Cabinet overheat - thermal fuse will need replacing
The E73 error code on Keston boilers indicates cabinet overheat - thermal fuse will need replacing.
Keston E73 means: cabinet overheat - thermal fuse will need replacing. The usual cause is air possibly due to a blocked air inlet. any breakage or damage to the burner. A Gas Safe registered engineer should diagnose the root cause.
What Does E73 Mean
The E73 code is shown by the boiler’s control board when it detects cabinet overheat - thermal fuse will need replacing. 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 E73 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 2 technical sources.
| Cause |
|---|
| air possibly due to a blocked air inlet. Any breakage or damage to the burner |
Safety Warning
E73 is a gas-related fault. Never attempt to repair or adjust the gas valve, burner assembly, or flue system yourself. All gas work must be carried out by a Gas Safe registered engineer (UK) or licensed HVAC technician (US). Repeated resets after ignition failure can cause unburnt gas to accumulate inside the combustion chamber — this is a serious safety hazard. If you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property, and call your gas emergency service immediately. Warning: this fault code may relate to gas supply, flue or combustion issues. Do not attempt repair — contact a Gas Safe registered engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
Check the circulation pump — verify it is running and water is flowing (feel the pipes either side of the pump for temperature difference)
Check system valves — ensure all isolation valves and zone valves are fully open
Check the system for air locks — bleed radiators and check for gurgling noises
Inspect the heat exchanger — check for limescale build-up or sludge
Test the flow and return thermistors — compare the temperature reading with an external thermometer
Check pump speed setting — increase if necessary
If the system has a filter (magnetic or inline), check if it needs cleaning
How to Fix It
Bleed the system of air
flush the heating system if sludge is present (consider a power flush)
replace the pump if it is not circulating
descale or replace the heat exchanger if blocked with limescale
open any closed valves
replace faulty thermistors
Refer to the Keston service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Keston models that can display fault code E73:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Keston models | — | Possible |
Frequently Asked Questions
Bleed the system of air. According to the technical documentation, this is the first check on the list for E73.
E73 is classified as a high-severity fault: it involves gas, combustion or flue components. Do not keep resetting the boiler and do not attempt the repair yourself — call a Gas Safe registered engineer.