Boiler programmer or timer not working
A faulty boiler programmer or timer is one of the most common reasons heating won't come on when you expect it. Whether you've got an older mechanical dial model or a modern digital display, these devices control when your boiler fires up — so when they go wrong, you're left in the cold. Power cuts are a frequent culprit, especially with older programmers that lose their settings, but dead batteries, incorrect settings, and loose wiring cause plenty of problems too. The good news is that most programmer faults are straightforward to diagnose and fix without calling an engineer. However, if the programmer itself is faulty and needs replacing, you'll want to know when to hand the job over to a Gas Safe registered engineer.
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Most likely cause & what to check
Check the programmer display is showing the correct time — a power cut will reset the clock on older programmers.
Check the programme settings: ensure heating is set to ON for the current time period, not TIMED or OFF.
For battery-powered programmers: replace the batteries.
Try running the heating in CONSTANT/OVERRIDE mode to check whether the boiler itself is working — if it fires up, the programmer settings are the issue.
Check all wiring connections at the back of the programmer are secure.
If the programmer display has gone blank or is showing errors, it may need replacing. Most are plug-and-play replacements and don't need an engineer.
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Frequently asked questions
Why has my boiler programmer reset after a power cut?
Older mechanical and some digital programmers don't have battery backup, so a power cut clears their memory and resets the clock to 12:00. You'll need to set the correct time and reprogram your heating schedule. Modern programmers usually have a small battery backup to prevent this, but it's worth checking your manual.
Can I replace a boiler programmer myself?
Yes — most programmers are plug-and-play replacements and don't require a Gas Safe engineer unless your system has a fault that needs diagnosing first. However, if you're unsure about the wiring or your boiler isn't responding, it's safer to get a professional in rather than risk damaging the system.
What's the difference between TIMED and CONSTANT mode on a programmer?
TIMED mode runs your heating on a schedule you've set — it'll only fire up during programmed periods. CONSTANT (or OVERRIDE) mode turns the heating on immediately and keeps it running continuously, which is useful for testing whether your boiler actually works or if it's just a programming issue.
How often should I replace boiler programmer batteries?
Battery-powered programmers typically need new batteries every 1–2 years, though this varies depending on usage and battery type. If your programmer has a low battery warning light, replace them straight away — don't wait until the display goes blank.
My programmer display is blank — does it need replacing?
A blank display usually means dead batteries or a power supply issue — try replacing the batteries first or checking the plug and socket are working. If the display still won't light up after that, the programmer itself has likely failed and will need replacing.
Should I call a Gas Safe engineer if my programmer isn't working?
Most programmer faults you can handle yourself by following these steps, but call a Gas Safe engineer if the boiler won't fire up even in CONSTANT mode, or if you're uncomfortable working with electrical connections. You'll also need a professional if the fault is with the boiler itself rather than the programmer.