What does the Atag STOP fault code mean?

Your boiler is telling you that the internal water pressure is too low, so you need to open your filling loop to top it up to the correct level.

How much does it cost to fix Atag error code STOP?

Cost varies. Contact a Gas Safe registered engineer for a quote.

Locate the filling loop, which is usually a silver flexible hose with one or two valves located underneath your boiler.

Filling up slowly to 1.5 to 1.7 bar is reached; close filling loop

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Six most common causes of dropping pressure.

Step-by-step using the filling loop.

Safety-critical — turn off, read this.