The E09 fault code appears on 8 boiler brands — Andrews, Baxi, Biasi, Johnson & Starley, Keston, Main, Potterton and Ravenheat — 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.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
E09 brand by brand
The E09 fault code on Andrews (Andrews Water Heaters by Baxi) boilers signals that the boiler's main control board (PCB) is not receiving a valid feedback signal from the gas valve. In practical terms, the PCB expects confirmation that the gas valve is operating correctly; when that confirmation is absent or interrupted, the boiler locks out as a safety precaution. The fault points specifically to the gas valve connection cable circuit — either the wiring between the valve and the PCB, or the gas valve itself. Because Andrews Water Heaters is now part of the Baxi group and shares control architecture with Baxi-family products, the E09 code carries the same meaning across the related product range.
Causes, fixes & costs for Andrews E09 → · All Andrews codes
The E09 code on a Baxi boiler signals that the control board has detected a problem with the gas valve connection cable or its associated wiring. As a safety response, the boiler locks out completely and will not attempt to fire until the fault is cleared. Because this fault directly involves the gas valve — a safety-critical component — it cannot be resolved by the homeowner and must be investigated by a Gas Safe registered engineer.
Causes, fixes & costs for Baxi E09 → · All Baxi codes
The Biasi E09 (also displayed as ER 09 on some models) fault code indicates that the boiler's control board has detected a problem with the flue temperature NTC probe — a thermistor sensor that monitors the temperature of gases leaving through the flue. When this sensor gives a reading outside expected parameters, or stops responding altogether, the boiler locks out as a safety precaution and will not restart until the fault is resolved. Note: a small number of Biasi models use E09 to flag a water pressure error rather than a flue probe fault — always cross-reference with your specific model's manual if you are unsure which applies to you.
Causes, fixes & costs for Biasi E09 → · All Biasi codes
E09 on a Johnson & Starley unit is a valve feedback error: the control board monitors the gas valve's electrical feedback to confirm the valve is genuinely doing what it has been commanded, and that confirmation has failed. Either the feedback signal is absent, contradicts the command (valve told to close but reading open, or vice versa), or is electrically implausible. Because valve feedback is how the controller proves gas is only flowing when it should, a feedback failure produces an immediate lockout. The suspects are the valve's feedback wiring and connectors, the valve's internal switch/coil feedback, and the board's monitoring input.
Causes, fixes & costs for Johnson & Starley E09 → · All Johnson & Starley codes
The E09 code on a Keston boiler indicates a valve drive error — the boiler's control system has detected that the gas valve actuator is not behaving as expected. This is a lockout-level fault, meaning the boiler will shut down and refuse to restart automatically. You must press the reset button to attempt a restart after the underlying issue has been resolved. In practice, E09 points to either the gas valve itself or the PCB (printed circuit board) that sends it signals, though fan and airflow problems can also trigger this code in some circumstances.
Causes, fixes & costs for Keston E09 → · All Keston codes
The E09 fault code on Main boilers (also seen on Baxi and Potterton boilers, which share the same BDR Thermea control platform) indicates that the boiler's control board has lost communication with the gas valve or detected a problem with the gas valve's connection cable. In plain terms, the PCB sent a signal to the gas valve and either received no response, or received an unexpected one. As a safety measure the boiler locks out immediately. Attempting a reset will usually result in instant re-lockout until the underlying wiring or component fault is resolved. Because the fault directly involves the gas valve — a safety-critical component — all diagnosis and repair work must be carried out by a Gas Safe registered engineer.
Causes, fixes & costs for Main E09 → · All Main codes
The E09 fault code on a Potterton boiler indicates that the boiler's control board has detected a problem with the gas valve connection. This could be an intermittent signal, a weak connection, or no signal at all between the PCB and the gas valve. Because the boiler cannot reliably control gas flow to the burner, it treats this as a safety risk and shuts down completely — locking out rather than continuing to run. You will need a Gas Safe registered engineer to diagnose and resolve this fault; there is no safe DIY fix for E09.
Causes, fixes & costs for Potterton E09 → · All Potterton codes
The E09 fault code on Ravenheat boilers indicates that the overheat thermostat (also referred to as the high-limit sensor) has registered an excessively high temperature and triggered a safety lockout. When this happens, the boiler shuts itself down to prevent unsafe operation, and you may notice a red warning light flashing on the control panel. The lockout can be caused by a genuine overheating event within the boiler, or by the sensor itself failing and sending a false signal to the PCB. Either way, the boiler will not resume normal operation until the underlying cause has been identified and resolved. Note: this code is sometimes displayed as 09E on certain Ravenheat control panels — both refer to the same overheat lockout condition.
Causes, fixes & costs for Ravenheat E09 → · All Ravenheat codes
Frequently asked questions
Is the E09 fault code the same on every boiler?
Broadly, yes — on Andrews, Baxi, Biasi, Johnson & Starley, Keston, Main, Potterton and Ravenheat the E09 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 E09 fault myself?
Not usually. On every brand listed, E09 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 E09 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.
