Low power supply lockout
The E34 error code on Johnson-Starley boilers indicates low power supply lockout.
Johnson-Starley E34 means: low power supply lockout. The usual causes are faulty or worn ignition electrode, cracked or dirty ignition lead or gas supply interruption or low gas pressure. You can safely reset the boiler once — if E34 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does E34 Mean
The E34 code is shown by the boiler’s control board when it detects low power supply lockout. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows E34 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 1 technical sources.
| Cause |
|---|
| Faulty or worn ignition electrode |
| cracked or dirty ignition lead |
| gas supply interruption or low gas pressure |
| blocked condensate drain preventing ignition |
| defective gas valve not opening |
| faulty PCB (printed circuit board) not sending ignition signal |
Safety Warning
E34 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
Verify gas supply — check gas meter is on and other gas appliances work
Inspect the ignition electrode — look for cracks, carbon build-up, or incorrect gap (typically 3-4mm)
Check the ignition lead — ensure it is securely connected and not damaged
Test the gas valve — measure voltage at the gas valve terminals during ignition attempt (should read 24V AC or mains depending on model)
Inspect the flue system — check for blockages, birds' nests, or debris
Check the condensate trap and drain — a blocked condensate pipe can prevent ignition
Verify the ionisation probe is clean and correctly positioned
If all above pass, test the PCB by measuring outputs during ignition sequence
How to Fix It
Clean or replace the ignition electrode and lead
clear any condensate blockage (pour warm water over frozen external pipe in winter)
reset the boiler after checking all components
if the gas valve is not opening, replace the gas valve (Gas Safe registered engineer required)
if the PCB is not sending ignition signal, replace the PCB. Important: all gas work must be carried out by a Gas Safe registered engineer
Refer to the Johnson-Starley service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Johnson-Starley models that can display fault code E34:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Johnson-Starley models | — | Possible |
Frequently Asked Questions
Clean or replace the ignition electrode and lead. According to the technical documentation, this is the first check on the list for E34.
E34 is not classified as a high-severity fault, so the boiler can be reset once. If E34 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.