This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Vaillant F.93 fault codewhat it means and how to fix it
Vaillant print F.93 as "check gas route": combustion quality is outside the permitted range. The causes are the gas injector, the gas group, flue gas recirculation and a blocked pressure point in the venturi.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.93
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.93 means
F.93 is the ecoTEC plus ..6/5-5 telling you that its combustion is measurably wrong. The table reads "fault: check gas route — combustion quality outside the permitted range", and the engineer's list is short and specific: the gas injector, which must suit the gas group; flue gas recirculation; the gas group itself; and the internal pressure measuring point in the venturi for blockage — with Vaillant's standing warning not to use any lubricant on the venturi O-ring.
The row belongs to the plus alone; the pro, exclusive, ecoFIT, ecoTEC sustain and 2022 CS/1-5 tables do not print F.93, the exclusive reporting its combustion faults through F.55 to F.57 and the CS/1-5 through F.040 and F.044. The four causes sort into two kinds. The injector and the gas group are configuration: a boiler converted or commissioned for the wrong gas, or with the wrong injector fitted, will not burn cleanly.
Recirculation and the blocked pressure point are the boiler's surroundings and its own venturi: flue products drawn back into the intake dilute the air, and a blocked tapping gives the board the wrong pressure to regulate against. Because one of Vaillant's four causes is a flue fault, the page carries the carbon monoxide flag. There is no householder step; every item is behind the case.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flue gas recirculation | Exhaust re-entering the air intake through a poor flue joint or a terminal in a sheltered position. On Vaillant's list and the reason for the CO flag. | Common |
| Pressure measuring point in the venturi blocked | The tapping the board reads to regulate combustion is obstructed. Cleared during a venturi inspection, without lubricating the O-ring. | Common |
| Gas injector wrong for the gas group | An injector for natural gas on LPG or the reverse, usually after a conversion. Vaillant's "gas injector (suitable for the gas group)". | Occasional |
| Gas group set wrongly | The gas family entered at commissioning does not match the supply. Corrected through the gas family check in the installer level. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£380
Time on site
1 hour
A venturi clean with a combustion check is £100 to £170. An injector or configuration correction after a conversion should be the converting firm's. Flue work is priced on inspection; a gas valve is £280 to £380.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant ecoTEC gas valve
0020053968£110–£210Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.57Get it checkedThe combustion regulation's measuring programme has found a fault — on the exclusive, a badly corroded ignition electrode; on the 2022 CS/1-5, F.057 means the regulation and its fallback mode have both failed.Related: combustion control
- F.56Stop — call an engineerA safety shutdown because the boiler's combustion regulation measured carbon monoxide in the flue gas above its limit. Vaillant say that if it comes back after a reset, the gas valve is defective.Related: combustion control
- F.55Get it checkedThe CO sensor — Vaillant's "all-gas sensor" that lets the ecoTEC exclusive regulate its own combustion — is defective or short-circuited. The boiler will not run without it.Related: combustion control
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: same brand
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: same brand
- F.28Stop — call an engineerThe boiler tried to light and failed every attempt it is allowed — three on an ecoTEC, five on the 2022 CS/1-5 — so it has shut the gas off and locked out.Related: same brand
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.93
Is F.93 dangerous?
It is the boiler detecting that its combustion is off and stopping, which is right. It is not a report of CO in the room. Make sure your alarm works, do not repeat resets, and book a Gas Safe engineer.
Why "check gas route"?
Vaillant's title for the row covers everything between the gas valve and the flame: the injector, the venturi and the air the gas mixes with. F.93 says the result of that route is outside what the board will accept.
Can a reset clear it?
One press of the fault clearance key is allowed. Combustion that is wrong for a configuration reason will be wrong again on the next firing, and the code returns.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.