Keston E99 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E99 fault code mean?
The E99 code on a Keston boiler signals a communication breakdown between the front fascia (display panel) and the internal control box (main PCB). The two components are no longer exchanging data correctly, causing the boiler to lock out and refuse to operate until the fault is resolved. Keston's own documentation describes E99 as carrying the same definition as E41 — a fascia-to-control-box communication fault — and the manufacturer's direct guidance is to check the wiring and cables connecting these two components. Because Keston boilers use a twin-flue, modulating design that many engineers find unfamiliar, diagnosis can take longer than on mainstream boilers. The fault will remain active until manually reset; simply switching the boiler off and on will not clear it.
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
- Loose or damaged wiring between fascia and control box Common
The most direct explanation for E99, and precisely what Keston's own documentation tells engineers to check. Vibration over years of operation, or work carried out inside the boiler casing, can loosen connectors or cause wiring to chafe and short. Even a slightly unseated plug on the ribbon or loom running between the display panel and the main board is enough to drop communication entirely.
- Faulty main control board (PCB) Common
If the wiring checks out, the main PCB itself may have developed an internal fault or lost its programmed parameters. Multiple Keston error codes in the E12–E22 range also point to control box failures, and E99 follows the same logic. Engineers should test the board under load before condemning it, as diagnosis is not always straightforward.
- Water ingress or leak damage to the PCB Sometimes
A boiler leak — from a heat exchanger, pump seal, or pipe joint — can drip onto electrical components inside the casing. A PCB damaged by water may intermittently lose communication before failing completely. It is important to trace and fix any underlying leak before replacing the PCB, otherwise the new board may suffer the same fate.
- Faulty fascia PCB Sometimes
The fascia board itself can fail independently. The closely related Keston code E39 specifically instructs engineers to replace the fascia PCB, indicating this component is a known weak point on these models. If the main board is healthy and wiring is intact, the fascia board is the next component to test and replace.
- Sensor fault causing PCB lockout Rare
In some cases the main PCB shuts down communication as a protective response to erroneous data from flow or return temperature sensors. If a sensor is sending implausible temperature readings, the control board may interpret this as an unsafe condition and lock out, displaying E99 rather than a dedicated sensor code. The engineer should log live sensor values to rule this out.
How to fix it
- Reset the boiler using the fascia button DIY safe
Locate the reset button on the boiler's front panel — on most Keston models it carries a spanner symbol. Press and hold it for 3–5 seconds until the display responds. Do not attempt to reset more than two or three times. If the code returns immediately or within a short time, resetting will not solve the underlying issue and further attempts may mask a more serious fault.
- Check there are no active boiler leaks DIY safe
Open the boiler casing cover (if accessible without tools) and visually inspect the area around the pump, heat exchanger connections, and pipe joints for any sign of dripping water, limescale tide-marks, or rust streaks. If you find a leak, report this to the engineer before they begin electrical diagnosis — water damage is a leading cause of PCB failure on Keston boilers.
- Inspect and reseat all wiring between the fascia and control box Gas Safe engineer
A Gas Safe engineer should carefully trace the wiring loom and ribbon cable running from the display panel to the main control board. Each connector should be unplugged, inspected for corrosion or burn marks, and firmly reseated. Any chafed or broken wires must be repaired or replaced. This is the specific check Keston's documentation recommends for E41/E99 faults.
- Test the main PCB and fascia PCB for damage or corrosion Gas Safe engineer
The engineer should inspect both circuit boards under good lighting for signs of water ingress, corroded tracks, bulging capacitors, or burn marks. Where possible, live voltage and signal readings should be taken at the board connectors to confirm whether communication signals are present. A board that shows physical damage should be replaced rather than repaired in the field.
- Repair or replace the faulty PCB Gas Safe engineer
Once the defective component is identified — whether the main control board, the fascia PCB, or associated wiring — the engineer should fit a genuine Keston replacement part. Specialist suppliers such as Greenheat UK carry an extensive range of Keston spares. The engineer should confirm the boiler fires correctly and that E99 does not return before leaving site.
- Contact a Gas Safe registered engineer with Keston experience Gas Safe engineer
Keston boilers are considered complex appliances due to their twin-flue configuration and modulating pump system. Not every heating engineer is familiar with them. Use the engineer-finder on keston.co.uk or call Keston's technical helpline to be directed to someone with hands-on Keston experience in your area. Always verify Gas Safe registration before work begins.
Parts you may need
- Keston main control PCB · from £280
- Keston fascia display PCB · from £120
- Fascia-to-control-box wiring loom · from £35
- Flow/return temperature sensor · from £25
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–£500, depending on the underlying cause.
Frequently asked questions
Can I fix Keston E99 myself by just resetting the boiler?
You can try a single reset — press and hold the spanner button for 3–5 seconds — and the boiler may restart if the fault was a one-off communication glitch. However, if E99 returns after resetting, the cause is an underlying wiring or PCB fault that only a Gas Safe engineer can safely diagnose and fix. Do not keep resetting repeatedly, as this will not resolve a hardware fault.
How much does it cost to replace a PCB on a Keston boiler?
For most Keston models, a PCB replacement including parts and labour typically costs in the region of £350–£500. In some cases — particularly if the main control box is involved and diagnosis is lengthy — costs can reach £600–£700. Always ask for a written quote before authorising any work. If your boiler is over 12–15 years old, it is worth comparing the repair cost against the price of a modern replacement, since a repair exceeding roughly half the cost of a new boiler is generally a signal to replace.
What is the difference between Keston fault codes E99 and E41?
Keston's own manual defines E99 as having the same meaning as E41 — both indicate a communication fault between the fascia display panel and the internal control box. In practice they point to the same root causes: loose or damaged wiring between the two units, or a failure of either the main PCB or the fascia PCB itself. An engineer diagnosing either code should follow the same inspection procedure.
Why do Keston boilers need a specialist engineer?
Keston boilers use a twin-flue system and a modulating pump arrangement that differs significantly from mainstream brands such as Worcester Bosch or Vaillant. Many general heating engineers are unfamiliar with the wiring layout and fault-code logic specific to Keston. Using an engineer who has worked on Keston appliances before reduces the risk of misdiagnosis and unnecessary parts being fitted. Keston's website and technical helpline can direct you to approved engineers in your area.