve, the outdoor fan or the pressure switch are their job rather than yours.