Burner off contact has been triggered this could be because of contact thermostat or condensate pump
The S39 error code on Vaillant boilers indicates burner off contact has been triggered this could be because of contact thermostat or condensate pump.
Vaillant S39 means: burner off contact has been triggered this could be because of contact thermostat or condensate pump. On this component the documented failure modes are no water circulation: pump blocked, seized or not running. A Gas Safe registered engineer should diagnose the root cause.
What Does S39 Mean
The S39 code is shown by the boiler’s control board when it detects burner off contact has been triggered this could be because of contact thermostat or condensate pump. 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 S39 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 5 different manufacturers.
| Cause |
|---|
| No water circulation: pump blocked, seized or not running |
Safety Warning
S39 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. Note the exact error code and check the manufacturer's installation and service manual for specific guidance. 2. Check all visible wiring connections for security and condition. 3. Power cycle the boiler — switch off at the mains for 30 seconds, then switch back on. 4. If the fault recurs, check the fault history for related or recurring codes. 5. Inspect the boiler for any obvious signs of damage, leaks, or unusual sounds
How to Fix It
Reset the boiler after investigation
repair any loose connections or damaged wiring
replace the faulty component identified during diagnosis
if the fault persists after basic checks, contact a qualified heating engineer with the exact error code
Refer to the Vaillant service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Vaillant models that can display fault code S39:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Vaillant models | — | Possible |
Frequently Asked Questions
Reset the boiler after investigation. According to the technical documentation, this is the first check on the list for S39.
S39 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.