Error A13 in Cointra boiler: causes and solution

What the A13 fault means

The A13 error in a Cointra boiler indicates an anomaly in the flue (fume) sensor circuit. It is a safety lockout: when the control electronics read an out-of-range or incoherent signal from that circuit, the appliance stops operation to prevent unsafe combustion or improper evacuation of gases. This is not a simple informational message but a preventive shutdown designed to protect the installation and the home.

Common causes

• Damaged sensor: sensors operate near hot areas and suffer wear from heat, vibration and thermal cycles, which can produce erratic or invalid readings.
• Short-circuited wiring: pinched conductors, melted insulation or accidental shorts can alter the sensor signal and trigger the lockout.
• Interrupted wiring: a broken conductor, poor terminal contact or an open circuit prevents the board from receiving a valid signal.

How the fault typically appears

In practice A13 can produce a complete lockout, a failed start, or a brief ignition attempt followed by shutdown. The behaviour may be consistent or intermittent: persistent faults usually point to the sensor itself, while intermittent or temperature-dependent faults often indicate wiring or contact issues.

What to check before assuming a major breakdown

• Perform a careful visual inspection of the sensor area, connectors and harness for loose plugs, heat marks, pinched cables or accumulated dirt.
• Observe the fault pattern: does it happen on every start, only when cold/hot, or after vibration? This helps distinguish sensor from wiring problems.
• Do not rely on repeated resets. Temporary clearing of the code does not fix the root cause and the lockout will return if the abnormal signal remains.

When it’s the sensor and when it’s the wiring

Sensor: if the error is constant and the wiring looks intact, the sensor is the main suspect—aging or internal damage can make readings incoherent.

Wiring: if the error is intermittent, changes with temperature or vibration, or there are visible connector issues, a damaged conductor, poor contact or short is more likely.

Accurate diagnosis requires checking the sensor, continuity and harness condition; the same A13 code can hide different origins.

Recommended diagnostic and repair steps

1. Isolate and power down the boiler before any inspection.
2. Visual check: connectors, clamps, cable insulation and sensor mounting.
3. Measure continuity and insulation of the sensor circuit with a multimeter; check for shorts to ground or between conductors.
4. If wiring faults are found, repair or replace the affected section of the harness and secure connectors.
5. If wiring is good but the signal remains invalid, replace the flue sensor with a part compatible with the exact Cointra model.
6. After repairs, restore power and run a controlled start-up to verify stable readings and normal operation.

Model differences and part compatibility

Cointra sells several ranges; component locations and access vary by model. In compact wall-mounted units access may be more restricted and require removing covers or nearby parts. Always identify the exact model so replacements and procedures match the unit’s internal architecture.

Why you must not force the boiler

Do not repeatedly force starts, disconnect/reconnect or keep resetting the appliance. Forcing operation does not remove the anomaly and can increase wear on electronics or sensors. The lockout prevents potential risks linked to improper gas combustion and poor flue evacuation; ignoring it can lead to higher costs or safety issues.

Quick reference

• Code: A13 — Description: Flue gas / fume sensor anomaly
  • Causes: Damaged sensor, short-circuited wiring, interrupted wiring
  • Suggested response: Check insulation, connectors and continuity; replace sensor if defective; repair or replace harness as needed

If the fault persists

If after inspection and basic checks A13 remains, replace the defective component (sensor or harness section) with the correct, model-compatible part or call a qualified technician. If you have a problem with your boiler, you can use our free error code finder to identify codes and guide repairs more effectively.