Johnson & Starley E21 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley E21 fault code mean?
E21 on a Johnson & Starley unit is an ADC lockout: the analogue-to-digital converter — the circuit on the control board that turns sensor voltages into the numbers the processor works with — has failed its self-check. Every temperature and pressure reading passes through the ADC, so a controller that cannot trust its converter cannot trust any measurement it makes, and the only safe response is a full lockout. Unlike a single failed sensor, this is a board-level fault: the practical outcomes are a recoverable glitch (cleared by a full power-cycle), or a genuine converter failure requiring control board replacement.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Transient glitch after a supply disturbance Common
A spike or brownout can upset the converter's reference; a full power-cycle restores it if no permanent damage was done.
- ADC or reference circuit failure on the board Sometimes
A failed converter or drifted voltage reference fails the self-test on every start — a definitive PCB replacement.
- Board damage from moisture or heat Rare
Condensation or overheating around the control compartment can degrade the measurement circuitry; the engineer checks for the cause before fitting a new board.
How to fix it
- Power-cycle fully once DIY safe
Switch off at the fused spur for a full minute, then back on. A transient converter upset clears; a hardware failure brings E21 straight back.
- Have a Gas Safe registered engineer complete the documented checks Gas Safe engineer
If the code returns, the control board's measurement stage has failed and the PCB is replaced and configured for the model, followed by a full commissioning check. The engineer should also look for a cause — moisture ingress or supply problems — so the new board is not sent the same way.
Parts you may need
- Control PCB · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £180–£400, depending on the underlying cause.
Frequently asked questions
What does E21 mean on my Johnson & Starley?
The board circuit that digitises all the sensor readings (the ADC) failed its self-check — so the controller cannot trust any measurement and locks out completely. Try one full power-cycle; if E21 returns, the control board needs replacing.
Why does one circuit failing stop everything?
Every sensor on the unit is read through that one converter. With it suspect, temperature limits, flame data and pressure readings are all unreliable at once — the lockout is the only safe state.
Is it worth repairing on an old unit?
Board replacement is a £180–£400 job, so on an elderly warm-air heater it is fair to ask for a repair-versus-replace comparison. Warm-air specialists — including Johnson & Starley's own service operation — can advise on both.