E15 Boiler Fault Code: What It Means on Every Brand

The E15 fault code appears on 6 boiler brands — Alpha, Andrews, Baxi, Johnson & Starley, Potterton and Warmflow — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 6 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What E15 means	Severity	DIY?
AAlpha	The E15 code appears when your Alpha boiler's PCB has issued a command to the gas valve — either to open or close — but the valve has not responded as expected. Full guide →	Lockout — engineer likely	No
AAndrews	The E15 code on an Andrews boiler indicates that the boiler's electronic controls have detected a problem with the gas valve circuit — meaning the valve isn't opening, closing, or… Full guide →	Lockout — engineer likely	No
BBaxi	The E15 code on a Baxi boiler signals a permanent lockout caused by a breakdown in the command signal between the boiler's printed circuit board (PCB) and the gas valve. Full guide →	Lockout — engineer likely	No
JSJohnson & Starley	E15 on a Johnson & Starley unit is a sensor drift lockout: one of the temperature sensors the controller cross-checks has drifted outside the tolerance it expects — the reading is still… Full guide →	Lockout — engineer likely	No
PPotterton	The E15 code on a Potterton boiler signals a problem with the gas valve connection. Full guide →	Lockout — engineer likely	No
WWarmflow	The E15 fault code on Warmflow Agentis combination boilers indicates a problem with the central heating return NTC temperature sensor. Full guide →	Lockout — engineer likely	No

E15 brand by brand

Alpha E15

The E15 code appears when your Alpha boiler's PCB has issued a command to the gas valve — either to open or close — but the valve has not responded as expected. Because the boiler cannot confirm that gas flow is being properly controlled, it locks out as a safety measure, cutting off heating and hot water. The fault sits firmly in the safety-critical category: it involves the component responsible for regulating gas entering the appliance. Some Alpha models (particularly older units with LED displays rather than digital screens, such as the CD18R) may indicate the same underlying valve-command problem through a different flash sequence — if in doubt, cross-reference with your model's manual, but the causes and remedies described here apply across the Alpha Intec, E-Tec, and CD ranges.

Causes, fixes & costs for Alpha E15 → · All Alpha codes

Andrews E15

The E15 code on an Andrews boiler indicates that the boiler's electronic controls have detected a problem with the gas valve circuit — meaning the valve isn't opening, closing, or responding as it should. As a safety measure, the boiler locks out and stops the ignition sequence entirely, preventing gas from reaching the burner until the fault is resolved. Andrews boilers sit within the Baxi Group family, so E15 carries the same meaning across related Baxi-platform products.

Causes, fixes & costs for Andrews E15 → · All Andrews codes

Baxi E15

The E15 code on a Baxi boiler signals a permanent lockout caused by a breakdown in the command signal between the boiler's printed circuit board (PCB) and the gas valve. In plain terms, the boiler's brain is either not receiving confirmation that the gas valve is responding correctly, or it has lost reliable communication with it altogether. Because this falls within the E04–E17 range of permanent gas-valve-related lockouts on Baxi boilers, the unit will not restart until the underlying fault is resolved. A single reset attempt is reasonable, but if the code returns, the boiler needs professional attention — this is not a fault that clears itself over time.

Causes, fixes & costs for Baxi E15 → · All Baxi codes

Johnson & Starley E15

E15 on a Johnson & Starley unit is a sensor drift lockout: one of the temperature sensors the controller cross-checks has drifted outside the tolerance it expects — the reading is still changing, but no longer agrees with the physics of the appliance or with its companion sensors. Modern controls run plausibility checks precisely to catch a slowly failing thermistor before it causes a temperature-control problem, and E15 is that check firing. The remedy is diagnosis with a multimeter rather than guesswork: each candidate sensor's resistance is compared against the temperature table and the drifting one is replaced. Sensors are cheap; running a heater on a lying sensor is not.

Causes, fixes & costs for Johnson & Starley E15 → · All Johnson & Starley codes

Potterton E15

The E15 code on a Potterton boiler signals a problem with the gas valve connection. In plain terms, the boiler's control system has detected that the gas valve is not responding or communicating correctly — it may not be opening or closing as commanded. Because the gas valve is a critical safety component that governs the flow of gas into the burner, the boiler will immediately lock out and refuse to fire until the fault is resolved. This code is closely related to the E09 code (which points specifically to a gas valve wiring fault), and both indicate issues in the gas valve circuit that must be investigated by a Gas Safe registered engineer. If at any point you smell gas, do not touch any electrical switches — leave the property immediately and call the National Gas Emergency Service on 0800 111 999.

Causes, fixes & costs for Potterton E15 → · All Potterton codes

Warmflow E15

The E15 fault code on Warmflow Agentis combination boilers indicates a problem with the central heating return NTC temperature sensor. This thermistor sits on the return pipework and continuously monitors the temperature of water flowing back into the boiler. It works by changing its electrical resistance as water temperature rises or falls, allowing the PCB to modulate the burner accurately and prevent overheating. When the boiler's control board detects that the sensor is giving an implausible reading — or no reading at all — it triggers E15 and shuts the boiler down as a safety precaution. The root cause is almost always a short circuit, an open circuit (disconnected wiring), or a physically displaced sensor probe rather than anything more serious.

Causes, fixes & costs for Warmflow E15 → · All Warmflow codes

Frequently asked questions

Is the E15 fault code the same on every boiler?

Broadly, yes — on Alpha, Andrews, Baxi, Johnson & Starley, Potterton and Warmflow the E15 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a E15 fault myself?

Not usually. On every brand listed, E15 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a E15 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.