What F84 means
Your boiler has two temperature sensors on its heating pipework. The flow sensor measures the water leaving the boiler for the radiators, and the return sensor measures the water coming back. When the burner is heating, the water going out should be warmer than the water coming back. The board compares the two readings all the time.
Vaillant's ecoTEC plus installation and maintenance instructions (document 0020244997) list F84 as "Fault: NTC temp. diff. implausible". The 2021 instructions for the open-vent ecoTEC plus (document 0020238195) are more precise: the flow and return values are "not consistent", with a difference below minus 6 K. A kelvin difference is the same size as a degree Celsius difference, so in practice F84 means the return reading has come out more than 6 degrees warmer than the flow reading. That should not happen in normal heating, so the boiler assumes it cannot trust its sensors and stops. It uses these two readings to control the burner and protect the heat exchanger, so it will not run on figures that contradict each other.
What causes F84
The flow and return sensors have been swapped
The 2021 manual puts it plainly: "Flow and return temperature sensors have been inverted." If the two sensors or their plugs were swapped during a repair, the boiler sees the readings the wrong way round and reports F84.
A sensor is not fitted correctly on its pipe
The ecoTEC plus ../5-5 manual tells the engineer to check the sensors for correct thermal connection. These sensors clip onto the outside of the pipes. One that has slipped, sits loose or has been fitted in the wrong place will read the wrong temperature.
A sensor giving a false reading
Less directly, a sensor that has drifted out of range can also produce numbers that do not agree with its partner. The engineer will test each sensor once the fitting has been checked.
| Clue | What it suggests |
|---|---|
| F84 appeared soon after a sensor, pump or board was replaced | Sensors or plugs swapped, or a sensor not refitted properly |
| F84 on a boiler with no recent work | A sensor that has moved on its pipe or is reading falsely |
| F84 alongside F00 or F01 | Possibly one sensor failing, upsetting the comparison |
Which Vaillant boilers use F84
- ecoTEC plus VU and VUW ../5-5 regular, system and combi models (combi 825 to 838), document 0020244997.
- ecoTEC pro 24 and 28 combis, document 0020244995.
- ecoTEC plus VU ../6-5 OVZ open-vent regular boilers, document 0020238195 (2021).
F84 is not in the fault table of the ecoTEC plus VU 246/6-5 OVZ LPG manual (document 0020239015) that we checked. If you have that boiler and see F84, confirm the code with an engineer.
F84 and the other Vaillant sensor codes
Vaillant has a family of codes built around the flow and return sensors, and the differences are useful when you speak to an engineer:
- F83: the temperature difference is too small. The 2021 manual calls this "Dry fire" and links it to too little water or a sensor in the wrong position.
- F72: the difference between flow and return is too great.
- F84: the difference is implausible, with the return warmer than the flow.
- F85: the sensors are fitted incorrectly and return incorrect or implausible values.
- F00 and F01: a fault with the flow or the return sensor itself.
What you can safely do
The fix for F84 is inside the casing, so your part is gathering information and ruling out the simple things.
- Think back over the last few weeks. Has anyone worked on the boiler? Note the date and what was changed.
- If there was recent work, contact that engineer first. Sensors fitted the wrong way round are something they should put right.
- Check the pressure reading. It is not the cause Vaillant names for F84, but low water also upsets flow and return readings, so it is worth ruling out. Top up through the filling loop if it is below 0.8 bar cold.
- Press reset as your manual describes. If F84 returns, stop after two or three attempts.
- Write down any other codes the boiler shows, in the order they appear.
The sensors are inside the casing, next to hot pipes and live wiring. Do not remove the casing to look at them or try to refit them. That is a job for a Gas Safe registered engineer.
What the engineer will check, and typical costs
Vaillant's measure for F84 is to check the flow and return sensors for correct thermal connection and to check whether they are inverted. The engineer will look at where each sensor sits and how it is clipped on, confirm the plugs go to the right places, and then test the readings against the actual pipe temperatures. If a sensor is faulty, it is replaced.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Refitting or correcting sensor connections (minor repair) | Typically £100 to £180 |
| Flow or return temperature sensor (NTC) replacement | Typically £100 to £200 |
If F84 is the result of a recent repair, ask the original engineer to return before paying for a new call-out, and keep the job sheet so you know exactly which parts were changed. If the boiler is still under guarantee, speak to Vaillant or your installer first. When you speak to them, mention the code, when it started, what was replaced and whether the boiler runs at all before it locks out. The other ecoTEC sensor codes are listed on our Vaillant fault codes hub.