Complete reference guide for all Vaillant boiler fault codes. Find your error code below for detailed causes and step-by-step solutions. Select your boiler series and locate your fault code.
Listed below are all Vaillant fault codes as described by the boiler manufacturer along with possible causes. Click “View Solution” for detailed step-by-step fixes, DIY tips, and when to call a Gas Safe engineer.
Vaillant boilers use different code types:
– F codes = Fault codes requiring attention or lockout
– S codes = Status codes showing current operation
– D codes = Diagnostic codes for engineers
– P codes = Test program codes
Find your boiler model below and locate your fault code.
| Fault Code | Description | Possible Cause | Solution |
|---|---|---|---|
| F.22 | Safety switch-off – low water pressure | System pressure below 0.5 bar | View Solution → |
| F.28 | Ignition unsuccessful during start-up | Gas supply, electrode, or frozen condensate pipe | View Solution → |
| F.29 | Re-ignition unsuccessful during operation | Flame loss – gas supply or condensate blockage | View Solution → |
| F.75 | No pressure change when pump starts | Pump fault or pressure sensor issue | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F.00 | Flow NTC cable defective or broken | View Solution → |
| F.01 | Flow NTC cable defective or broken | View Solution → |
| F.10 | NTC fault | View Solution → |
| F.11 | NTC fault | View Solution → |
| F.13 | NTC fault | View Solution → |
| F.20 | Boiler exceeded maximum temperature | View Solution → |
| F.22 | Dry fire / low water / overheat | View Solution → |
| F.24 | Low water / pump failure / blockage | View Solution → |
| F.28 | Insufficient gas, defective electrode or ignitor | View Solution → |
| F.29 | Flame extinguished | View Solution → |
| F.60 | Gas valve control defective | View Solution → |
| F.61 | Gas valve control defective | View Solution → |
| F.62 | Defective gas valve closure mechanism | View Solution → |
| F.63 | EEPROM error – electronic fault | View Solution → |
| F.64 | Electronic system or sensor fault | View Solution → |
| F.65 | Electronic system temperature too high | View Solution → |
| F.67 | Electronic fault in flame system | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F.1 | Lack of gas, ignition fault, or flue issue | View Solution → |
| F.4 | Ignition fault | View Solution → |
| F.5 | System overheating fault | View Solution → |
| F.6 | Flow NTC fault | View Solution → |
| F.17 | Check system polarity | View Solution → |
| F.24 | Pump failure, overrun, or airlock | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F.00, .04, .05, .10, .14, .15 | NTC faults – change all NTC for diagnosis | View Solution → |
| F.11 | Blockage preventing flow | View Solution → |
| F.12 | Blockage affecting system pressure | View Solution → |
| F.13 | Blockage due to water deficiency | View Solution → |
| F.21 | Pilot flame failed | View Solution → |
| F.22 | Insufficient water in system | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F.00 / F.0 | Flow temperature sensor interruption | View Solution → |
| F.01 / F.1 | Return temperature sensor interruption | View Solution → |
| F.10 | Flow NTC short circuit | View Solution → |
| F.11 | Return NTC short circuit | View Solution → |
| F.20 | Safety switch-off – temperature limiter | View Solution → |
| F.22 | Safety switch-off – low water pressure | View Solution → |
| F.23 | Safety switch-off – temperature difference too great | View Solution → |
| F.24 | Safety switch-off – temperature rising too fast | View Solution → |
| F.25 | Safety switch-off – flue gas temperature too high | View Solution → |
| F.26 | Error with gas valve – no function | View Solution → |
| F.27 | Safety switch-off – flame simulation | View Solution → |
| F.28 | Ignition unsuccessful during start-up | View Solution → |
| F.29 | Re-ignition unsuccessful during operation | View Solution → |
| F.32 | Fan error | View Solution → |
| F.49 | eBUS error – possible short circuit | View Solution → |
| F.61 | Error with gas valve regulation | View Solution → |
| F.62 | Gas valve switch-off delay error | View Solution → |
| F.63 | EEPROM error | View Solution → |
| F.64 | Electronics / NTC error | View Solution → |
| F.65 | Electronic temperature error – overheating | View Solution → |
| F.67 | Electronics flame signal error | View Solution → |
| F.68 | Unstable flame signal – air in gas | View Solution → |
| F.70 | Invalid device specific number (DSN) | View Solution → |
| F.71 | Flow NTC error – constant value | View Solution → |
| F.72 | Flow/return NTC error – temperature difference too great | View Solution → |
| F.73 | Water pressure sensor signal too low | View Solution → |
| F.74 | Water pressure sensor signal too high | View Solution → |
| F.75 | No pressure change when pump starts | View Solution → |
| F.76 | Overheating protection – heat exchanger responded | View Solution → |
| F.77 | Flue non-return flap / condensate pump error | View Solution → |
| F.78 | Interruption to DHW outlet sensor | View Solution → |
| F.83 | Flow and/or return temperature sensor error | View Solution → |
| F.84 | Flow/return temperature sensor changing error | View Solution → |
| F.85 | Flow/return sensor incorrectly fitted | View Solution → |
| CONNECTION | No communication with PCB | View Solution → |
Note: S codes show boiler status during operation – these are NOT faults:
| Status Code | Description | Action Required |
|---|---|---|
| S.00 | Heating mode – no heat demand | No action – normal |
| S.01 | Heating mode – fan start-up | No action – normal |
| S.02 | Heating mode – water pump running | No action – normal |
| S.03 | Heating mode – ignition sequence | No action – normal |
| S.04 | Heating mode – burner ignited | No action – normal |
| S.06 | Heating mode – fan overrun | No action – normal |
| S.07 | Heating mode – pump overrun | No action – normal |
| S.08 | Heating cut-off time XX minutes | No action – normal |
| S.10 | Hot water demand via impeller sensor | No action – normal |
| S.11 | DHW mode – fan start-up | No action – normal |
| S.13 | DHW mode – ignition | No action – normal |
| S.14 | Hot water demand via fan sensor | No action – normal |
| S.15 | DHW mode – pump and fan overrun | No action – normal |
| S.16 | DHW mode – fan overrun | No action – normal |
| S.17 | DHW mode – pump overrun | No action – normal |
| S.20 | DHW mode – burner on | No action – normal |
| S.21 | DHW mode – fan start-up | No action – normal |
| S.22 | DHW demand – pump running | No action – normal |
| S.23 | DHW ignition | No action – normal |
| S.24 | DHW mode – burner on | No action – normal |
| S.25 | DHW mode – pump or fan overrun | No action – normal |
| S.26 | DHW mode – fan overrun | No action – normal |
| S.27 | DHW mode – pump overrun | No action – normal |
| S.28 | Hot water burner anti-cycling time | No action – normal |
| S.30 | Room thermostat blocking heating mode | No action – normal |
| S.31 | No heat demand – summer operating mode | No action – normal |
| S.32 | Waiting period – fan speed deviation | No action – normal |
| S.34 | Heating mode frost protection | No action – normal |
| S.39 | Burner off contact triggered | Check thermostat/condensate pump |
| S.40 | Comfort safety mode active | No action – normal |
| S.41 | Water pressure > 2.8 bar | Reduce system pressure |
| S.42 | Flue non-return flap blocks burner | Check flue system |
| S.46 | Comfort safety mode – flame loss | Monitor boiler |
| S.53 | Waiting period – water shortage | Check system pressure |
| S.54 | Waiting period – operation blocking | Monitor boiler |
| S.57 | Waiting period – comfort safety mode | No action – normal |
| S.58 | Modulation limitation – noise/wind | No action – normal |
| S.61 | Gas family check unsuccessful | Call engineer |
| S.85 | Service message – circulation water quantity | Service due |
| S.96 | Return sensor error | Call engineer |
| S.98 | Flow/return sensor check running | No action – normal |
| Status Code | Description | Solution |
|---|---|---|
| S.0 | Heating issue – no heat demand | No action – normal |
| S.1 | Fan start-up error | Monitor – call if persists |
| S.2 | Pump flow error | Monitor – call if persists |
| S.3 | Heating mode – ignition problem | View Solution → |
| S.4 | Heat mode – burner on error | Monitor boiler |
| S.6 | Heating mode – fan overrun error | No action – normal |
| S.7 | Heating mode – pump overrun error | No action – normal |
| S.8 | Heating cut off for XX time | No action – normal |
| S.31 | No heat demand – summer mode | No action – normal |
| S.34 | Heating mode frost protection | No action – normal |
| S.20 | Hot water requirement error | Monitor boiler |
| S.22 | Hot water pump flow error | Monitor – call if persists |
| S.24 | Hot water operation – burner on | No action – normal |
| S.85 | Service message – circulation water | Service due |
Note: D codes are diagnostic information for engineers – not faults you need to fix:
| Diagnostic Code | Description | Notes |
|---|---|---|
| D.0 | Heating part load error | Engineer diagnostic |
| D.1 | Water pump overrun time | Engineer diagnostic |
| D.2 | Max burner anti-cycling period | Engineer diagnostic |
| D.3 | Hot water flow temperature reading | Engineer diagnostic |
| D.4 | Current temperature for warm stat sensor | Engineer diagnostic |
| D.7 | Warm start temperature target value | Engineer diagnostic |
| D.8 | External controls heat demand | Engineer diagnostic |
| D.10 | Status internal heating pump | Engineer diagnostic |
| D.22 | Hot water demand | Engineer diagnostic |
| D.23 | Summer/Winter function | Engineer diagnostic |
| D.30 | Control signal for gas valves | Engineer diagnostic |
| D.33 | Fan speed target value | Engineer diagnostic |
| D.34 | Fan speed actual value | Engineer diagnostic |
| D.40 | Flow temperature target value | Engineer diagnostic |
| D.41 | Return temperature actual value | Engineer diagnostic |
| D.76 | Remaining burner anti-cycling time | Engineer diagnostic |
| D.80 | Operating hours heating | Engineer diagnostic |
| D.81 | Operating hours hot water generation | Engineer diagnostic |
| D.82 | Burner start-ups in heating mode | Engineer diagnostic |
| D.83 | Burner start-ups in hot water mode | Engineer diagnostic |
| D.84 | Maintenance indicator – hours until service | Service reminder |
| D.98 | Telephone a heating engineer | Call for service |
| Test Code | Description | Notes |
|---|---|---|
| P.00 | Purging heating and hot water circuits | Engineer test mode |
| P.01 | Maximum load test program | Engineer test mode |
| P.02 | Minimum load test program | Engineer test mode |
| P.06 | Filling mode test program | Engineer test mode |
The most common Vaillant fault. Usually an easy DIY fix by repressurising your heating system to 1.0-1.5 bar.
Very common in winter. Almost always caused by a frozen condensate pipe blocking the boiler's drainage system.
No pressure change detected when pump starts. Requires professional diagnosis of pump or pressure sensor.
If your Vaillant boiler is showing F.28 or F.29 error codes during cold weather, your condensate pipe is frozen. This is the #1 cause of Vaillant boiler breakdowns in winter months.
**Quick Fix You Can Do:**
1. Locate the condensate pipe outside – usually white or grey plastic, 21-32mm diameter
2. Pour warm (NOT boiling) water slowly along the entire length of the pipe
3. You’ll hear gurgling sounds when the ice blockage clears
4. Reset your boiler and it should fire up normally
**Why This Happens:**
The condensate pipe carries acidic water from your boiler to an outside drain. In freezing temperatures, this water can freeze solid, blocking the pipe and preventing the boiler from operating.
**Permanent Solution:**
Have the pipe properly insulated by a Gas Safe engineer. Costs around £50-100 and prevents future freezing, saving you emergency callout fees in winter.
**Still not working after thawing?** Call us: 01304 381999
❌ NEVER Attempt DIY:
These require Gas Safe registered engineers with Vaillant training and diagnostic equipment.
✅ You CAN Try:
How to Repressurise Vaillant Boiler:
1. Turn off boiler
2. Locate filling loop (usually under boiler – built-in key or external braided hose)
3. If key type: Insert key and turn slowly
4. If tap type: Open both taps slowly
5. Watch pressure gauge reach 1.0-1.5 bar (green zone)
6. Close valves/remove key immediately
7. Turn boiler back on
Important: If F.28 or F.29 returns after thawing condensate pipe, or if F.22 returns within days, call a professional. Repeated faults indicate an underlying issue.
Our Gas Safe registered engineers are Vaillant-approved installers and service providers, serving Kent and surrounding areas. We stock genuine Vaillant parts for same-day repairs.
✓ Same-day emergency callouts available
✓ All major brands serviced and repaired
**Filling Key Type:**
1. Turn off boiler
2. Insert filling key (usually stored under boiler)
3. Turn key slowly anticlockwise
4. Watch pressure gauge reach 1.0-1.5 bar
5. Turn key back clockwise and remove
6. Turn boiler back on
**External Filling Loop Type:**
1. Turn off boiler
2. Find two small taps with braided hose
3. Open both taps slowly
4. Watch pressure gauge reach 1.0-1.5 bar
5. Close both taps immediately
6. Turn boiler back on
**Important:** Never exceed 1.5 bar – over-pressurisation can damage components.
**Standard Reset:**
Press the reset button (flame symbol with a cross) for 2 seconds.
**Power Cycle Reset (for persistent faults):**
1. Turn boiler mode switch to OFF
2. Wait 30 seconds
3. Turn back to heating or auto mode
4. Boiler should restart
**After F.28/F.29:**
After thawing condensate pipe, always reset once. If fault returns, don’t keep resetting – call an engineer.
Your condensate pipe is usually:
– White or grey plastic pipe (21-32mm diameter)
– Runs from boiler to outside drain
– Often visible on exterior wall near boiler
– May run through wall cavity
– Commonly freezes where it exits the wall
If you can’t locate it or can’t access it safely, call an engineer.
**Annual Service Recommended:**
– Maintains Vaillant warranty
– Prevents faults developing
– Keeps efficiency high (saves money)
– Ensures safe operation
– Required for landlord safety certificates
**Service Includes:**
– Full inspection and cleaning
– Flue gas analysis
– Safety checks
– Pressure testing
– Filter cleaning
– Fault code history review
F.22 means low water pressure – your system pressure has dropped below the safe operating level (typically below 0.5 bar). The boiler has shut down to protect itself from dry-running. To fix: repressurise your system to 1.0-1.5 bar using the filling loop or filling key. If pressure drops again within days, you have a leak and need an engineer to locate and repair it.
F.28 is an ignition fault – the boiler tried to light but failed. In winter, this is almost always a frozen condensate pipe. Other causes include: no gas supply, faulty ignition electrode, low gas pressure, blocked flue, or air in gas supply. Try thawing the condensate pipe first (pour warm water along it), check your gas is on, then reset once. If F.28 returns, call a Gas Safe engineer immediately – don’t keep resetting.
F codes are faults that need attention – they indicate something is wrong and often lock out the boiler. S codes are status codes showing what the boiler is currently doing – they’re informational, not errors. For example, S.04 means “burner is on” which is normal operation. S.31 means “summer mode – no heating demand” which is also normal. Only F codes indicate actual problems.
Press the reset button (marked with a flame symbol with a cross through it) for 2 seconds and release. The display will change and the boiler will attempt to restart. For persistent faults, try a power cycle: turn the mode switch to OFF, wait 30 seconds, then turn back to heating mode. Important: If the same fault code returns within minutes, there’s an underlying problem – don’t keep resetting, call an engineer.
Vaillant owns Glow-worm – they’re both part of the Vaillant Group. Glow-worm boilers are essentially Vaillant boilers with different branding, sold at slightly lower price points. They share many components and fault codes are often identical. Both are high-quality German-engineered brands known for reliability and efficiency.
F.75 means the boiler didn’t detect a pressure change when the pump started. This usually indicates: a seized pump, faulty pressure sensor, air lock in the system, or closed isolation valve. You can try: checking isolation valves are open, bleeding radiators, and resetting once. However, F.75 typically requires an engineer to diagnose whether it’s the pump, sensor, or a blockage. Don’t ignore F.75 as a seized pump can cause serious damage if it continues.
Vaillant recommends annual servicing by a Gas Safe registered engineer, preferably Vaillant Advance accredited. Annual servicing: maintains your warranty validity, prevents expensive breakdowns, keeps efficiency high (saving on bills), ensures safe operation, provides landlord gas safety certificates. We offer annual service plans with reminders to ensure your boiler is serviced on time every year.
