Vaillant ecoMAX 600 and 800 series fault codes

Built 2000–2005

The condensing boilers before the ecoTEC. F.0 to F.67 on a two-character display; F.25 is a water switch fault, F.27 is "no demand to gas valve", F.60 and F.61 are the two sides of the gas valve control, and the reset is a one-second press with a three-attempt limit.

Codes on the ecoMAX 600 and 800 series follow the same Vaillant scheme as the rest of the range. Where a code behaves differently on this generation, the code page says so.

Vaillant fault codes

Every published Vaillant code. Open a code to see which models and years it applies to.

F.22The boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Get it checked
F.75The boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Get it checked
F.28The boiler tried to light and failed every attempt it is allowed — three on an ecoTEC, five on the 2022 CS/1-5 — so it has shut the gas off and locked out.Stop — call an engineer
F.29The boiler lit correctly and then lost the flame while it was running. It has shut the gas off and locked out, and it needs an engineer.Stop — call an engineer
F.20The overheat thermostat has tripped because the boiler got too hot. It has shut down to protect itself and it should not be restarted repeatedly.Stop — call an engineer
F.32The fan is not running at the speed the boiler expects, so combustion cannot be verified as safe and the boiler has locked out.Get it checked
F.23Vaillant call this a safety shutdown for temperature spread too great. It nearly always means the pump is not moving enough water.Get it checked
S.53Not a fault. Your boiler is in a deliberate waiting period because the gap between flow and return temperatures is too wide, which points at poor circulation.Get it checked
F.62The gas valve is not shutting off when the boiler tells it to. This is a gas safety fault and the boiler must not be used until an engineer has seen it.Stop — call an engineer
F.35The air intake or flue is blocked. Something is physically obstructing the path combustion products take out of the building.Get it checked
F.73The pressure sensor signal is out of range at the low end. Usually the sensor or its wiring, rather than the pressure itself.Get it checked
F.83The burner lit but neither sensor saw the temperature rise it should have. Either water is not moving through the boiler, or a sensor is misplaced.Get it checked
F.24Temperature is rising faster than it should when the burner fires. That is a circulation problem caught a step earlier than an overheat.Get it checked
F.0The flow temperature sensor circuit is broken. The boiler cannot read how hot the water leaving it is, so it has stopped rather than fire blind.Get it checked
F.61The gas valve control circuit has faulted. The boiler will not fire and this is firmly an engineer's job.Get it checked
F.63Vaillant reports an EEPROM error, meaning the boiler cannot reliably read its own stored settings. The printed circuit board is at fault and needs replacing.Get it checked
F.27The boiler is detecting a flame when the burner should be off. Switch it off and get a Gas Safe engineer out rather than resetting it.Stop — call an engineer
F.53The gas is not flowing through the valve the way the boiler expects. Vaillant's list runs from low gas pressure to a wet or blocked filter under the venturi cap.Get it checked
F.74The water pressure sensor is sending a signal that is too high to be genuine. This is a wiring or sensor fault, not high system pressure.Get it checked
F.54A gas supply fault, usually shown alongside F.28 or F.29. It means gas is reaching the appliance at too low a pressure.Get it checked
F.77The condensate pump or the flue non-return flap has not responded. The boiler cannot clear its condensate or confirm the flue, so it has shut down.Get it checked
F.72The flow and return sensors are reporting a temperature difference that cannot be real. The boiler is blaming the sensors themselves rather than the water.Get it checked
S.08Not a fault. S.08 is the burner anti-cycling time: the boiler has just finished a heating burn and is deliberately waiting, up to about twenty minutes, before it is allowed to relight.Usually simple to fix
F.70The boiler does not know which model it is. The device specific number is missing or wrong, which nearly always follows a board or display replacement.Get it checked
S.76A service message, not a lockout: the heating water pressure has dropped and Vaillant want it topped up. The boiler is still running, but below 0.5 bar it will stop with F.22.Get it checked
F.25On a current ecoTEC this is a flue gas temperature safety shutdown. The same number meant a cable break on the 2009 ecoTEC and a water switch fault on the ecoMAX.Get it checked
S.31Not a fault. S.31 is summer mode: heating has been switched off at the boiler while hot water carries on. If you want the radiators back, turn the heating flow temperature up from "Heating off".Usually simple to fix
F.1The return temperature sensor circuit is open: the boiler is getting no reading from the sensor on the pipe bringing water back in, so it has stopped.Get it checked
S.30Not a fault. S.30 means the room thermostat or programmer is not asking for heat, so the boiler is idle on purpose. If the house is cold, look at the thermostat, not the boiler.Usually simple to fix
F.10The flow temperature sensor has short-circuited. The boiler is reading an impossible value from the sensor on its outlet pipe and will not fire on it.Get it checked
F.76The overheating protection built into the primary heat exchanger has responded. A thermal fuse on the heat cell, or its wiring, has opened, and the boiler has stopped.Get it checked
F.11The return temperature sensor has short-circuited. Vaillant treat it as a wiring or sensor fault, and the boiler stays off until the circuit reads correctly again.Get it checked
S.34Not a fault. S.34 is frost protection: the boiler has fired by itself because its flow temperature dropped below 5 °C, and it will warm the water to about 30 °C and stop.Usually simple to fix
F.68The flame is lighting but its signal is unstable. Vaillant's causes run through the gas supply, the air ratio, the condensate route and the flue, ending with the electrode and the board.Get it checked
F.49The voltage on the eBUS, the two-wire link between the boiler and its Vaillant controls, has dropped too low. Usually a short or a wiring clash on that bus, not a boiler fault.Get it checked
F.84The flow and return sensors disagree in a way that cannot be true — the return reading warmer than the flow. Vaillant's first explanation is that the two sensors have been swapped.Get it checked
F.64An electronics or sensor fault: the board cannot make sense of a safety-relevant sensor signal. Vaillant's causes are a shorted flow or return sensor or a defective board.Get it checked
S.41A status, not a lockout: the heating water pressure is too high, above about 2.8 bar. Usually an over-filled system or a tired expansion vessel, and the safety valve will discharge above 3 bar.Get it checked
F.71The flow sensor is returning a value the boiler does not believe — typically one that never changes. Vaillant's first suspect is a sensor that has slipped off the pipe.Get it checked
F.65The electronics have got too hot. Vaillant say either something outside the boiler is heating the board, or the board itself is faulty; the boiler stops until it cools.Get it checked
F.26Vaillant call this "fuel valve not working": the stepper motor that drives the gas valve is not responding, so the boiler cannot open the gas and will not light.Get it checked
F.13The cylinder or warm-start temperature sensor has short-circuited. On a system boiler that is the sensor in your hot water cylinder; on a combi it is the warm-start sensor.Get it checked
F.67The flame signal the board is receiving is not believable. Vaillant treat F.67 as an electronics fault in the flame-monitoring circuit rather than a real flame problem.Get it checked
S.32A protective wait, not a lockout: the fan speed has strayed outside tolerance and the boiler is holding off before it tries to start again. If the wait keeps recurring, F.32 is next.Get it checked
F.33An air pressure switch fault: the boiler cannot prove the fan is moving air through the flue. Vaillant check the flue route first, so treat it as a flue fault until proved otherwise.Stop — call an engineer
F.85Vaillant say the flow and return sensors have been installed on the same pipe, or on the wrong pipe. The boiler will not run until they are where it expects them.Get it checked
S.40Not a lockout, but not normal either: comfort protection mode. The boiler has detected a fault and is running on with reduced comfort. Its fault memory will say what it found.Get it checked
conThe display has lost contact with the main board. "con" on a Vaillant is a communication fault between the user interface and the PCB in the electronics box, not a fault in the heating itself.Get it checked
F.57The combustion regulation's measuring programme has found a fault — on the exclusive, a badly corroded ignition electrode; on the 2022 CS/1-5, F.057 means the regulation and its fallback mode have both failed.Get it checked
F.42A fault with the coding resistor, the small component that tells the board which gas the boiler is set up for. The boiler will not run until it knows.Get it checked
F.88The board cannot see the gas valve. Vaillant print F.88 as the valve not connected or connected incorrectly, or a harness fault, and the boiler will not run until the circuit is proved.Get it checked
S.39A protective wait, not a lockout: the "burner off" contact has opened — an underfloor thermostat or a condensate pump alarm wired to the boiler — and heating is blocked until it closes again.Get it checked
S.88Not a fault. S.88 means the purging programme is running: the pump is cycling to push air out of the boiler, 7.5 minutes per circuit, and the burner stays off until it finishes.Usually simple to fix
F.78A wiring-configuration code on UK combis: the external control's link box is connected but the hot water sensor input has not been bridged. Vaillant say the boiler itself has not malfunctioned.Get it checked
F.97Vaillant's main PCB has failed the check it runs on itself. The table gives one cause, "main PCB defective", and one remedy: replace the PCB.Get it checked
F.12The hot water outlet sensor has short-circuited. A combi or storage-combi code: heating may continue, but the boiler cannot control the hot water temperature.Get it checked
F.3The cylinder temperature sensor circuit is open. The boiler has no reading from the probe in the hot water cylinder, so it cannot tell when to charge it and will not try.Get it checked
F.347A 2022 ecoTEC plus has detected condensate inside its combustion chamber and stopped the burner. Vaillant's cause is a blocked condensate discharge — the siphon or the pipe — backing up into the boiler.Get it checked
F.87The ignition transformer, or on the ecoFIT pure the electrodes, are not connected as the board expects. A wiring fault on the ignition side; the boiler will not attempt to light.Get it checked
S.42A protective wait, not a lockout: an external device — a flue non-return flap through the VR40 module, or a condensate pump — is telling the boiler not to fire. Heating waits until it clears.Get it checked
F.2The hot water outlet sensor circuit is open. On a combi that is the sensor on the water leaving for your taps; on an ecoTEC plus storage combi it is the shift-load cylinder sensor.Get it checked
F.325The heating pump in a 2022 ecoTEC plus has reported a fault — blocked, or defective. F.320, F.321, F.324 and F.330 are its neighbours in the same pump family, and this page covers them too.Get it checked
F.89The pump is not connected, is connected incorrectly, or is not the pump the boiler expects. On the 2022 CS/1-5, F.089 means the fitted pump does not match the unit type.Get it checked
F.195A 2022 ecoTEC plus code: the boiler has measured a significant undervoltage in its mains supply. F.196 is the overvoltage twin. Vaillant's first instruction is to check the mains, not the boiler.Get it checked
F.323The heating pump in a 2022 ecoTEC plus is dry running — turning with no water in it. Vaillant name air in the boiler as the cause; a pump run dry may need replacing.Get it checked
F.56A safety shutdown because the boiler's combustion regulation measured carbon monoxide in the flue gas above its limit. Vaillant say that if it comes back after a reset, the gas valve is defective.Stop — call an engineer
F.93Vaillant print F.93 as "check gas route": combustion quality is outside the permitted range. The causes are the gas injector, the gas group, flue gas recirculation and a blocked pressure point in the venturi.Get it checked
S.96Not a fault. S.96, S.97 and S.98 are the boiler testing its own sensors — return, water pressure, flow and return — with heating demands blocked for the few seconds it takes.Usually simple to fix
F.322The heating pump in a 2022 ecoTEC plus has overheated, and running it more gently did not bring the temperature down. Vaillant's remedy is to check and if necessary replace the pump.Get it checked
F.334A commissioning code on the 2022 ecoTEC plus: the flue system configuration was never completed in the installer settings. F.336, F.337 and F.338 are its neighbours — a configuration set wrongly.Get it checked
F.55The CO sensor — Vaillant's "all-gas sensor" that lets the ecoTEC exclusive regulate its own combustion — is defective or short-circuited. The boiler will not run without it.Get it checked
F.90Communication with the actoSTOR hot water module — or on the 2022 CS/1-5 the internal cylinder — has been interrupted. A wiring fault on the link, or a setting left on after the module went.Get it checked
F.94The vortex and differential pressure sensors — the two that measure flow through the heat exchanger on an ecoTEC plus or exclusive — are returning values the board does not believe.Get it checked
F.342The 2022 ecoTEC plus measures the air its fan moves, and F.342 means that air-mass flow is outside the permitted limits. Vaillant's list runs from fan speed and plugs to a blocked flue.Get it checked
F.86An ecoFIT code: the underfloor heating contact is open. A floor-protection thermostat wired to the boiler has tripped, or its sensor is disconnected, and heating is blocked until it closes.Get it checked
F.91On an ecoTEC plus or exclusive, a sensor on the actoSTOR module is defective. On an ecoFIT, F.91 is the cylinder temperature sensor short-circuited. Two different sensors under one number.Get it checked
F.92The gas type the boiler has been told about does not match the coding resistor on its board. A commissioning or conversion fault; the boiler will not run on a gas it cannot confirm.Get it checked
F.343The air-mass flow sensor on a 2022 ecoTEC plus is giving a signal the board does not believe. Vaillant's causes are its plugs and harness, a dirty or failed sensor, or the PCB.Get it checked
F.60On an ecoMAX or the first 2004 ecoTEC, the positive side of the gas valve control has faulted — loom, valve coils or board. On a turboMAX plus, F.60 is simply "electronic fault".Get it checked
F.37A turboMAX plus 837 or thermoCOMPACT 637 code only: the fan is turning too fast or too slow for the board, and Vaillant blame the air pressure switch, the fan or the electronics.Get it checked
F.41A turboMAX plus 837 or thermoCOMPACT 637 code only: the board has no gas type stored, usually after a PCB change, and an engineer must set natural gas or LPG in the P programmes.Get it checked
F.66A turboMAX plus and thermoCOMPACT ../2 E code only: the multifunction display has faulted, or lost its connection to the board. The boiler may still be running normally behind it.Get it checked

Other Vaillant models

ecoFIT pure and ecoFIT sustain
ecoMAX pro 18E and 28E
ecoTEC exclusive and exclusive Green
ecoTEC plus ..6/5-5 and 48–64 kW
ecoTEC plus 612–837 and pro 24/28
ecoTEC plus CS/1-5 (three-digit display)
ecoTEC pro ..6/5-3
ecoTEC sustain
thermoCOMPACT 142–282 /1 E and turboMAX pro
turboMAX plus 824–837 E and thermoCOMPACT 615–637 ../2 E