Ignition failure: the furnace failed to ignite after several attempts
The E203 error code on Lennox boilers indicates ignition failure: the furnace failed to ignite after several attempts.
Lennox E203 means: ignition failure: the furnace failed to ignite after several attempts. The usual causes are configuration flow chart. see page 70, no appliance capacity code selected. check for or proper configuring under: unit size codes for fur. A Gas Safe registered engineer should diagnose the root cause.
What Does E203 Mean
The E203 code is shown by the boiler’s control board when it detects ignition failure: the furnace failed to ignite after several attempts. 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 E203 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 2 technical sources.
| Cause |
|---|
| configuration flow chart. See Page 70. |
| No appliance capacity code selected. Check for |
| proper configuring under: Unit Size Codes for Fur |
| nace on configuration guide or in installation instruc |
| tions. Critical Alert. Cleared after valid unit code is |
| read following a reset. (Remove thermostat from |
Safety Warning
E203 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. Gas-related fault. Do not open the furnace cabinet or work on the gas valve, burner, igniter or venting yourself. Repeated resets are unsafe. Have it diagnosed and repaired by a licensed HVAC technician.
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 Lennox service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
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Affected Models
Lennox models that can display fault code E203:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Lennox 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 E203.
E203 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.