20 Boiler Fault Code: What It Means on Every Brand

The 20 fault code appears on 3 boiler brands — ATAG, Intergas and Viessmann — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 3 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What 20 means	Severity	DIY?
AATAG	Fault code 20 on an ATAG boiler signals that the boiler's control system has detected a problem with the flow sensor, known internally as T1. Full guide →	Lockout — engineer likely	No
IIntergas	Fault code 20 on an Intergas boiler signals a problem with the S2 temperature sensor — one of two NTC (Negative Temperature Coefficient) sensors mounted on the heat exchanger. Full guide →	Lockout — engineer likely	No
VViessmann	Fault code 20 on a Viessmann boiler is a fan and burner control fault that causes the boiler to enter a lockout or emergency operating mode. Full guide →	Lockout — engineer likely	No

20 brand by brand

ATAG 20

Fault code 20 on an ATAG boiler signals that the boiler's control system has detected a problem with the flow sensor, known internally as T1. Specifically, the temperature or signal reading coming from T1 has fallen outside the expected operating range, so the boiler cannot confirm that water is circulating correctly through the primary circuit. The boiler locks out as a precaution to prevent it running dry or overheating. The root cause could be the sensor itself, the wiring connecting it to the PCB, a circulation problem preventing adequate flow, or a blockage in the pipework. A Gas Safe registered engineer will need to diagnose which of these is responsible.

Causes, fixes & costs for ATAG 20 → · All ATAG codes

Intergas 20

Fault code 20 on an Intergas boiler signals a problem with the S2 temperature sensor — one of two NTC (Negative Temperature Coefficient) sensors mounted on the heat exchanger. The S2 sensor sits at the bottom of the heat exchanger and continuously feeds water-temperature readings back to the PCB. When the PCB loses a valid signal from S2, it cannot safely manage combustion and shuts the boiler down in lockout. Codes 21 and 22 are closely related and also point to an S2 sensor fault (21 indicates the sensor is reading abnormally high; 22 indicates abnormally low), so if your display shows any of these three codes the diagnosis and repair path is essentially the same. The fix is usually straightforward — either a wiring issue or a failed sensor — and because the S2 sensor itself is an inexpensive part, this tends to be one of the cheaper boiler repairs.

Causes, fixes & costs for Intergas 20 → · All Intergas codes

Viessmann 20

Fault code 20 on a Viessmann boiler is a fan and burner control fault that causes the boiler to enter a lockout or emergency operating mode. Exactly what triggers it depends on your specific model. On most Viessmann boilers, the burner shuts down because the fan has not returned to a standstill after an auto-reset attempt, or because a plug-in wiring connection to the fan or burner control unit is unstable. On Vitodens 200-W, 222-F, and models running Vitotronic 200, 300, or 050 controllers, the boiler may instead switch to a fixed 'constant mode' output because the outdoor temperature sensor has developed a short-circuit. On the Vitodens 100-W specifically, the fault is often linked to DIP switch S2 being set incorrectly. If your display shows code 20, identify your boiler model first — the underlying problem and the repair route can differ significantly between series.

Causes, fixes & costs for Viessmann 20 → · All Viessmann codes

Frequently asked questions

Is the 20 fault code the same on every boiler?

Broadly, yes — on ATAG, Intergas and Viessmann the 20 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a 20 fault myself?

Not usually. On every brand listed, 20 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a 20 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.