- Boilers
- Gas boiler
F39
Cointra F39 on a gas boiler: outdoor temperature probe fault
Risk level
Low
Usually safe to check without specialist tools.
Urgency
Medium
Quick answer
Error F39 on a Cointra SUPERLATIVE SUPRA C condensing boiler indicates an outdoor sensor fault: either the probe or its wiring is shorted, or the probe was disconnected after enabling adaptive temperature. If no outdoor probe is fitted, set curve CU to 0; if one is installed, call a technician.
Safety first
Stop and check this first
- Do not climb ladders to reach an outdoor wall sensor or strip wiring yourself; restrict your check to the control panel menu and leave wiring repairs to a qualified technician.
- Always isolate the boiler from the electrical supply before opening the terminal box, and remember that all connections on the low-voltage terminal strip must be dry contacts (never 230 V).
Safe checks you can perform
- Check whether an outdoor sensor is actually installed
- Check or disable curve CU in the Adaptive Temperature menu
- Inspect outdoor sensor 138 and terminal block wiring
Leave these tasks to a qualified professional
- Do not attempt this professional task: Reconnect the outdoor probe or disable adaptive temperature
- Do not attempt this professional task: Repair shorted wiring or replace the outdoor sensor
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.
Reconnect the outdoor probe or disable adaptive temperature
What they doThe technician checks whether an outdoor sensor (138) is part of the system: if disconnected, they reconnect it at the terminal strip; if none is fitted, they disable adaptive temperature by setting curve CU to 0.
They must show youthe sensor connection at the terminal block or the CU curve setting in the adaptive temperature menu.
In writingcode F39, whether the probe was reconnected or adaptive curve CU was set to 0, and the final curve setting.
Repair shorted wiring or replace the outdoor sensor
What they doWith mains power disconnected, the technician tests the wiring between the boiler terminal block and the outdoor probe, repairs any short circuit or replaces a failed probe, and confirms transient fault F39 clears.
They must show youthe repaired cable section or the replaced outdoor sensor and the boiler display free of code F39.
In writingthe wiring and sensor 138 test readings, any part replaced and the service date.
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.
Check whether an outdoor sensor is actually installed
- Who:
- you.
- How:
- check whether F39 is flashing on the boiler display alongside the outdoor sensor icon (17 on the control panel) and confirm whether your home actually has an external wall sensor fitted.
- Result that matters:
- knowing whether F39 was triggered by enabling adaptive mode without a sensor or by a fault on an existing probe.
- If no outdoor sensor is installed on the property:
- move to the next check to disable adaptive temperature by setting curve CU to 0.
- If an outdoor sensor is installed:
- do not touch the external wiring and book a qualified technician.
- Evidence to keep:
- a photo of the screen showing F39 and confirmation of whether an outdoor sensor is fitted.
Evidence: Cointra SUPERLATIVE SUPRA C, instrucciones de uso, instalación y mantenimiento (cod. 3541V210)
Check or disable curve CU in the Adaptive Temperature menu
- Who:
- you, at the boiler control panel or remote timer if no outdoor probe is fitted.
- How:
- if no outdoor probe is installed, the manual states you can hold the Reset key (6) for 5 seconds to enter the Adaptive Temperature menu (CU flashes), use the DHW keys (1 and 2) to select curve 0 (which disables adaptive regulation), and hold Reset for 5 seconds again to exit (if a remote timer is connected, make this adjustment from its menu).
- Result that matters:
- disabling outdoor sensor regulation when no external probe is connected.
- If setting curve CU to 0 clears F39:
- the boiler will regulate heating using the fixed flow temperature setpoint.
- If you do have an outdoor sensor and want weather compensation active:
- leave your chosen curve (1 to 10) set and call a technician to inspect the probe and cable.
- Evidence to keep:
- the CU curve number that was set before you checked or changed it.
Evidence: Cointra SUPERLATIVE SUPRA C, instrucciones de uso, instalación y mantenimiento (cod. 3541V210)
Inspect outdoor sensor 138 and terminal block wiring
- Who:
- qualified heating technician.
- How:
- with the boiler isolated from the mains supply, open the terminal box and inspect the outdoor probe wiring (item 138 on fig. 45) for a short circuit or loose terminal, then test the sensor resistance.
- Result that matters:
- determining whether fault F39 is caused by the cable run or by the outdoor sensor capsule.
- If the cable was loose or shorted:
- repair and reconnect it at the dry-contact terminal block and confirm F39 clears automatically.
- If the cable is sound but the probe itself is faulty:
- replace the outdoor sensor with an original Cointra part.
- Evidence to keep:
- the resistance measured across sensor 138 leads and the condition of the external cable run.
Accessing the boiler terminal box or an external wall sensor requires isolating the 230 V supply first, so leave this inspection to a heating technician.
Evidence: Cointra SUPERLATIVE SUPRA C, instrucciones de uso, instalación y mantenimiento (cod. 3541V210)
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
- Gas boiler
Known code variants
- Cointra boiler error F39
- Cointra F 39
- Cointra F39
- error F39 caldera Cointra
- F 39
- F39
- Superlative Supra F39
What it means
In the Cointra SUPERLATIVE SUPRA C manual (Table 11), fault code F39 signals an anomaly on the optional outdoor temperature probe (component 138 on the wiring diagram). Because F-series codes cause a transient lockout, the boiler recovers automatically as soon as the reading returns to its normal operating range. Seeing F39 flashing on the display does not by itself prove the outdoor sensor is broken, as a wiring short circuit or leaving adaptive temperature enabled with the probe disconnected triggers the same code.
Warnings and stop conditions
- Mediumeditorial warning
Do not climb ladders to reach an outdoor wall sensor or strip wiring yourself; restrict your check to the control panel menu and leave wiring repairs to a qualified technician.
Evidence: Cointra SUPERLATIVE SUPRA C, instrucciones de uso, instalación y mantenimiento (cod. 3541V210)
- Highsource verified
Always isolate the boiler from the electrical supply before opening the terminal box, and remember that all connections on the low-voltage terminal strip must be dry contacts (never 230 V).
Evidence: Cointra SUPERLATIVE SUPRA C, instrucciones de uso, instalación y mantenimiento (cod. 3541V210)
Symptoms and causes
Probable causes · in order
- Outdoor probe disconnected after enabling adaptive temperatureCommon
Cointra's fault table notes that F39 appears if the outdoor probe is disconnected after weather-compensated (adaptive temperature) control has been activated.
Evidence: Cointra SUPERLATIVE SUPRA C, instrucciones de uso, instalación y mantenimiento (cod. 3541V210)
- Short circuit in the outdoor probe wiringCommon
The manual lists a short circuit in the cable linking the external weather sensor to the boiler terminal block as a direct cause of fault F39.
Evidence: Cointra SUPERLATIVE SUPRA C, instrucciones de uso, instalación y mantenimiento (cod. 3541V210)
- Faulty outdoor temperature sensorPossible
Cointra also states that the optional outdoor probe itself (item 138 on wiring diagram fig. 45) may have failed.
Evidence: Cointra SUPERLATIVE SUPRA C, instrucciones de uso, instalación y mantenimiento (cod. 3541V210)
Virtual technician
Does F39 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.
Sources and technical references
- 1
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