There’s a particular moment in every facilities manager’s week when they get a call from reception: “The AC unit’s got a funny code on the display and it’s stopped working.” The code is a string of letters and numbers that means nothing to anyone except the people who fix these things for a living.
Here’s the shortcut. We’ve listed the error codes we see most often on the three brands that dominate London’s commercial buildings — Daikin, Mitsubishi Electric, and Fujitsu. This isn’t every code (each manufacturer has hundreds), but it covers the faults that generate the majority of our callouts.
Before You Read the Codes: The Pattern Matters
If you manage a building with multiple indoor units — ceiling cassettes across an office floor, wall units in a restaurant, ducted systems in a hotel — the pattern of errors tells you more than the code itself.
One unit showing an error, rest working fine. The fault’s on that specific indoor unit. Fan motor, expansion valve, thermistor, drain issue. Lower urgency unless it’s above the server rack or the CEO’s office.
Multiple units showing the same error at the same time. The fault’s almost certainly on the outdoor unit or the shared refrigerant circuit. This is the urgent one — it usually means an entire floor or zone has lost cooling.
Different errors on different units. Could be a communication bus fault rippling through the system, or a power issue causing cascading trips. Note down all the codes and report the lot.
Knowing this distinction before you call us saves time and helps us send the right engineer with the right parts.
Common Daikin Error Codes
We’re a Daikin D1 partner, so we see these daily across London’s offices and restaurants.
E1 — PCB Fault. Internal fault on the circuit board. Worth trying a power cycle first — switch it off at the isolator (not the remote), wait 30 seconds, switch back on. Some transient faults clear themselves. If E1 comes back, the board likely needs replacing. On a VRV system, an E1 on one indoor unit won’t affect the rest.
E7 — Fan Motor. The indoor fan has failed or isn’t responding. The unit won’t run because it can’t circulate air. In a building with 20 cassettes, one dead fan motor is an inconvenience, not an emergency — unless it’s above the boardroom during a client meeting.
U0 — Low Refrigerant. The system’s detected low pressure, which almost always means a leak. Do not keep resetting the unit. Running a system with low gas damages the compressor, and compressor replacement costs ten times what a leak repair costs. On VRV systems, a U0 on the outdoor unit will affect every indoor unit connected to it.
U4 — Communication Error. Indoor and outdoor units aren’t talking to each other. Damaged signal cable, loose connection, or a PCB fault. On VRV systems where the communication bus is daisy-chained through multiple indoor units, a single break can take out every unit downstream. We had a case in a Shoreditch office where a data cabling contractor had cut through the Daikin comms wire while running CAT6. Took out AC to the whole floor.
A3 — Condensate Drain Issue. The float switch has detected that water isn’t draining away. The unit shuts down to stop water pouring through the ceiling — which is better than the alternative, but still leaves a zone without cooling. This is the most preventable fault on the list: a routine drain flush during a PPM visit stops it happening entirely.
Common Mitsubishi Electric Error Codes
E6 — Indoor/Outdoor Communication Error. Same story as Daikin’s U4 — the units have lost communication. On City Multi VRF systems, this can cascade across multiple indoor units if the main bus is disrupted.
E9 — Serial Communication Fault. The wired wall controller can’t talk to the indoor unit. Usually a wiring problem — damaged cable, reversed polarity, or water ingress into the controller’s terminal block. We see this a lot after office refurbishments where ceiling work has disturbed the cabling.
P8 — Pipe Temperature Sensor. A thermistor (small temperature sensor clipped to the refrigerant pipe) is reading out of range or has failed. Without it, the system can’t regulate itself properly. Quick fix — clip on a new one. No need to touch the refrigerant circuit.
Common Fujitsu Error Codes
Fujitsu units — especially the older ones — tend to use LED flash patterns rather than alphanumeric codes. Newer Airstage models show codes on the wired controller.
Operation and Timer LEDs flashing together. Both the green (operation) and orange (timer) LEDs flashing simultaneously usually means an outdoor unit fault. The specific flash count tells us which fault, but from your end, the message is: this isn’t going to clear itself with a reset.
E1:00 or EE:EE. Significant system fault — communication failure between indoor and outdoor, or a major component failure on the outdoor unit. The system won’t run until an engineer’s diagnosed and repaired it.
What to Do When You See a Code
Try a power cycle. Isolator off, 30 seconds, back on. Not the remote — the actual isolator switch. Voltage spikes and transient sensor glitches can throw a code that clears on restart.
If the code comes back:
- Photograph the display. The exact code, including any sub-codes. This saves us 15 minutes of on-site diagnosis.
- Walk the floor. Check every unit you can see. How many are affected? This one piece of information tells us more than anything else about whether it’s a local or system-wide fault.
- Check the outdoor unit if you can access it. Fan running? Compressor humming? Error code on the outdoor display panel?
- Don’t keep resetting it. If the same code keeps coming back, the system’s trying to protect itself. Overriding that protection — or repeatedly cycling the power — can turn a £300 repair into a £3,000 one.
Most of These Are Preventable
This is the part nobody wants to hear, but it’s true: most of the error codes we clear on emergency callouts could have been caught during a routine service visit.
- A3 condensate faults — prevented by drain flushing every visit
- E7 fan motor faults — early signs (bearing noise, high current draw) are detectable during inspection
- U0 low refrigerant — pressure checks catch slow leaks months before the system trips
- Icing and airflow faults — regular filter cleaning prevents the restriction that causes them
A structured maintenance programme across all units in a building catches these systematically, rather than waiting for a code to appear and a zone to go offline.
Need an Engineer?
If your AC system’s showing an error code and you need it diagnosed, call us on 020 3974 1419. We work on Daikin, Mitsubishi, Fujitsu, Samsung, and all other commercial split and VRF systems across London. Daikin D1 partner with direct access to their technical support line and parts supply.