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A51

Cointra A51 on a gas water heater: air or flue duct blocked

Verified with sources2 sourcesReviewed Oct 4, 2026
Technician coming? See the 2 steps they must show you
Two wall-mounted tankless gas water heaters with their pipework
BehratCC BY-SA 3.0Image source

Risk level

High

Professional inspection is recommended.

Urgency

High

Quick answer

A51 on Cointra CPE T and Supreme E TS water heaters is a poor combustion lockout whose documented cause is a blocked air intake or flue duct. Do not use it until a gas technician checks the flue; at most, try a single reset.

Safety first

Stop and check this first

  • If there is a gas smell alongside A51, do not attempt a reset: open windows, do not touch switches, plugs or flames, and leave the house. Call the gas emergency number once you are outdoors.
  • A51 points to a blocked flue, a situation in which combustion gases may not be expelled outdoors. With a carbon monoxide alarm, headache or dizziness, leave the home and call for help from outside.

Safe checks you can perform

  • Consider one reset only with no symptoms
  • Report any changes near the terminal
  • Request a flue inspection

Leave these tasks to a qualified professional

  • Do not attempt this professional task: Locate and clear the obstruction
  • Do not attempt this professional task: Check sealing and combustion after clearing

What a technician must be able to justify

Technician visit

These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.

These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.

  1. Locate and clear the obstruction

    What they doThe authorised gas technician inspects the air intake and exhaust ducts along their whole route, including the terminal, and removes whatever blocks them.

    They must show youthe point in the duct where the blockage was.

    In writingwhere the blockage was found, what was removed and the condition of the terminal.

  2. Check sealing and combustion after clearing

    What they doOnce the duct is clear, the technician confirms that the air and flue ducts have no leaks and that combustion is correct before returning the appliance to service.

    They must show youthe combustion reading or draught test performed.

    In writingthe result of that measurement and confirmation that A51 does not return.

Before accepting a part or repair

Nothing has been shown yet. Do not accept a part swap based on the code alone.

Follow these steps

Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.

  1. Consider one reset only with no symptoms

    No special tools

    Who:
    you, only if there is no gas smell, no CO alarm and nobody feels unwell.
    How:
    press RESET for one second, as the manual describes for A codes, and do it only once.
    Result that matters:
    whether the lockout stays or comes back.
    If A51 returns:
    do not reset again and leave the heater off until it is inspected.
    If it does not return:
    still tell the technician, as the blockage may be intermittent.
    Evidence to keep:
    a photo of A51 and how it responded to the reset.

    Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)

  2. Report any changes near the terminal

    No special tools

    Who:
    you, without climbing the facade or touching the duct.
    How:
    recall whether there has been building work, scaffolding, awnings, balcony enclosures or storms near the flue outlet.
    Result that matters:
    helping the technician locate the blockage.
    If there were changes:
    mention them when booking the visit.
    If there were none:
    say so anyway.
    Evidence to keep:
    approximate dates of that work or those events.

    Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)

  3. Request a flue inspection

    Tools required

    Who:
    an authorised gas technician for the A51 flue.
    How:
    they follow the intake and exhaust ducts to the terminal and check that both are sealed.
    Result that matters:
    finding the exact point of the A51 blockage.
    If they find it:
    it must be cleared and combustion measured.
    If nothing is found:
    they should leave the appliance out of service or explain why not.
    Evidence to keep:
    a job sheet naming where the blockage was.

    Do not climb the facade or remove the terminal; the flue is for the technician to inspect.

    Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)

Still not solved

If the error persists, stop here. See what the technician must do and show you.

Where this code applies

A code is meaningful only inside the right product and version context.

Primary scope

System
Boilers
Brand
Cointra
Product type
Tankless gas water heater

Known code variants

  • A 51
  • calentador Cointra A51
  • Cointra A51
  • Cointra water heater A51
  • CPE T A51
  • Supreme E TS A51

What it means

A51 is a permanent lockout for poor combustion. Unlike A21, which follows repeated F20 faults, Cointra's table gives A51 one specific cause: an obstruction in the duct through which the appliance draws air or expels flue gases. The manufacturer's remedy is to check the flue. Because the water heater is room-sealed, that duct is its only route for air and exhaust, so the lockout guards against inadequate combustion.

Warnings and stop conditions

  • Criticaleditorial warning

    If there is a gas smell alongside A51, do not attempt a reset: open windows, do not touch switches, plugs or flames, and leave the house. Call the gas emergency number once you are outdoors.

    Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)

  • Criticaleditorial warning

    A51 points to a blocked flue, a situation in which combustion gases may not be expelled outdoors. With a carbon monoxide alarm, headache or dizziness, leave the home and call for help from outside.

    Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)

Symptoms and causes

Probable causes · in order

  • Blocked air intake or flue ductCommon

    This is the cause Cointra lists for A51. It may be in the air intake or the exhaust section, coaxial or twin pipe, and the table calls for the complete flue to be checked up to the outdoor terminal.

    Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)

Virtual technician

Does A51 still appear after these checks?

Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.

Start diagnosis

Sources and technical references

  1. 1

    Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03)

    Cointra · 2018 · manual

  2. 2

    Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)

    Cointra · 2016 · manual

Sources: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)reviewed 2026-10-04 · verified

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