Loss of flame during operation
The 128 error code on Baxi boilers indicates loss of flame during operation.
Baxi 128 means: loss of flame during operation. On this component the documented failure modes are no gas reaching the burner, or the gas supply is shut off, flue or air intake blocked or restricted or ignition or detection electrode dirty, worn or out of position. A Gas Safe registered engineer should diagnose the root cause.
What Does 128 Mean
The 128 code is shown by the boiler’s control board when it detects loss of flame during operation. 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 128 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 6 different manufacturers.
| Cause |
|---|
| No gas reaching the burner, or the gas supply is shut off |
| Flue or air intake blocked or restricted |
| Ignition or detection electrode dirty, worn or out of position |
| Faulty gas valve, or its wiring and connector |
| Control board (PCB) faulty |
Safety Warning
128 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 flue and air intake along their whole length, including the outside terminal, and clear any obstruction or debris.
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.
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.
Only after every other check: verify the board’s supply and connections, and replace the board — fitting the correct boiler chip card or coding key where the model requires one.
Refer to the Baxi service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Baxi models that can display fault code 128:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Baxi 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 128.
128 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.