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Heat pumps · Error codes · Mitsubishi Electric
L9Mitsubishi Electric

L9 on a Mitsubishi Electric heat pump: Low primary circuit flow rate detected by flow switch

Short answer

On a Mitsubishi Electric heat pump, L9 means: Low primary circuit flow rate detected by flow switch. 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 · 8 sources

What it means

On the Ecodan

The manuals for this range word it in different ways:

  • Low primary circuit flow rate detected by flow switch.
  • Low primary circuit flow rate detected by flow switch (flow switches 1, 2, 3).
  • Low primary circuit flow rate detected by flow sensor or flow switch (flow switches 1, 2, 3).

Part involved: Flow switch (IN3); Flow switches 1 (heat source), 2 (zone 1, IN3), 3 (zone 2, IN7); Flow sensor (heat source side) / flow switches 2 (zone 1) and 3 (zone 2).

When it shows: Flow switch (IN3) detects low flow (LED4 lit, LED5 off) Flow sensor/switch detects low flow for 10 s (ignored for 1 min after pump 1 starts); request code 569 shows 1 = heat source side, 2 = zone 1, 3 = zone 2.

Likely causes:

  • 1 Insufficient system head (pump undersized); 2 Reduced flow in primary water circuit (faulty pump, insufficient air purge, blocked strainer/magnetic filter, leak in water circuit); 3 Valve operation fault (valves not installed level); 4 2-way valve (local supply) actuator fault; 5 3-way valve actuator fault; 6 Connector/terminal wire detached (CN1A / CN2F, IN2, IN3, IN7); 7 Flow sensor or flow switch fault; 8 Incorrect SW2-2 / SW3-2 / SW3-3 setting; 9 FTC board failure.

On the Ecodan Co2 Quhz

The manual says: Low primary circuit (heat source side) flow rate detected by flow sensor (flow sensors 1, B).

Part involved: Flow sensor 1 (CN1A) / flow sensor B (CN2B).

When it shows: Flow sensor detects low flow for 10 s (request code 569 = 4 for sensor 1, 5 for sensor B).

Likely causes:

  • Reduced flow in primary water circuit (faulty pump, insufficient air purge, blocked strainer, leak); valve operation fault; 2-way (local) / 3-way valve actuator fault; connector CN1A/CN2B detached; flow sensor 1/B fault; FTC board failure.

On the Ecodan Hydrodan

The manual says: Low primary circuit flow rate detected by flow sensor or flow switch (flow switches 1, 2, 3).

Part involved: Flow sensor (heat source side) / flow switches 2 (zone 1) and 3 (zone 2).

When it shows: Flow sensor/switch detects low flow for 10 s (ignored for 1 min after pump 1 starts); request code 569 shows 1 = heat source side, 2 = zone 1, 3 = zone 2.

Likely causes:

  • 1 Insufficient system head (pump undersized); 2 Reduced flow in primary water circuit (faulty pump, insufficient air purge, blocked strainer/magnetic filter, leak in water circuit); 3 Valve operation fault (valves not installed level); 4 2-way valve (local supply) actuator fault; 5 3-way valve actuator fault; 6 Connector/terminal wire detached (CN1A / CN2F, IN2, IN3, IN7); 7 Flow sensor or flow switch fault; 8 Incorrect SW2-2 / SW3-2 / SW3-3 setting; 9 FTC board failure.

Is it safe?

On the Ecodan

Does the heat pump stop?Yes
SeverityFault — 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
SeverityFault — 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: Press F4 (Reset) then F3 (Yes) on the main remote controller, or switch system off (installation manual).

On the Ecodan Hydrodan

Does the heat pump stop?Yes
SeverityFault — 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.

What you can check

  • 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 the primary circuit pressure gauge (typically about 1-2 bar when cold on sealed systems) and look for visible leaks; do not top up or bleed unless your installer has shown you how and the filling loop is then closed.

What the engineer will check

On the Ecodan

  • See action for L3 (water leakage, strainer blockage, pump function); if the flow switch itself does not work, replace it; check SW3-6 flow switch logic.
  • 1 Check pump against required head, add/replace pump if needed; 2 Check circulation pump, open purge valve to remove trapped air, check strainer for blockage, check primary circuit for leaks, check flow rate within recommended range; 3 Check valves installed level; 4-5 Electrically test valve actuators, operate 3-way valve manually from service menu, replace coil/valve; 6 Check connectors/terminals; 7 Electrically test flow sensor/switch, replace if faulty; 8 Check DIP switch setting; 9 Replace board. Caution: pump valves may be hot.

Who to ask for: Heat pump installer / heating engineer (hydraulic circuit); Mitsubishi Ecodan-trained engineer for FTC diagnostics.

On the Ecodan Co2 Quhz

  • Check pump, bleed air, strainer, leaks, flow; check valves; reattach connector; test/replace flow sensor; replace board. Install manual: run pump A and pump 1 in manual operation to flush debris on the flow sensor.

Who to ask for: Heat pump installer / heating engineer (hydraulic circuit).

On the Ecodan Hydrodan

  • 1 Check pump against required head, add/replace pump if needed; 2 Check circulation pump, open purge valve to remove trapped air, check strainer for blockage, check primary circuit for leaks, check flow rate within recommended range; 3 Check valves installed level; 4-5 Electrically test valve actuators, operate 3-way valve manually from service menu, replace coil/valve; 6 Check connectors/terminals; 7 Electrically test flow sensor/switch, replace if faulty; 8 Check DIP switch setting; 9 Replace board. Caution: pump valves may be hot.

Who to ask for: Heat pump installer / heating engineer (hydraulic circuit); Mitsubishi Ecodan-trained engineer for FTC diagnostics.

What you might notice

What owners and engineers report

23 owner and engineer reports mention this.

  • No heating on the Ecodan: 9 reports
  • No hot water on the Ecodan: 9 reports
  • Gurgling or rushing water noise on the Ecodan: 2 reports
  • Pressure keeps dropping on the Ecodan: 2 reports
  • Radiators cold at top / air in system on the Ecodan: 1 report

Sources

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026