ATAG 78 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 78 fault code mean?
Fault code 78 appears on ATAG boilers when the system water pressure has dropped below 1 bar, or when the boiler's control system cannot detect a pressure increase from the circulation pump after it has been running at full speed for around 5 seconds. In plain terms, the boiler is either running low on water pressure, or it cannot confirm that the pump is actually moving water around the system. Both conditions can trigger the same code, so a little detective work is needed to identify which is the true cause.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Low system water pressure Common
If the pressure gauge on your boiler reads below 1 bar, the boiler will lock out with code 78. This is the single most common trigger. Pressure can drop gradually through normal operation, or more quickly if there is a small leak somewhere on the system — at a radiator valve, a compression fitting, or around the boiler itself.
- Faulty or undercharged expansion vessel Common
The expansion vessel absorbs pressure changes as the system heats and cools. Over time the internal diaphragm can split or the air charge behind it can be lost, causing pressure to fluctuate erratically. If the vessel is no longer doing its job, pressure may fall and trigger code 78 even after you have topped the system up.
- Faulty water pressure sensor Sometimes
The pressure sensor feeds a live reading to the PCB. If the sensor becomes contaminated, corroded, or develops an electrical fault, it can report falsely low pressure even when the actual system pressure is perfectly normal. The engineer will typically check the wiring continuity to the sensor before condemning it.
- Circulation pump failure or airlock Sometimes
Because code 78 is also triggered by a lack of detectable pressure increase from the pump, a pump that has seized, is air-locked, or is running but not generating sufficient flow can cause the fault even when system pressure is adequate. An airlock can sometimes develop after draining work or a prolonged loss of pressure.
- Internal blockage or motorised valve fault Rare
Sludge, scale, or a stuck motorised valve can restrict flow enough that the pump cannot build the pressure differential the boiler expects to see. This is more common in older, unserviced systems that have not been chemically cleaned.
- PCB fault Rare
In rare cases where the pressure sensor, pump, wiring, and system pressure have all been confirmed as correct, the PCB itself may be misreading signals or failing to power the pump correctly. This is a last-resort diagnosis made by an engineer after all other possibilities have been ruled out.
How to fix it
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front of your ATAG boiler. It should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, low pressure is almost certainly the trigger for code 78 and you can address this yourself before calling anyone out.
- Top up the system pressure via the filling loop DIY safe
Locate the filling loop — usually a braided flexible hose with one or two valves underneath the boiler. Slowly open the valve(s) and watch the pressure gauge rise. Stop when it reaches approximately 1.2 to 1.5 bar, then close the valve(s) firmly. Never overfill above 2 bar. Once done, press the reset button on the boiler and allow it to restart.
- Reset the boiler DIY safe
Press and hold the reset button as described in your ATAG user guide. Allow the boiler a minute or two to run through its startup sequence. If the pressure was the only issue, the boiler should resume normal operation. Avoid resetting more than two or three times in succession — repeated lockouts suggest an underlying fault that needs professional attention.
- Monitor pressure over the next 24–48 hours DIY safe
Check the gauge the following morning and again after a full heating cycle. If the pressure has dropped again, there is likely a leak or a failing expansion vessel. Inspect visible pipework, radiator valves, and the area around the boiler for damp patches. Do not keep topping up repeatedly without finding the root cause — a persistent pressure loss needs an engineer.
- Check for a frozen or blocked condensate pipe DIY safe
In cold weather, ATAG condensing boilers can lock out because the plastic condensate pipe outside has frozen. This is separate from code 78 but worth ruling out. If the pipe is frozen, carefully pour warm (not boiling) water over the external section to thaw it, then reset the boiler.
- Have a Gas Safe engineer inspect the expansion vessel Gas Safe engineer
If pressure keeps dropping back after topping up, the expansion vessel is the most likely culprit. An engineer will check the Schrader valve air charge and test the diaphragm. A flat or burst vessel can sometimes be recharged; if the diaphragm has split, the vessel will need replacing. This is not a DIY task.
- Have a Gas Safe engineer test the pressure sensor and wiring Gas Safe engineer
If system pressure is consistently correct but code 78 persists, an engineer will check the wiring loom and connectors at the pressure sensor (including continuity from the PCB connector) for corrosion, damage, or loose pins. A faulty sensor will be replaced. Prices for the sensor itself are relatively modest, but correct diagnosis matters to avoid unnecessary parts changes.
- Have a Gas Safe engineer assess the circulation pump Gas Safe engineer
The engineer will check whether the pump is receiving correct voltage, spinning freely, and generating the expected pressure differential. An air-locked pump can sometimes be bled; a seized or burnt-out pump will need to be replaced. ATAG technical support may be consulted during diagnosis.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If any of the above steps have not resolved code 78, or if you are not comfortable carrying out the homeowner checks, contact a Gas Safe registered engineer. Ensure they are familiar with ATAG boilers — ATAG operates a UK technical support line that engineers can call during diagnosis. Note that ATAG's 10-year warranty requires annual servicing; using an unqualified person could void your cover.
Parts you may need
- Water pressure sensor · from £25
- Expansion vessel (internal replacement) · from £65
- Grundfos or Wilo circulation pump (compatible) · from £85
- Filling loop assembly · from £20
- PCB (main control board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix ATAG fault code 78 myself?
Partially. Topping up the system pressure via the filling loop and resetting the boiler are safe homeowner tasks and cost nothing. However, if the pressure keeps dropping, or if the boiler reads code 78 despite normal pressure, the underlying cause — a failing expansion vessel, pressure sensor, or pump — must be diagnosed and repaired by a Gas Safe registered engineer.
Why does my ATAG boiler keep losing pressure and showing code 78?
Repeated pressure loss almost always points to either a water leak somewhere on the system (check radiator valves, visible pipework joints, and the area directly under the boiler) or a failed expansion vessel diaphragm. A one-off top-up is fine, but if you are topping up more than once every few weeks, you need an engineer to find the root cause rather than just refilling the system each time.
How much does it cost to repair ATAG fault code 78 in the UK?
The cost depends entirely on what is actually wrong. Re-pressurising is free and takes five minutes. An expansion vessel recharge or replacement typically costs £180–£350 including labour. A new pressure sensor is usually under £30 for the part, with most of the bill being the engineer's time — expect around £120–£180 all in for a straightforward sensor swap. A circulation pump replacement tends to come in at around £200–£320 parts and labour. In rare cases where the PCB is at fault, parts alone can be £250–£365 plus VAT and labour, which pushes the total bill higher — but this is uncommon and should only be reached after all other causes have been eliminated.
Will ATAG's 10-year warranty cover a fault code 78 repair?
ATAG boilers come with a 10-year manufacturer's warranty as standard, but it is conditional on the boiler being serviced by a qualified engineer every year. If annual services have been carried out and documented, a covered component failure should be dealt with under warranty. Contact ATAG UK directly to check your warranty status before authorising any paid repair — having your service history to hand will speed things up considerably.