Main E168 Fault Code: Causes, Fixes & Repair Costs
What does the Main E168 fault code mean?
The E168 is Main's catch-all lockout code — sometimes called the 'mystery code' — because it doesn't point to a single specific component. Instead, it signals that the boiler's PCB has shut the appliance down after detecting a problem it cannot clearly categorise. In practice, E168 most often appears after a failed ignition sequence (the boiler tried to light the burner several times but the ionisation probe never confirmed a stable flame), after mains voltage fluctuated outside the acceptable ±10% of 230 V, or as a residual code left behind when an earlier fault (such as E119 low pressure or E133 ignition failure) was cleared by a reset without the underlying issue being resolved. Because E168 is shared across the Main, Baxi, and Potterton ranges from the same manufacturer group, guidance for one brand applies equally to the others. The boiler will not provide heating or hot water while this code is active.
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
- Masked or residual fault from a previous code Common
This is the most frequent trigger for E168. The boiler may have already displayed a more descriptive code — such as E119 (low system pressure), E133 (flame not established), E110 (overheat thermostat tripped), E20 (flow temperature sensor), E28 (flue gas temperature sensor), E156 (under-voltage), or E160 (fan fault) — before being reset. Once the original fault clears from memory but the root cause remains, the PCB locks out and displays E168 because it no longer has enough information to show the original code. Checking whether any other codes appeared before E168 is the most useful first step.
- Ignition or flame detection failure Common
Main boilers make several ignition attempts before locking out. If the ignition electrode produces a weak or absent spark, the gas valve does not open properly, or the ionisation probe fails to sense a stable flame, the PCB logs the sequence as a failed ignition and displays E168. Worn electrodes, a contaminated or mispositioned probe, a cracked ignition lead, or a reluctant gas valve can all cause this without necessarily triggering a more specific error first.
- Mains power interruption or voltage instability Common
The boiler's PCB monitors incoming mains voltage and locks out if it strays more than roughly 10% from 230 V — either too high or too low. A recent power cut, a tripped circuit breaker, loose wiring at the spur or boiler terminals, or interference from other appliances can all cause E168 to appear. The code can persist as a residual lockout even after power is restored.
- Gas supply problem Sometimes
If the gas meter is off, a supply valve has been accidentally closed, or there is a local network interruption, the burner will not light. After several failed attempts the boiler displays E168. Testing another gas appliance (a hob or gas fire) confirms whether the wider supply is the issue. Low gas pressure arriving at the boiler — which the boiler may not flag with its own pressure-specific code — can also trigger this fault.
- Internal component failure (pump, fan, diverter valve, wiring) Sometimes
A failing circulating pump, a sluggish fan, a sticking diverter valve, or degraded wiring connections can each create conditions where the PCB detects an abnormality but cannot assign it a precise code, defaulting instead to E168. These faults require hands-on diagnosis by an engineer with appropriate test equipment.
- Faulty PCB Rare
If the control board itself is damaged — through overheating, moisture ingress, or simple component ageing — it may produce spurious lockouts or fail to process sensor data correctly, causing it to default to E168. PCB failure tends to be a diagnosis of exclusion: an engineer will typically rule out all other components before condemning the board.
How to fix it
- Note any fault codes that appeared before E168 DIY safe
Before touching anything, think back to what the display showed earlier. Codes such as E119, E133, E110, E20, E28, E156, or E160 appearing before E168 can reveal the true root cause and are extremely useful information to give an engineer. Write down every code you have seen.
- Check that your gas supply is working DIY safe
Turn on a gas hob ring or check another gas appliance in the property. If nothing else is getting gas either, contact your gas supplier — the fault is external to the boiler and the E168 should clear once supply is restored.
- Check your fuse board for tripped switches DIY safe
Look at your consumer unit (fuse box) for any tripped RCDs or MCBs. If a switch has tripped, reset it. Also check that the boiler's isolation switch (usually on the wall nearby) is in the ON position. If you have had a recent power cut, the E168 may simply be a residual lockout from the supply interruption.
- Check the boiler pressure gauge DIY safe
The pressure gauge should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1 bar, top up the pressure using the filling loop (a pair of valves beneath the boiler connected by a short flexible hose). Open both valves slowly and watch the gauge, then close them once pressure reaches about 1.2 bar. Low pressure can trigger E119 which then leaves E168 behind as a residual code.
- Attempt a single boiler reset DIY safe
Press and hold the reset button for approximately 3 seconds, then release. Give the boiler up to 60 seconds to complete its ignition sequence. If E168 returns, do not attempt further resets — repeated resets without resolving the underlying fault can mask important diagnostic information and in some cases cause additional wear. A maximum of two or three resets total is reasonable before calling for help.
- Contact a Gas Safe registered engineer for diagnosis Gas Safe engineer
If the code persists after the checks above, a Gas Safe registered engineer is needed. They will use flue gas analysers and electrical test equipment to trace the fault through the ignition chain, sensors, wiring, and internal components. Do not attempt to open the boiler casing, adjust the gas valve, or interfere with any internal parts — this is both a legal and safety requirement for gas work in the UK.
- Allow the engineer to repair or replace the faulty component Gas Safe engineer
Common engineer repairs for E168 include cleaning or replacing the ignition electrode and ionisation probe, replacing a faulty gas valve, repairing wiring connections, replacing a pump or fan, or — as a last resort — fitting a new PCB. The engineer will advise on the most cost-effective course of action, particularly if the boiler is over 10 years old.
Parts you may need
- Ignition electrode · from £25
- Ionisation probe · from £20
- Ignition lead · from £18
- Gas valve · from £95
- Circulating pump · from £85
- Fan assembly · from £120
- PCB (printed circuit board) · from £220
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 £150–£400, depending on the underlying cause.
Frequently asked questions
Why does my Main boiler keep showing E168 after I reset it?
Repeated E168 appearances after resetting almost always mean the underlying fault has not been fixed. The most common reason is that an earlier fault code (such as E119 for low pressure or E133 for ignition failure) was reset before the root cause was addressed. Each time the boiler tries to run and hits the same problem, it locks out again with E168. If the code comes back more than once or twice, stop resetting and call a Gas Safe engineer to properly diagnose what is causing it.
Is E168 on a Main boiler the same fault as on a Baxi or Potterton?
Yes. Main, Baxi, and Potterton are all part of the same manufacturer group and share the same control systems. E168 has the same meaning and the same causes across all three brands. Guidance and parts are largely interchangeable, though you should always confirm compatibility with your specific model before ordering parts.
Could the E168 fault mean my PCB needs replacing?
It can, but a PCB replacement is a diagnosis of last resort. An engineer will first check all the more common and less expensive causes — ignition components, sensors, gas supply, wiring, pump, and fan — before concluding the PCB is at fault. PCB replacement including parts and labour typically costs £300–£700, putting it at the top end of the repair range. If your boiler is over 10 years old and the PCB has failed, it is worth getting a replacement boiler quote at the same time, as a new A-rated boiler may represent better long-term value.
Can I prevent E168 from coming back after it has been repaired?
Yes, to a reasonable degree. The most effective prevention is an annual Gas Safe service, which catches worn electrodes, degrading seals, loose connections, and other early-warning signs before they cause a lockout. You should also check boiler pressure monthly (it should sit at 1.0–1.5 bar when cold) and top it up after bleeding radiators, since dropping pressure is one of the most common triggers. If your home experiences frequent power fluctuations, a mains surge protector on the boiler's spur can help protect the PCB.