Loss of flame during calibration
The E129 error code on Potterton boilers indicates loss of flame during calibration.
Potterton E129 means: loss of flame during calibration. On this component the documented failure modes are no gas reaching the burner, or the gas supply is shut off, ignition or detection electrode dirty, worn or out of position or flue or air intake blocked or restricted. A Gas Safe registered engineer should diagnose the root cause.
What Does E129 Mean
The E129 code is shown by the boiler’s control board when it detects loss of flame during calibration. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows E129 and the boiler stops until the fault is cleared |
Common Causes
The manufacturer does not publish a cause list for this specific code. The causes below are the failure modes documented for the same component on other boilers, confirmed by the service literature of 11 different manufacturers.
| Cause |
|---|
| No gas reaching the burner, or the gas supply is shut off |
| Ignition or detection electrode dirty, worn or out of position |
| Flue or air intake blocked or restricted |
| Mains gas pressure too low |
| Faulty gas valve, or its wiring and connector |
Safety Warning
E129 is a gas-related fault. Never attempt to repair or adjust the gas valve, burner assembly, or flue system yourself. All gas work must be carried out by a Gas Safe registered engineer (UK) or licensed HVAC technician (US). Repeated resets after ignition failure can cause unburnt gas to accumulate inside the combustion chamber — this is a serious safety hazard. If you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property, and call your gas emergency service immediately. Gas-related fault. Do not open the boiler casing or attempt repairs to the gas valve, burner, ignition or flue. Repeated resets are unsafe. Diagnosis and repair must be carried out by a Gas Safe registered engineer.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
1. Check the ionisation probe — measure the ionisation current (should be above 1µA, typically 3-6µA). 2. Monitor flame behaviour — observe if the flame lifts or is uneven (indicates gas pressure or air supply issue). 3. Check gas pressure at the test point — dynamic pressure should be within manufacturer's specification. 4. Inspect the condensate trap — ensure it is not blocked. 5. Check the flue terminal outside — ensure it is not obstructed and has correct clearances. 6. Verify the earth connection to the boiler. 7. If flame loss occurs only in windy conditions, check flue terminal position and consider a wind guard
How to Fix It
Check that the gas service valve is open and that other gas appliances in the property work. If they do not, the problem is the gas supply, not the boiler.
Inspect the ignition and detection electrode: clean off any deposits, check the gap and position against the manual, and replace it if it is worn.
Inspect the flue and air intake along their whole length, including the outside terminal, and clear any obstruction or debris.
Have the inlet gas pressure measured at the boiler with a manometer and compared against the figure in the manufacturer’s data table.
Check the gas valve connector and wiring for continuity, then test the valve coil resistance against the manual; replace the valve if it is out of range.
Refer to the Potterton service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Potterton models that can display fault code E129:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Potterton models | — | Possible |
Frequently Asked Questions
Check that the gas service valve is open and that other gas appliances in the property work. If they do not, the problem is the gas supply, not the boiler. According to the technical documentation, this is the first check on the list for E129.
E129 is classified as a high-severity fault: it involves gas, combustion or flue components. Do not keep resetting the boiler and do not attempt the repair yourself — call a Gas Safe registered engineer.