Boiler
E03
The E03 error code on a Viessmann boiler indicates an ignition fault — the boiler has attempted to light but failed to detect a flame. This is a moderately serious fault that will lock out your boiler and leave you without heating or hot water until it’s resolved.
What does this error mean?
E03 means your Viessmann boiler tried to ignite and either couldn’t establish a flame or lost it immediately after lighting. The boiler’s control board monitors the burner via an ionisation sensor; if no flame signal is detected within a set number of attempts, it shuts down safely and displays E03. This is a safety lockout — the boiler is protecting you from unburned gas accumulating.
Est. repair cost
£150–£300 for electrode/sensor repair; £250–£450 for gas valve replacement; new boiler £2,000–£3,500 installed
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: make sure other gas appliances (e.g. your hob) are working. If they’re not, call your gas supplier — this is a supply issue, not a boiler fault.
Check your boiler’s gas isolation valve (the handle on the pipe feeding the boiler) is fully open — it should be inline with the pipe, not at 90 degrees.
Press the reset button on your Viessmann boiler — typically a flame symbol button on the front panel. Hold it for 3–5 seconds and wait for the boiler to attempt restart.
If the E03 returns after one reset, do not reset repeatedly. Multiple failed ignition attempts can allow unburned gas to accumulate — stop and call a Gas Safe registered engineer.
Do not attempt to inspect, clean, or replace the ignition electrodes, gas valve, or any internal components yourself. Viessmann boilers are gas appliances — internal work is illegal without Gas Safe registration and dangerous without the correct training and tools.
📞 When to call a professional
Call a Gas Safe registered engineer if the E03 error returns after a single reset attempt, or if your gas supply is confirmed as working but the fault persists. Tell the engineer: ‘My Viessmann boiler is showing error code E03, it’s locking out on ignition failure, and a reset didn’t clear it.’ This gives them enough information to arrive with the right parts — likely an electrode set, ionisation probe, or gas valve.
⚖️ Repair or replace?
Most E03 faults are repairable: an ignition electrode or flame sensor replacement typically costs £150–£300 all-in including labour; a gas valve replacement is £250–£450. If your Viessmann boiler is over 12–15 years old and facing a gas valve or PCB fault, replacement often makes more financial sense — a new A-rated boiler installed costs £2,000–£3,500 but will be more efficient and under warranty. For boilers under 10 years old in otherwise good condition, repair is almost always the right call.