What the F01 code means
Water leaves the boiler hot, travels round your radiators and comes back cooler. The return temperature sensor sits on the pipe bringing that water back into the boiler. On most ecoTEC models, Vaillant lists F01 as an interruption in the return temperature sensor circuit, which means the board is no longer getting a usable reading from it.
It might seem that only the outgoing temperature matters, but the return reading does real work. By comparing it with the flow reading, the boiler can tell how much heat the radiators are taking and whether water is moving properly. It also uses the return temperature to fine-tune the burner, which matters for a condensing boiler because it runs most efficiently when the returning water is cool enough to condense the flue gases. Without this information the boiler stops rather than guess. As with most sensor faults, it will not light while F01 is showing, so expect no heating and, on a combi, no hot water until it is fixed.
On most ecoTEC models a short circuit in this sensor has its own code, F11. F01 is the open circuit case, where the signal has been lost.
Common causes of F01
Plugs and connectors
The manual's first suggestion is that the sensor plug has not been pushed home or has worked loose, or that the multi-pin plug on the PCB is not seated correctly. Vibration, heat and work carried out inside the boiler can all disturb these connections.
Wiring damage
A wire that has chafed against the casing, cracked with age or pulled away from its terminal will interrupt the signal. This type of fault can be intermittent, appearing when the boiler warms up and the metal expands.
The sensor itself
Return sensors fail in the same way as other thermistors, usually giving an open circuit reading once they stop working. Replacing the sensor is a routine job.
Moisture
Water from a weeping joint or a condensate leak inside the case can corrode the contacts. If you have seen water around the boiler, that is useful for the engineer to know.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Plug not seated | F01 after recent work on the boiler | Gas Safe engineer |
| Damaged wiring | F01 comes and goes as the boiler warms up | Gas Safe engineer |
| Failed return sensor | F01 every time the boiler tries to run | Gas Safe engineer |
| Moisture on contacts | F01 with damp or drips near the boiler | Gas Safe engineer |
Safe checks you can do yourself
You cannot test or replace the sensor yourself, but these steps rule out a temporary glitch and give the engineer useful facts:
- Note the code and anything that happened just before, such as a service, a leak, a power cut or a storm.
- Make sure the fused spur supplying the boiler is on.
- Turn the boiler off at the fused spur for about a minute, then back on, so the board restarts.
- If F01 is still displayed, press reset in the way your manual describes.
- Check the floor and pipework under the boiler for water, without taking any covers off.
- Stop after two or three attempts in total. If F01 keeps coming back, leave the boiler off and book an engineer.
It is also worth checking the pressure on the gauge or display while you are there. It will not cause F01, but if it is low you can top it up so the engineer has one less thing to sort. Our guide on how to repressurise a boiler explains how.
When to call a Gas Safe engineer
Book a Gas Safe registered engineer when F01 will not clear or returns after a few days. The engineer will check the connectors at the sensor and on the PCB, test the wiring for continuity and measure the sensor's resistance against the expected value for the water temperature. That tells them quickly whether the sensor, the wiring or, less often, the board is the problem.
If F01 appears at the same time as F00, the two sensors share wiring and connectors, so the fault is more likely to be in the loom or the PCB plug than in both sensors at once.
Do not open the boiler to check connections yourself. The sensors sit beside the gas and combustion components. If you smell gas, 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
Return sensor faults are usually straightforward. These typical ranges include parts and labour.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Re-seating a plug or repairing a connector | Typically £80 to £130 |
| Replacing the return temperature sensor | Typically £90 to £180 |
| Repairing the wiring loom | Typically £100 to £200 |
How to stop F01 coming back
Keep the boiler and the space around it dry, and get any drips fixed quickly. Keep the cupboard clear so nothing knocks the casing. Have the boiler serviced every year, as an engineer can spot corrosion on connectors and replace a sensor that is starting to drift before it fails. If a sensor has been replaced, ask the engineer to run the heating and hot water for a while before leaving, so you know the fault has really cleared. Our boiler service cost guide explains what a service includes. If your boiler is showing a different code, our Vaillant fault codes page lists the others we cover.