Keston E30 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E30 fault code mean?
The E30 fault code on Keston boilers signals a problem that varies slightly by model. On the C40 and C55 — the most widely installed Keston models — it indicates that the temperature gap between the flow pipe (hot water leaving the boiler) and the return pipe (cooled water arriving back) has grown too large, pointing strongly to a circulation failure. The boiler detects this as a potential overheating risk and shuts itself down into lockout, meaning it will not restart until the reset button is pressed. On the C36 model, E30 can instead indicate a flow sensor short circuit or an unstable flame that has been lost five times within a four-minute window, in which case gas supply, combustion, air supply, and flue condition all need attention.
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
- Circulation pump failure or seizure Common
The circulating pump is responsible for pushing heated water from the boiler around the entire central heating system and back again. When the pump seizes or fails, hot water stagnates in the heat exchanger, the flow-to-return temperature difference spikes, and E30 is triggered. Seized impellers are particularly common in older systems or those that have stood idle over summer months.
- Airlock in the circulation pump or pipework Common
Air trapped in the pump or pipework prevents water from moving freely. The pump may sound unusually noisy — a grinding or humming tone — and some radiators may stay cold while others heat up. An airlock can mimic a full pump failure but is sometimes easier to resolve by bleeding the pump via its bleed screw.
- Sludge or debris restricting water flow Sometimes
Years of corrosion inside older radiators and pipework produce iron oxide sludge, which can partially or fully block the heat exchanger or pump. Restricted flow causes the boiler to overheat locally, widening the flow-return differential and triggering E30. A kettle-like boiling noise from the boiler is a common tell-tale sign.
- Wiring or electrical fault to the pump Sometimes
Faulty wiring, a failing relay, or a defective PCB output can prevent the pump from receiving the correct electrical signal, so it either runs intermittently or not at all despite appearing physically intact. This is harder to diagnose without specialist test equipment.
- Flow sensor short circuit (C36 models only) Rare
On the C36, a short circuit in the flow sensor or its wiring harness can generate an E30 code without any underlying circulation problem. The sensor connections and wiring loom should be inspected before assuming pump failure.
- Unstable flame or ignition issue (C36 models only) Rare
Also specific to the C36, repeated flame loss — five dropouts within four minutes — can trigger E30. Possible causes include insufficient gas pressure, a partially blocked flue, inadequate air supply, or a deteriorating burner or gas valve.
How to fix it
- Press the reset button once DIY safe
Locate the reset button on the boiler's control panel and press it firmly. Wait at least 30 seconds to see whether the boiler restarts and runs normally. You can attempt this up to two or three times, but repeated resets without a clear outcome will not fix the underlying fault and may mask it further.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home — a gas hob, for example — are working normally. If they are not, contact your gas supplier, as a supply interruption could be contributing to the fault, particularly on C36 models where flame instability is a factor.
- Inspect the system pressure gauge DIY safe
Look at the pressure gauge on the boiler. It should read between 1.0 and 1.5 bar when cold. If it has dropped below 1 bar, use the filling loop to top it up slowly to around 1.2 bar, then try resetting the boiler again. Low pressure reduces water flow and can worsen temperature differentials.
- Attempt to bleed an airlock from the pump (if it has a bleed screw) DIY safe
If your pump is making grinding or humming noises, check whether it has a bleed screw on the front face. If it does, place a cloth underneath, turn the screw very slightly anti-clockwise, and wait for any hissing to stop before re-tightening. This releases trapped air. If the pump has no bleed screw, do not attempt further intervention — call an engineer.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the boiler continues to display E30 after the checks above, a qualified engineer must attend. They will test whether the pump is running, measure flow rates, check for sludge using a system water sample, inspect the flow sensor and wiring on C36 models, and assess the PCB if an electrical fault is suspected. On C36 boilers with flame instability, they will also inspect the flue, air intake, burner, and gas valve. UK law requires all work on gas boiler components to be carried out by a Gas Safe registered engineer.
Parts you may need
- Central heating circulator pump (e.g. Grundfos UPS2 or equivalent) · from £120
- Flow temperature sensor · from £25
- Pump wiring loom or relay · from £35
- 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
Can I fix a Keston E30 fault myself?
There are a small number of safe checks you can carry out yourself: resetting the boiler, confirming your gas supply is on, topping up system pressure if it has dropped, and bleeding an airlock from the pump if it has a bleed screw. Beyond these steps, the fault almost always requires a Gas Safe registered engineer. Pump replacement, sludge clearance, sensor testing, and any work near the gas valve or PCB are all legally required to be carried out by a registered professional.
How much does it cost to fix a Keston E30 fault in the UK?
Most repairs fall in the £150–£400 range. A minor fix such as freeing a seized pump impeller or bleeding an airlock will sit at the lower end, while a full pump replacement including parts and labour typically costs £200–£350. A power flush to clear heavy sludge can add £300–£500 on top, and if the PCB is at fault, that part alone can cost £200 or more. Emergency call-outs outside normal working hours often attract a premium of £100–£200 above standard rates.
Why does the E30 code mean different things on different Keston models?
Keston updated its diagnostic logic across its C-series range. On the C40 and C55, E30 is dedicated to flagging an abnormally high temperature difference between the flow and return pipes — almost always a circulation issue. The older C36 platform uses the same code number for two distinct faults: a short circuit on the flow sensor, and repeated flame instability. If you are unsure which model you have, check the data plate on the front or inside the casing — the model name is printed there.
Will the Keston boiler restart on its own after an E30 lockout?
No. Once the boiler enters lockout following an E30 fault, it will not attempt to restart automatically. You must press the dedicated reset button on the control panel. Simply switching the boiler off and on at the programmer or isolator switch is not the same as a proper reset and will not clear the lockout.