What the F00 code means
Your boiler has several temperature sensors, known as thermistors or NTC sensors. The flow sensor sits on the pipe carrying hot water out of the heat exchanger towards your radiators. It tells the circuit board how hot that water is, many times a second, so the board can turn the burner up or down to hit the temperature you have set, and shut it off if things get too hot.
On most ecoTEC models, Vaillant's installation manual lists F00 as an interruption in the flow temperature sensor circuit. In practice this means the board is getting no sensible reading at all, as if the sensor had been disconnected. Without that reading the boiler has no way to control its temperature safely, so it refuses to light. You will usually lose heating, and on a combi you are likely to lose hot water too.
A related code, F10, is used on most ecoTEC models for a short circuit in the same sensor. F00 is the open circuit version: the signal has gone, rather than being shorted out.
Common causes of F00
A loose or unplugged connector
Vaillant's manual lists a sensor plug that is not plugged in or has worked loose as the first cause, along with the multi-pin plug on the PCB not being seated properly. If F00 appeared soon after a service or repair, a connector disturbed during the work is a strong possibility.
A break in the wiring
The wiring loom inside the boiler runs close to hot pipes and moving parts. Over time a wire can crack, chafe or pull out of its terminal, which interrupts the signal.
A failed sensor
Temperature sensors are fairly cheap parts that can fail with age or after repeated heating and cooling. A failed sensor usually reads as open circuit, which is exactly what F00 describes.
Water or corrosion on the contacts
A drip from a leaking joint inside the boiler can corrode the sensor terminals and break the connection.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Loose connector | F00 soon after a service or repair | Gas Safe engineer |
| Broken wiring | F00 comes and goes, especially when the boiler heats up | Gas Safe engineer |
| Failed sensor | F00 every time the boiler is asked for heat | Gas Safe engineer |
| Corroded contacts | F00 along with drips or damp under the boiler | Gas Safe engineer |
Safe checks you can do yourself
The fault is inside the boiler, so homeowner checks are about ruling out a glitch and helping the engineer:
- Write down the code exactly as shown and note whether it appeared after a service, a repair or a power cut.
- Check the fused spur for the boiler is switched on.
- Switch the boiler off at the fused spur, wait about a minute and switch it back on. This lets the board restart and take a fresh reading.
- Press reset as your manual describes if the code is still showing.
- Look under the boiler, without removing any covers, for drips or a damp patch. Our guide on a boiler leaking water explains what to look for.
- If F00 returns after two or three attempts in total, leave the boiler off and call an engineer.
If F00 showed up within a few days of a service or repair, call the engineer who did that work first. A connector knocked during the visit is a common cause, and most engineers will come back to check their own work.
When to call a Gas Safe engineer
F00 is not something you can fix yourself. Call a Gas Safe registered engineer if a power cycle and a reset do not clear it, or if it keeps coming back. Only a registered engineer should remove the casing, because the sensor sits right next to the gas and combustion parts.
The engineer will check that the sensor plug and the PCB connector are seated properly, test the wiring for breaks, and measure the resistance of the sensor to see if it has failed. If the sensor is at fault, replacing it is a quick job on most ecoTEC models. It is rare for F00 to need a new circuit board, but the engineer will confirm this once the sensor and wiring are ruled out.
Do not remove the boiler casing or try to reconnect anything yourself. If you smell gas at any point, turn off the supply at the meter if safe to do so and call the National Gas Emergency Service on 0800 111 999.
Typical repair costs in the UK
F00 is usually one of the cheaper internal faults to fix. These are typical ranges including parts and labour.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Reconnecting or repairing a loose connector | Typically £80 to £130 |
| Replacing the flow temperature sensor | Typically £90 to £180 |
| Repairing the wiring loom | Typically £100 to £200 |
| Replacing the main PCB (rarely needed) | Typically £250 to £500 |
How to stop F00 coming back
There is not much you can do to stop a sensor ageing, but you can reduce the risk of damage. Keep the boiler dry by getting drips and leaks fixed promptly, and avoid knocking or leaning things against the casing. A yearly service lets an engineer spot a corroded connector or a tired sensor before it fails completely, and our boiler service cost guide explains what a service usually includes. If you see F01 instead, that is the return sensor, covered on our Vaillant F01 page. Other ecoTEC codes are listed on our Vaillant fault codes page.