High
C 78 on Atag A325ECX

Your boiler is struggling to circulate water, either because there isn't enough water inside the system or because the internal pump has stopped working.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

What the manual says

Manufacturer manual

Water pressure outside of range or not connected or if water pressure is OK: Pump failure

From this model's manual
Verified against the Atag/Atag A200 System installation & service manual, page 36.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

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

Find a Gas Safe engineer
Search the official register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like C 78: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

All 32 fault codes on the A325ECX

Seeing a different code? Every code this model can display, in one place.

102040506178105117A0A1A2A4A5A6A9c 1 05C 20C 40C1 10C1 11c1 17c1 18C1 29C1 33C1 51c1 54COdEF1FILLHIGHSTOP

Related guides you might find useful

Hot water works but heating does not

On a combi it is usually the diverter valve. On a system boiler, the motorised valve.

Read →

What pressure should my boiler be?

1 to 1.5 bar when cold on most UK boilers, from the manuals.

Read →

Boiler keeps losing pressure

Six most common causes of dropping pressure.

Read →