- Boilers
- Gas boiler
F13
Vaillant F13 boiler fault: hot water cylinder sensor short
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Fault code F13 (shown on the display as F.13) on a Vaillant ecoTEC boiler indicates a short circuit in the domestic hot water cylinder temperature sensor or warm-start sensor circuit. Try at most one reset or switch off cylinder charging/comfort mode from the front controls, then book an engineer.
Safety first
Stop and check this first
- Never remove the boiler front casing, unplug internal wiring harnesses, or bridge the cylinder sensor with a resistor; internal electrical and gas-appliance work must be carried out only by a qualified engineer.
- If you smell gas at any point, do not operate electrical switches, plugs, or phones inside the building; open doors and windows wide, turn off the gas isolation valve if safe, leave the property immediately, and call the gas emergency service from outside.
Safe checks you can perform
- Check whether the display alternates F.13 with F.91
- Turn off comfort mode or cylinder charging via the user controls
- Perform at most one single boiler reset
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check of the cylinder or warm-start NTC temperature sensor
- Do not attempt this professional task: Wiring harness continuity and earth-short inspection
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.
Check of the cylinder or warm-start NTC temperature sensor
What they doWith the boiler isolated from the mains, the technician checks the cylinder or warm-start NTC sensor, its connector and its wiring as the manual indicates.
They must show youThe sensor checked, and the old sensor if it was replaced.
In writingResult of the sensor and wiring check, and the genuine Vaillant sensor part reference fitted.
Wiring harness continuity and earth-short inspection
What they doInspect the sensor plug, trace the cable harness back to the main PCB, and test for a short circuit between conductors or between the sensor lead and the appliance chassis.
They must show youThe damaged section of harness or corroded plug, or the insulation test reading confirming the wiring is clear of earth shorts.
In writingHarness insulation test result, connector repair or replacement details, and confirmation that fault memory was cleared.
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 the display alternates F.13 with F.91
- Who:
- you, looking at the front display without tools.
- How:
- Watch the boiler screen for several seconds without removing any casing panels to see whether F.13 is displayed on its own or alternating with F.91.
- Result that matters:
- Identifies whether an integrated actoSTOR storage module is reporting a combined sensor fault.
- If only F.13 is shown, note down your exact ecoTEC model and whether an external hot water cylinder is installed.
- If F.13 alternates with F.91, record both codes so the engineer knows the actoSTOR cylinder sensor circuit is involved.
- Evidence to keep:
- Clear photo of the display showing F.13 (and F.91 if present).
Evidence: Vaillant ecoTEC plus installation and maintenance fault table · Vaillant ecoTEC exclusive VMW ..6/5-7, instrucciones de instalación y mantenimiento 0020198146_03
Turn off comfort mode or cylinder charging via the user controls
- Who:
- you, using only the external control panel buttons.
- How:
- As described in the operating instructions, press the hot water temperature button; on models with an external cylinder, scroll down to cylinder charging off (Carga acum. desconec.) and confirm, or on a combi boiler switch off comfort warm-start mode (Confort desconectado).
- Result that matters:
- Checks whether central heating can continue operating safely while hot water preheating is disabled.
- If space heating continues to work normally with comfort or cylinder charging switched off, leave it in that setting until the engineer attends.
- If the F.13 fault remains active and blocks operation, do not tamper with any sensor cables or internal plugs.
- Evidence to keep:
- Short note confirming whether disabling comfort mode or cylinder charging kept heating available.
Evidence: Vaillant ecoTEC exclusive VM y VMW ..6/5-7, instrucciones de funcionamiento 0020196912_02
Perform at most one single boiler reset
- Who:
- you, pressing the fault clearance button once.
- How:
- Press the fault clearance (reset) button once for 1 second and observe whether the display returns to the basic screen.
- Result that matters:
- Rules out a one-off transient glitch without subjecting the control board to repeated resets.
- If F.13 clears and domestic hot water heats normally, monitor the display over the next few days.
- If F.13 reappears after that single reset attempt, stop resetting the appliance and contact a qualified heating engineer or Vaillant service.
- Evidence to keep:
- Timestamp of the single reset attempt and the code shown afterwards.
Evidence: Vaillant UK boiler fault codes list · Vaillant ecoTEC exclusive VM y VMW ..6/5-7, instrucciones de funcionamiento 0020196912_02
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
- Vaillant
- Product type
- Gas boiler
Known code variants
- A13
- anomalía F13 Miele
- anomalie F13 Miele
- BlueHelix F13
- caldera Vaillant F.13
- code erreur F13 lave-vaisselle Miele
- ecoTEC F13
- Error F13 Ferroli
- error F13 lavavajillas Miele
- error F13 Vaillant
- errore F13 lavastoviglie Miele
- F 13
- Fehlercode F13 Miele
- Ferroli A13
- Ferroli F13
- guasto F13 Miele
- Miele dishwasher F13
- Miele F13
- Miele fault F13
- Miele Geschirrspüler Fehler F13
- Vaillant ecoTEC F13
- Vaillant F13
- Vaillant fault code F.13
What it means
On Vaillant ecoTEC condensing boilers, display code F.13 means the electronics have detected a short circuit in the domestic hot water cylinder temperature sensor circuit or, on combi models documented in the Spanish ecoTEC exclusive manual, in the warm-start sensor (appearing together with F.91 on units with an actoSTOR module). The code alone never proves the NTC sensor itself has failed, because a pinched wire touching the metal casing or a shorted wiring harness triggers the exact same fault.
Warnings and stop conditions
- Highsource verified
Never remove the boiler front casing, unplug internal wiring harnesses, or bridge the cylinder sensor with a resistor; internal electrical and gas-appliance work must be carried out only by a qualified engineer.
Evidence: Vaillant ecoTEC exclusive VM y VMW ..6/5-7, instrucciones de funcionamiento 0020196912_02
- Highsource verified
If you smell gas at any point, do not operate electrical switches, plugs, or phones inside the building; open doors and windows wide, turn off the gas isolation valve if safe, leave the property immediately, and call the gas emergency service from outside.
Evidence: Vaillant ecoTEC exclusive VM y VMW ..6/5-7, instrucciones de funcionamiento 0020196912_02
Symptoms and causes
Probable causes · in order
- Defective domestic hot water cylinder or warm-start NTC sensorCommon
The NTC thermistor monitoring the domestic hot water cylinder or combi warm-start temperature has developed an internal short circuit due to thermal ageing or moisture ingress.
Evidence: Vaillant ecoTEC plus installation and maintenance fault table · Vaillant UK boiler fault codes list · Vaillant ecoTEC exclusive VMW ..6/5-7, instrucciones de instalación y mantenimiento 0020198146_03
- Short circuit in the cable harness or wire-to-housing contactPossible
Chafed insulation on the sensor lead or main wiring harness causes a direct short between the signal conductors or against the metallic boiler casing.
Evidence: Vaillant ecoTEC plus installation and maintenance fault table · Vaillant ecoTEC exclusive VMW ..6/5-7, instrucciones de instalación y mantenimiento 0020198146_03
- Faulty electrical plug connection at the cylinder sensor or actoSTOR moduleLess common
Corrosion, water droplets, or a damaged plug on the cylinder sensor lead (or on the actoSTOR sensor connection when paired with F.91) shorts the low-voltage sensing line.
Evidence: Vaillant UK boiler fault codes list · Vaillant ecoTEC exclusive VMW ..6/5-7, instrucciones de instalación y mantenimiento 0020198146_03
Virtual technician
Does F13 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
Vaillant ecoTEC plus installation and maintenance fault table
Vaillant UK · 2020 · manual
- 2
Vaillant UK boiler fault codes list
Vaillant Group · 2026 · official_doc
- 3
- 4
Vaillant ecoTEC exclusive VM y VMW ..6/5-7, instrucciones de funcionamiento 0020196912_02
Vaillant · 2016 · manual
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