Vaillant ecoTEC Fault Codes
Complete Diagnostic Reference Guide
When you’re cold and the boiler isn’t working, you need answers quickly. We’ve compiled this directory Vaillant ecoTEC fault codes as a fast, searchable reference to help you identify exactly what your boiler’s display is trying to tell you.
Crucial Safety Notice: This guide is designed for diagnostic reference. Modern condensing boilers are complex, pressurised systems mixing water, electricity, and combustible gas. Identifying a fault code is the first step; safely rectifying it is the second. Never attempt to remove the boiler casing or work on gas-carrying components yourself. Always use a Gas Safe registered engineer.
Is your Vaillant ecoTEC broken down? > If you are based in North East Lincolnshire and need your heating back online today, bypass the call centres. Book a Vaillant boiler repair directly with your local Gas Safe engineer. Call 07825 121000 for an immediate response across Grimsby, Cleethorpes, Louth, and surrounding areas.
Vaillant ecoTEC Fault Code Directory
| Code | Diagnostic Meaning | Primary Causes |
| F.00 / F00 | Flow temperature sensor interruption | Loose NTC plug, incorrect PCB connection, defective NTC, or cable harness break. |
| F.01 / F01 | Return temperature sensor interruption | Loose NTC plug, incorrect PCB connection, defective NTC, or cable harness break. |
| F.02 / F02 | Interruption cylinder charging sensor actoSTOR (NTC) | Defective NTC, damaged cable, or faulty plug connections (only in combination with F.91). |
| F.03 / F03 | Interruption cylinder sensor actoSTOR (NTC) | Defective NTC, damaged cable, or faulty connections to the actoSTOR electronics (with F.91). |
| F.10 / F10 | Flow NTC short circuit | Defective NTC or a short circuit in the cable harness/casing. |
| F.11 / F11 | Return NTC short circuit | Defective NTC or a short circuit in the cable harness/casing. |
| F.12 / F12 | Short circuit at cylinder charging sensor (NTC) | Defective NTC or cable short circuit (only in combination with F.91). |
| F.13 / F13 | Short circuit warm start/cylinder sensor | Defective NTC or short circuit in the cable harness. |
| F.20 / F20 | Safety switch-off: Temperature limiter | Incorrect earth connection, defective flow/return NTC, or discharge via ignition components. |
| F.22 / F22 | Safety switch-off: Low water pressure | System pressure too low, defective water pressure sensor, or damaged sensor cabling. |
| F.23 / F23 | Safety switch-off: Temperature difference too great | Blocked pump, poor pump output, air in the system, or interchanged flow/return NTCs. |
| F.24 / F24 | Safety switch-off: Temperature rise too fast | Blocked/failing pump, air in the boiler, low system pressure, or jammed non-return valve. |
| F.25 / F25 | Safety switch-off: Flue gas temperature too high | Break in the plug connection for the flue gas safety thermostat (STB) or cable harness. |
| F.26 / F26 | Gas valve without function | Stepper motor disconnected/defective, incorrect PCB connection, or defective electronics. |
| F.27 / F27 | Safety switch-off: Flame simulation | Moisture on the PCB, defective flame monitor electronics, or a leaking gas solenoid valve. |
| F.28 / F28 | Failure during start-up: Ignition unsuccessful | Defective gas meter/regulator, air in gas, low pressure, blocked condensate, ignition system failure, or interrupted ionisation current. |
| F.29 / F29 | Failure during operation: Re-ignition unsuccessful | Gas supply interruption, flue gas recirculation, blocked condensate duct, or faulty earthing. |
| F.32 / F32 | Fan fault | Disconnected plug, fan blockage, defective Hall sensor, or PCB failure. |
| F.35 / F35 | Air/flue gas duct fault | Blockage within the air or flue gas duct. |
| F.42 / F42 | Coding resistor fault | Short circuit or interruption in the coding resistor output range. |
| F.49 / F49 | eBUS fault | Short circuit on the eBUS or conflicting power supplies/polarities. |
| F.52 / F52 | Mass flow sensor connection fault | Sensor disconnected or incorrectly plugged in. |
| F.53 / F53 | Mass flow sensor fault | Faulty sensor, wet/blocked venturi filter, or low gas flow pressure. |
| F.54 / F54 | Gas pressure fault | Insufficient gas supply pressure or closed gas valve (often paired with F.28/F.29). |
| F.56 / F56 | Mass flow sensor regulation | Defective gas valve or damaged cable harness to the gas valve. |
| F.57 / F57 | Fault during comfort safety mode | Highly corroded ignition electrode. |
| F.61 / F61 | Gas valve regulation fault | Short circuit/short to earth in the gas valve cable harness, or defective PCB. |
| F.62 / F62 | Gas valve switch-off delay | Delayed extinguishing of the flame signal, leaking gas valve, or defective electronics. |
| F.63 / F63 | EEPROM error | Defective electronics/PCB. |
| F.64 / F64 | Electronics/NTC fault | Supply or return NTC short-circuited, or defective electronics. |
| F.65 / F65 | Electronic temperature fault | Electronics overheating due to external heat sources or an internal PCB fault. |
| F.67 / F67 | Electronics/flame fault | Implausible flame signal registered; electronics defective. |
| F.68 / F68 | Unstable flame signal | Air in gas, low flow pressure, wrong air ratio, blocked condensate, or ionisation interruption. |
| F.70 / F70 | Invalid Device Specific Number (DSN) | Display and PCB replaced simultaneously without setting the DSN, or wrong/missing coding resistor. |
| F.75 / F75 | No pressure change detection on pump start | Defective pump/pressure sensor, air in the system, or low pressure. Check the adjustable bypass. |
| F.76 / F76 | Overheating protection on primary heat exchanger | Defective primary heat exchanger or compromised connections for its safety fuse. |
| F.77 / F77 | Flue non-return flap / condensate pump fault | Defective condensate pump or no response from the flue non-return flap. |
| F.78 / F78 | Interruption to DHW outlet sensor | UK link box connected, but hot water NTC is not bridged. |
| F.83 / F83 | Flow/return sensor temperature change fault | Insufficient water in the boiler, or sensors not positioned correctly on the pipe. |
| F.84 / F84 | Flow/return sensor temperature difference implausible | Sensors have been inverted or incorrectly fitted to the pipework. |
| F.85 / F85 | Flow and return sensors incorrectly fitted | Sensors fitted to the wrong pipes or the same pipe. |
| F.92 / F92 | Coding resistor fault | PCB coding resistor does not match the entered gas family. |
Need Professional Diagnostics?
Trying to bypass safety lockouts or resetting a boiler repeatedly without fixing the underlying fault can cause catastrophic damage to the system.
If you are dealing with a persistent fault code, contact one of our engineers and book your Vaillant boiler repair with iWarm.
- Call iWarm Direct: 07825 121000
- Gas Safe Registered: 932713
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