E6 on a Mitsubishi Electric heat pump: Indoor (FTC)/outdoor communication failure - reception error (FTC receives no/incomplete signal for 6 min after power-on or 3 min in operation)
Short answer
On a Mitsubishi Electric heat pump, E6 means: Indoor (FTC)/outdoor communication failure - reception error (FTC receives no/incomplete signal for 6 min after power-on or 3 min in operation). The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the Ecodan
The manual says: Indoor (FTC)/outdoor communication failure - reception error (FTC receives no/incomplete signal for 6 min after power-on or 3 min in operation).
Part involved: FTC-outdoor connecting wire S1/S2/S3.
When it shows: Indoor (FTC)/outdoor communication failure - reception error (FTC receives no/incomplete signal for 6 min after power-on or 3 min in operation).
Likely causes:
- 1 Contact failure / short circuit / miswiring of connecting wire; 2 Outdoor unit transmission/receiving board fault; 3 FTC transmission/receiving board fault; 4 Electrical noise on FTC-outdoor cable.
On the Ecodan Co2 Quhz
The manuals for this range word it in different ways:
- Communication abnormality between cylinder unit and outdoor unit (reception malfunction detected by cylinder unit and outdoor microcomputers).
- Indoor/outdoor communication failure - reception error (FTC receives no/incomplete signal for 5 s after power-on or 3 min in operation).
- Outdoor unit emergency stop - short/disconnection of discharge temperature thermistor TH1.
- Outdoor unit emergency stop - short/disconnection of defrost temperature thermistor TH2.
- Outdoor unit emergency stop - short/disconnection of ambient temperature thermistor TH3.
- Outdoor unit emergency stop - short/disconnection of hot water temperature thermistor TH4.
- Outdoor unit emergency stop - excessive ambient temperature (TH3 detects 50C or higher).
- Outdoor unit emergency stop - disconnection of outdoor inverter PCB temperature thermistor.
- Outdoor unit emergency stop - abnormality of compressor phase current.
- Outdoor unit emergency stop - abnormality of DC (bus) voltage.
- Outdoor unit emergency stop - fan cutoff (fan motor abnormality).
- Outdoor unit emergency stop - no power to outdoor unit / inverter PCB (both LEDs out).
- Outdoor unit emergency stop - overcurrent abnormality (secondary current).
- Outdoor unit emergency stop - overheat of inverter PCB temperature (board thermistor 80C or more for 1 s).
- Outdoor unit emergency stop - primary current abnormality (defective primary current sensor for 1 s).
- Ambient air protection operation in progress.
- Discharge temperature high-temperature protection operation in progress.
- Fan protection stopped (compressor does not stop) - propeller fan drive obstruction, e.g. strong winds.
- PAM control stop in progress (transient/persistent power waveform distortion or noise, or defective inverter PCB).
- Primary current protection in operation (compressor speed does not increase).
- Primary current protection in operation (compressor speed reduced).
- Primary voltage detection error protection (power waveform distortion/noise or defective inverter PCB).
- Secondary stall control in progress.
- Service mode (outdoor unit run with compressor relay connector disconnected).
Part involved: Cylinder unit-outdoor power transmission line; Thermal store FTC-outdoor wiring; TH1 (CN641); TH2 (CN641); TH3 (CN643); TH4 (CN664); Ambient thermistor TH3; Board temperature thermistor; Inverter PCB; Outdoor fan motor; Power supply S1/S2/S3, reactor L61, relay X64, inverter PCB; Inverter PCB / LEV / compressor; Inverter PCB / fan motor; Primary current sensor on inverter PCB; Outdoor inverter PCB protection control.
When it shows: Reception malfunction detected on both microcomputers As description; check LED display on outdoor board Disconnection -5C or less for 1 s (from 10 min after start); short for 1 s Disconnection -47C or less for 1 s at start; short 153C or more for 1 s Disconnection -44C or less for 1 s at start; short for 1 s TH3 >= 50C Disconnection for 1 s from 1 min after start, or short for 1 s Compressor current abnormality detected Bus voltage abnormality detected Fan motor abnormality detected Both outdoor LEDs out Overcurrent detected by secondary current Board thermistor >= 80C for 1 s Defective primary current sensor detected for 1 s Ambient air protection operation in progress Discharge temperature high-temperature protection operation in progress Fan protection stopped (compressor does not stop) - propeller fan drive obstruction, e.g. strong winds PAM control stop in progress (transient/persistent power waveform distortion or noise, or defective inverter PCB) Primary current protection in operation (compressor speed does not increase) Primary current protection in operation (compressor speed reduced) Primary voltage detection error protection (power waveform distortion/noise or defective inverter PCB) Secondary stall control in progress Service mode (outdoor unit run with compressor relay connector disconnected).
Likely causes:
- 1 Defective power transmission line between cylinder and outdoor unit (not connected, disconnection, defective contact); 2 Defective cylinder unit board; 3 Defective outdoor inverter PCB.
- Contact failure/short/miswiring; outdoor or FTC transmission board fault; electrical noise.
- 1 CN641 connection; 2 TH1 open/short; 3 Inverter PCB.
- 1 CN641 connection; 2 TH2 open/short; 3 Inverter PCB.
- 1 CN643 connection; 2 TH3 open/short; 3 Inverter PCB.
- 1 CN664 connection; 2 TH4 open/short; 3 Inverter PCB.
- 1 Environment outside operating temperature; 2 Defective ambient thermistor TH3.
- 1 Defective board temperature thermistor; 2 Defective microcomputer on inverter PCB.
- 1 Defective outdoor inverter PCB.
- 1 Obstacle contacting propeller fan; 2 Defective fan motor (wiring disconnected, locked); 3 Defective inverter PCB.
- 1 No power to outdoor unit (cylinder unit earth leakage breaker off, power line/transmission line not connected); 2 No power to outdoor inverter PCB (loose TB S1/S2/S3, reactor L61 or relay X64 terminals); 3 Defective outdoor inverter PCB.
- 1 Defective inverter PCB; 2 Overcurrent due to LEV malfunction (loose coil, CN724, coil disconnection, LEV unit); 3 Compressor malfunction (locked, out-of-phase, phase interruption); 4 Power waveform distortion/noise.
- 1 Defective fan motor; 2 Defective outdoor inverter PCB.
- Protection control operating temporarily (see description).
Is it safe?
On the Ecodan
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated as auto-resetting; code remains until reset.
Manual reset: Installation manuals: press F4 (RESET) then F3 (Yes) on the main remote controller (older controllers: hold button E for 3 s); 'To cancel error codes please switch system off'. Fix the cause first.
On the Ecodan Co2 Quhz
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated as auto-recovering.
Manual reset: Outdoor error cancelled by switching the cylinder unit earth leakage circuit breaker OFF, waiting until the outdoor board LEDs go out (about 1 minute), then ON again (service manual GHH15010).
What you can check
On the Ecodan
- Note the code, unit and ref. address shown on the main remote controller and contact your installer (Mitsubishi user manuals: 'Make a note of the error code number and contact installer'). Check the heat pump breaker/isolator at the consumer unit is on and read the primary system pressure gauge (if fitted) so you can report it. A single reset from the main remote controller (RESET, then Yes) may be tried; if the code returns, do not keep resetting. Do not open the outdoor unit, cylinder/hydrobox or any casing. A power cycle is a Mitsubishi first step for communication codes: switching the heat pump off and on at its isolator (outdoor and indoor supplies) once is acceptable.
On the Ecodan Co2 Quhz
- Note the code, unit and ref. address shown on the main remote controller and contact your installer (Mitsubishi user manuals: 'Make a note of the error code number and contact installer'). Check the heat pump breaker/isolator at the consumer unit is on and read the primary system pressure gauge (if fitted) so you can report it. A single reset from the main remote controller (RESET, then Yes) may be tried; if the code returns, do not keep resetting. Do not open the outdoor unit, cylinder/hydrobox or any casing. A power cycle is a Mitsubishi first step for communication codes: switching the heat pump off and on at its isolator (outdoor and indoor supplies) once is acceptable.
- Note the code, unit and ref. address shown on the main remote controller and contact your installer (Mitsubishi user manuals: 'Make a note of the error code number and contact installer'). Check the heat pump breaker/isolator at the consumer unit is on and read the primary system pressure gauge (if fitted) so you can report it. A single reset from the main remote controller (RESET, then Yes) may be tried; if the code returns, do not keep resetting. Do not open the outdoor unit, cylinder/hydrobox or any casing.
- Note the code, unit and ref. address shown on the main remote controller and contact your installer (Mitsubishi user manuals: 'Make a note of the error code number and contact installer'). Check the heat pump breaker/isolator at the consumer unit is on and read the primary system pressure gauge (if fitted) so you can report it. A single reset from the main remote controller (RESET, then Yes) may be tried; if the code returns, do not keep resetting. Do not open the outdoor unit, cylinder/hydrobox or any casing. Also check from a distance that the outdoor unit air inlet/outlet is not blocked by snow, ice, leaves or other debris (never reach into the fan).
- Note the code, unit and ref. address shown on the main remote controller and contact your installer. Do not reset repeatedly: this is an electrical/compressor protection code. If the breaker or RCD has tripped, or there is a burning smell, leave the heat pump switched off at its isolator and call an engineer. Do not open any casing.
- Note the code, unit and ref. address shown on the main remote controller and contact your installer (Mitsubishi user manuals: 'Make a note of the error code number and contact installer'). Check the heat pump breaker/isolator at the consumer unit is on and read the primary system pressure gauge (if fitted) so you can report it. A single reset from the main remote controller (RESET, then Yes) may be tried; if the code returns, do not keep resetting. Do not open the outdoor unit, cylinder/hydrobox or any casing. Check that the heat pump's dedicated breaker/isolator (and, where fitted, the separate indoor-unit supply) is switched on; if it trips again, leave it off.
What the engineer will check
On the Ecodan
- Check LED display on outdoor board (A-Control service tool PAC-SK52ST; EA-EC codes); 1 Check connections and cable; 2-4 Power indoor and outdoor off/on; if E6 persists replace FTC and/or outdoor board.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring).
On the Ecodan Co2 Quhz
- 1 Check/connect/replace/correct power transmission line; 2 Replace cylinder unit board; 3 Replace outdoor inverter PCB.
- Check connections and wiring; power off/on; replace FTC and/or outdoor board.
- Connect CN641; check/replace TH1; replace PCB.
- Connect CN641; check/replace TH2; replace PCB.
- Connect CN643; check/replace TH3; replace PCB.
- Connect CN664; check/replace TH4; replace PCB.
- 1 Improve installation environment; 2 Replace TH3.
- Replace outdoor inverter PCB.
- 1 Remove obstacle; 2 Replace fan motor; 3 Replace inverter PCB.
- 1 Check earth leakage breaker, power and transmission line connections; 2 Check TB, L61, X64 connections; 3 Replace inverter PCB. Caution: capacitors C61-C65 may hold >300 V.
- 1 Replace inverter PCB; 2 Check/refit/replace LEV coil or replace outdoor unit; 3 Replace outdoor unit; 4 Check power waveform.
- 1 Check/replace fan motor; 2 Replace inverter PCB.
- No action for temporary protection; for persistent waveform distortion check power supply waveform or replace outdoor inverter PCB; for service mode turn cylinder breaker OFF before reconnecting compressor relay connector.
Who to ask for: Mitsubishi Ecodan-trained heat pump engineer (controls / wiring); Mitsubishi Ecodan-trained heat pump engineer (electrical / inverter diagnostics).
Certification: Competent electrician (BS 7671 / Part P); heat pump inverter experience.
What you might notice
What owners and engineers report
40 owner and engineer reports mention this.
- Controller / thermostat problem on the Ecodan: 14 reports
- No heating on the Ecodan: 9 reports
- No hot water on the Ecodan: 9 reports
- Controller display blank on the Ecodan: 5 reports
- Intermittent fault on the Ecodan: 2 reports
- Outdoor fan not spinning on the Ecodan: 1 report
Sources
- Ecodan EHPT20X-MEHEW Service Manual (OCH814A) ↗ · OCH814A · page 63 (10-4 Self-diagnosis and action) · Reviewed 23 September 2026
- Ecodan QUHZ-W40VA Service Manual (GHH15010) ↗ · GHH15010 · pages 16, 11, 10, 12, 17 · Reviewed 23 September 2026
- Ecodan EHPT20Q VM2EA Service Manual (GCH15010C) ↗ · GCH15010C · page 40 (10-4 Self diagnosis and action) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026