The E12 fault code appears on 7 brands — Alpha, Andrews, Baxi, Biasi, Johnson & Starley, Potterton and Warmflow — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
E12 brand by brand
The E12 code on an Alpha boiler indicates that the differential water flow switch is stuck in the open position. This safety switch monitors whether water is genuinely circulating through the boiler when it fires. Under normal operation, water movement causes the switch to close, confirming to the boiler's controls that it's safe to run the burner. When the switch fails to close — remaining open even when water should be flowing — the boiler locks out to protect the heat exchanger. A related code, E13, is the opposite condition: the same switch stuck in the closed position. If you see E12, the boiler cannot confirm adequate water flow and has shut down as a precaution.
Causes, fixes & costs for Alpha E12 → · All Alpha codes
The E12 fault code on Andrews boilers indicates that the differential water flow switch is reading as open when the boiler expects it to be closed during normal operation. In plain terms, the boiler cannot confirm that water is circulating adequately through the system, so it locks out as a safety measure. This is distinct from the E13 code, which triggers when the same switch is stuck in the closed position — E12 is specifically the 'switch still open' variant. Andrews boilers share the same E-code platform as Baxi (both sit under the BDR Thermea group), so engineers familiar with Baxi fault logic will recognise this code immediately.
Causes, fixes & costs for Andrews E12 → · All Andrews codes
The E12 code on most Baxi boilers signals a differential water flow switch fault where the switch is stuck in the open position. In plain terms, the boiler's internal sensor cannot confirm that water is actually moving through the system, so the boiler refuses to fire as a safety precaution. A closely related code — E13 — is the opposite condition, where the same switch is stuck closed rather than open. It is worth noting that on certain older Baxi ranges (including some Solo and EcoBlue LED-only models), E12 and E3 can instead point to a printed circuit board (PCB) fault rather than a flow switch issue, so always cross-reference with your specific model's documentation if the standard flow-switch checks draw a blank.
Causes, fixes & costs for Baxi E12 → · All Baxi codes
The E12 code appears on Biasi boilers when the control board cannot obtain a valid signal from the central heating return temperature sensor — a thermistor known as an NTC (Negative Temperature Coefficient) probe. This probe sits on the return pipe of the primary heat exchanger and continuously feeds temperature data to the PCB so the boiler can regulate heating safely. When the reading falls outside acceptable parameters, or disappears entirely, the boiler locks out and displays E12. On some Biasi models the display alternates between 'Er' and 'RESET', indicating a safety lockout that requires a confirmed repair before normal operation can resume.
Causes, fixes & costs for Biasi E12 → · All Biasi codes
E12 on a Johnson & Starley unit is an EEPROM lockout: the control board's non-volatile memory — where it stores its configuration, parameters and safety-relevant settings — has failed a checksum or read test. A controller that cannot trust its stored settings cannot trust its own behaviour, so it locks out. The practical causes are a genuine memory failure on the board, corruption after a severe supply disturbance, or (least often) a recoverable glitch that clears with a full power-cycle. The realistic remedy where the fault persists is control board replacement with full reconfiguration for the model.
Causes, fixes & costs for Johnson & Starley E12 → · All Johnson & Starley codes
The E12 code appears on several Potterton boiler models and relates to a water circulation problem detected by the boiler's safety controls. There are two closely linked interpretations depending on the specific model: the boiler has sensed an unusually large difference between the flow temperature (water leaving the boiler) and the return temperature (water coming back), or the differential water flow switch has failed to close as expected during operation. In both cases, the boiler is detecting that water is not moving through the system as it should, and it has shut down to protect itself from overheating. You may notice the boiler locking out, cycling on and off rapidly, making unusual noises, or failing to heat radiators properly.
Causes, fixes & costs for Potterton E12 → · All Potterton codes
The E12 code appears on the LCD display of the Warmflow Agentis Combination Boiler and points to a problem with the domestic hot water (DHW) tank control NTC temperature sensor. NTC stands for Negative Temperature Coefficient — it describes a thermistor that continuously feeds water temperature readings back to the boiler's PCB. When the PCB detects that the signal from this sensor is missing, out of range, or implausible, it triggers E12 and shuts the boiler down to protect the system. In most cases the underlying cause is a straightforward wiring or sensor issue rather than a major component failure. Note that E12 is specific to the Agentis Combination Boiler range; older Warmflow oil boiler models without an LCD display use indicator lights and lockout conditions rather than alphanumeric fault codes. Because the Warmflow Agentis is an oil-fired appliance, any hands-on repair work must be carried out by an OFTEC-registered technician, not a Gas Safe engineer.
Causes, fixes & costs for Warmflow E12 → · All Warmflow codes
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.
