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What to do if…
you suddenly have no hot water and the water heater shows an error or warning light

Short answer

First do a safety check: if you smell gas, a carbon monoxide (CO) alarm sounds, or anyone feels suddenly unwell, get into fresh air and call the National Gas Emergency Service on 0800 111 999. If there are no danger signs, photograph the error/warning and contact your landlord/agent or a Gas Safe registered engineer.

Do not do these things

  • Do not keep resetting the boiler/water heater again and again if it locks out or faults again.
  • Do not remove covers, open the sealed case, or attempt internal repairs.
  • Do not ignore a CO alarm or sudden symptoms (headache, dizziness, nausea), especially near a boiler/water heater.
  • If you smell gas, do not use naked flames or operate electrical switches (on or off) before you’ve safely ventilated/left as needed.
  • Do not let anyone who isn’t Gas Safe registered work on gas appliances.

What to do now

  1. Check for immediate danger signs (30 seconds).

    • Smell gas? Open doors/windows, shut off the gas at the meter control valve if you know where it is and it’s safe, and call the National Gas Emergency Service: 0800 111 999.
    • CO alarm sounding or anyone feels suddenly unwell near the appliance? Open doors/windows, move everyone into fresh air, and call 0800 111 999. In a medical emergency, call 999 immediately.

  2. Pause use and make the area safe.

    • Stop using hot taps/showers for now.
    • Keep the area around the appliance clear and well ventilated.
    • If there’s any water near electrical parts, don’t touch switches or the appliance—just keep people away.

  3. Capture the information you’ll need (prevents missteps).

    • Take a clear photo of the fault code / flashing lights and the make/model label.
    • Note what changed just before it failed (power cut, timer changes, very cold weather, recent work).

  4. Do only external supply checks (no tools, no covers).

    • Check whether your electricity has tripped (consumer unit).
    • If you have gas and you do not smell gas, check whether another gas appliance (e.g., hob) works.
    • If you have a combi boiler, glance at the pressure gauge and note the reading (don’t adjust settings if you’re unsure).

  5. Contact the right person (and send the photos).

    • Tenant / landlord-managed home: report “no hot water + warning light/code” to your landlord/agent and attach the photo(s).
    • Homeowner: contact a Gas Safe registered engineer (gas systems). For electric-only systems (e.g., immersion heater), contact a qualified heating/plumbing engineer or electrician as appropriate.

  6. If you try a reset, keep it to one attempt.

    • Only if there are no gas/CO danger signs, you can try one reset using the manufacturer’s user control (e.g., a reset button on the front panel).
    • If it faults again, stop and wait for professional help.

What can wait

  • You don’t need to diagnose the code, buy parts, or dismantle anything right now.
  • You don’t need to decide “repair vs replace” until a qualified engineer confirms the cause and safety.
  • You can wait to do non-essential troubleshooting (timers, settings, optimisations) until you have hot water back and the system is confirmed safe.

Important reassurance

It’s common for boilers/water heaters to shut down to protect you when a sensor detects a problem. Focusing on the safety check, capturing the fault info, and arranging the correct engineer is enough for now.

Scope note

These are first steps only to reduce risk and avoid unsafe DIY fixes. Your exact next steps depend on whether you have a combi boiler, hot water cylinder, or immersion heater.

Important note

This is general safety and first-response information, not a substitute for professional inspection or emergency services. If you suspect gas or carbon monoxide, prioritise fresh air and urgent help.

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