Vaillant Boiler F.20 Error Code

Overheat safety cut-out has tripped on Vaillant boilers.

F.20

May need a pro

Water in the boiler got hotter than the safety limit, so the cut-out shut the gas valve. Low water, a pump that isn't moving water, a stuck diverter valve or a faulty temperature sensor are the usual reasons; Vaillant also lists sparking from the ignition parts.

Compatible models

Shown on Vaillant's general UK fault code page and in the fault table for ecoTEC plus VU 126 to 356/6-5 OVZ models.

Also shown as

No other codes are listed for this fault.

Difficulty

Try the basic checks, but the usual fix is a technician's job.

How to fix
When to call a pro
Parts
FAQ

Not sure you are reading the code right? See how to read Vaillant boiler codes.

Safety first: never take the casing off a gas boiler. If you smell gas, open windows, don't use light switches, turn the gas off at the meter if you can and call your gas emergency number from outside. A carbon monoxide alarm going off means the same: switch the boiler off, get fresh air and call.

How to fix Vaillant F.20

Check the pressure first
Low water is on Vaillant's cause list, so look at the gauge. If it's under 0.8 bar, top up with the filling loop as the operating manual shows.
Let it cool and reset once
Leave it off until it has cooled down, then hold reset for more than 3 seconds.

When to call a technician

An overheat lockout that returns needs a Gas Safe engineer to check the pump, diverter valve and sensors. Don't keep resetting it.

Parts linked to F.20

These are the parts an engineer usually checks for this code. They sit inside the sealed casing, so leave them to a Gas Safe registered engineer.

Frequently asked questions

Which Vaillant models show the F.20 code?

Shown on Vaillant's general UK fault code page and in the fault table for ecoTEC plus VU 126 to 356/6-5 OVZ models.

Can I fix Vaillant F.20 myself?

Only the outside checks. The steps above are the ones Vaillant's user instructions allow; if they don't clear F.20, the fix is inside the boiler and needs a Gas Safe registered engineer (RGII in Ireland).

Other Vaillant boiler codes

Last reviewed 2026-10-21. Codes and steps can vary by model; check your owner's manual.

Sources we checked