Overview
The F.13 fault on a Vaillant Turbomax Plus 824/828E indicates a problem with a cylinder temperature sensor (an NTC thermistor) — usually the boiler reports a short circuit on that sensor. In plain terms the control electronics are seeing an abnormally low resistance (a short) or a wiring fault to the cylinder or related warm-start sensor, so the boiler cannot trust the temperature reading and locks out or displays the error. Severity: this is an electrical/sensor fault rather than an immediate gas-safety emergency. It will typically stop hot water or interfere with boiler operation, but it is not the same as a gas leak or flame failure. However, because it involves internal wiring, sensors and the boiler electronics, and because gas appliances must be worked on only by qualified engineers, this fault should be diagnosed and repaired by a Gas Safe registered engineer unless you are fully competent and authorised to work on gas boilers.
Troubleshooting Steps
Safety precautions:
- Do not work on gas components. If you are not a qualified engineer, do not attempt to repair internal gas or sealed electrical parts. Call a Gas Safe registered engineer.
- Before any basic checks, switch the boiler off at the mains and isolate power. Allow the boiler to cool. If you open casing panels you may expose live terminals and gas components; only proceed if you are confident and competent.
- If you smell gas at any time, evacuate and call the gas emergency number immediately.
Initial homeowner checks (safe, visual, non-invasive):
1. Note the exact error code (F.13) and any other codes displayed (sometimes F.91 appears with it). Record when it happened and whether a reset clears it temporarily.
2. Try a simple reset: switch the boiler off at the mains, wait 30 seconds, switch on and try to restart. If the fault returns immediately or shortly after, further checks are needed.
3. Check for visible water leaks or signs of condensate damage around the bottom of the boiler, beneath the condensate trap, or around sensors. Water ingress can corrode sensor connectors and cause short circuits.
4. Check system water pressure on the boiler gauge. Very low pressure can cause many faults; top up only if you know how and are comfortable doing so (follow the boiler handbook). Low pressure is not a typical direct cause of F.13 but is useful to report to your engineer.
5. Inspect any visible sensor wiring and plug connectors outside the sealed area for loose, corroded or damaged connections. Do not pull or force connectors attached to live circuitry.
If you are competent with basic electrical testing and prepared to follow the safety precautions (isolate mains first), you can do one diagnostic step with a multimeter:
1. Isolate the boiler from the mains power. Confirm the boiler is off and cool.
2. Locate the cylinder/plate heat exchanger sensor connector. On many Vaillant combi/combination appliances this sensor is accessible when the lower front cover is removed, but if you are unsure stop and call an engineer.
3. Carefully disconnect the sensor plug from the wiring harness. Set a multimeter to measure resistance (kΩ scale).
4. Measure the resistance across the sensor terminals. At room temperature a typical NTC for these Vaillant sensors is approximately 10–12 kΩ. Interpretation:
- A reading close to 0 Ω (very low) indicates a short circuit — consistent with F.13.
- A very high or infinite reading indicates an open circuit (broken sensor or break in wiring).
- A resistance roughly in the expected range (around 10 kΩ) suggests the sensor itself may be OK and the fault could be in the wiring/connector or the control PCB.
5. If the sensor reads as shorted or open, replace the sensor assembly. If the sensor reads normal but the fault persists, the wiring loom or the PCB/connector may be at fault.
Next steps and fixes (professional work recommended):
- If you confirmed a shorted or failed sensor, replacement of the cylinder/NTC sensor is the normal remedy. If replacement is required, a Gas Safe engineer should fit the correct part and test the system.
- If there is evidence of water/condensate damage or corrosion around the sensor or wiring, the engineer will clean/replace affected parts and investigate the source of the leak (condensate trap/sump/pipework or plate heat exchanger issues).
- If the sensor measures OK but the error persists the engineer will check the wiring harness, connector continuity and eBUS/PCB inputs. Faulty wiring, a short to earth, or a PCB fault can trigger F.13.
When to call a professional and what to tell them:
- Call a Gas Safe registered engineer if the fault returns after a reset, if you find a short/open on the sensor, if you see water damage, or if you are not confident performing the checks above.
- When you contact the engineer give the boiler model (Vaillant Turbomax Plus 824/828E), the fault code (F.13), any additional codes (e.g. F.91), what you observed (leaks, condensate damage), whether reset cleared the fault and any resistance reading you measured if you performed a multimeter test.
Notes and cautions:
- Do not attempt to repair gas valves, the PCB or internal wiring unless you are a qualified gas engineer.
- Replacing sensors or working inside the boiler without correct certification may be illegal and unsafe. Always use a Gas Safe registered engineer for repairs and any work that affects gas safety or the boiler’s sealed components.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant Turbomax Plus 824/828E.