Overview
F.00 on a Vaillant Ecomax Pro 828E is an NTC fault meaning the boiler has detected a problem with one or more temperature sensors (NTC thermistors) used to measure flow/pipe temperature. The boiler uses these sensors to control combustion and pump operation; if the control board sees an open circuit, short circuit or implausible reading it will lock out and show F.00 to protect the appliance. Common causes are a disconnected or loose sensor plug, damaged wiring harness, a failed thermistor (open or short), or less commonly an electronic/PCB fault. Severity: the fault is protective rather than dangerous — the boiler will usually lock out so you will lose heating and/or hot water until the issue is fixed. It should be treated promptly because repeated attempts to run without valid temperature feedback can cause overheating or other faults. Some basic checks are safe for a competent homeowner (power-cycle, visual checks, checking pressure), but sensor replacement, wiring repairs inside the boiler, or any work involving gas must be carried out by a Gas Safe qualified engineer. If you are not experienced with electrical testing and boiler internals, call a professional.
Possible Cause: All of these are NTC faults, and are best resolved by first changing all NTC to make diagnosis earlier.
Troubleshooting Steps
Safety precautions
- If you are not competent with electrical work or gas appliances, call a Gas Safe engineer — do not attempt internal repairs. Working on gas and sealed parts is hazardous.
- Before opening the boiler casing isolate the mains electricity at the isolator or consumer unit and allow the boiler to cool. Turn off the gas supply at the appliance gas tap if you will be disturbing gas-work (but do not attempt any gas repair yourself).
- Use insulated tools and a multimeter rated for domestic use. Wear eye protection and take care with sharp sheet metal inside the casing.
Initial homeowner checks (safe, quick checks)
1. Note the code and when it appears (on start, continuously, after power cycle). Try a reset: use the boiler reset button if present or switch the boiler off at the isolator for 10–30 seconds and switch it back on. If the code does not clear or returns immediately proceed to the next steps.
2. Check system pressure at the gauge — if pressure is very low or zero that can cause unrelated faults; top up to the recommended level (usually ~1–1.5 bar) if you know how.
3. Look for obvious signs: water leaks, corroded connectors, damaged cables, or a recently disturbed wiring loom or sensors (recent servicing/works can leave a plug loose).
Specific diagnostic and fix steps (for someone comfortable with basic electrical checks)
1. Isolate mains power to the boiler and remove the front cover following the manufacturer instructions. Ensure the boiler is cool.
2. Locate the flow and return NTC sensor connectors — typically clipped to the flow and return pipes near the heat exchanger. Visually inspect connectors and wiring for chafing, broken wires, corrosion or disconnected plugs. Re-seat any loose connectors firmly.
3. With power still off, perform a continuity check of the wiring loom if you suspect a broken lead: disconnect the sensor plug at the sensor and at the PCB connector and check continuity between the two ends of the same wire. If continuity is open, the harness is faulty and should be repaired or replaced.
4. Measure the sensor resistance with a multimeter across the two sensor pins (sensor unplugged). NTCs show higher resistance when cold and lower when warm (resistance falls as temperature rises). Many domestic boiler NTCs are around 10 kΩ at ~20–25 °C, but values vary by model — check the service manual or compare to another known-good sensor if possible. An infinite/open reading indicates a broken/open sensor; a very low reading (close to 0 Ω) indicates a short. Either condition means the sensor should be replaced.
5. If the sensor reads plausibly and wiring continuity is good, re-seat connectors at the PCB and check for corrosion on the PCB pins. Power the boiler and see whether the fault persists (only power up if you are confident connections are secure). Note: do not touch live circuits while powered.
6. If you replace a failed NTC, follow the manufacturer’s recommended replacement part and fit in the correct location (flow vs return). After replacement isolate/restore power and reset the boiler per the manual. Observe for normal operation. The manufacturer notes that changing all NTCs can speed diagnosis, so if one sensor has failed and others are original it is reasonable for the engineer to replace multiple sensors to avoid repeat callouts.
When to call a professional
- If the fault cannot be cleared by reseating connectors or replacing a clearly failed sensor, or if wiring harness continuity is lost and you are not confident repairing it.
- If you find PCB damage, burnt connectors, intermittent or unexplained sensor readings, or if the fault recurs after sensor replacement.
- Any time work on gas pipework, the burner, electrodes or sealed parts is required — only a Gas Safe registered engineer should do this.
Additional notes
- Keep a record of what you tried, the exact fault messages and any changes made — this helps a visiting engineer diagnose faster.
- Do not attempt to hide or bypass safety interlocks. If you are unsure, stop and call a Gas Safe engineer. They will check sensors, wiring, and the PCB and replace parts correctly and safely.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant Ecomax Pro 828E.