Johnson & Starley 99 Fault Code: Causes, Fixes & Repair Costs
What does the Johnson & Starley 99 fault code mean?
Fault code 99 on Johnson & Starley warm air boilers indicates that the MMI (Man Machine Interface — the control panel you interact with) and the ESYS (Electronic System — the main control board) have lost the ability to communicate with one another. When this internal communication link breaks down, the boiler shuts itself off as a precaution and displays 'E 99' on the screen. Because both components need to exchange data continuously for safe, controlled operation, any disruption to that link will prevent the boiler from firing. This fault almost always requires a qualified engineer to diagnose, as it involves the internal electronics of a warm air heating appliance.
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 disconnected wiring harness Common
The most frequent root cause. The cable loom or individual connector plugs linking the MMI panel to the ESYS board can vibrate loose over months or years of operation. Even a partially unseated connector is enough to break the communication signal and trigger code 99.
- Corroded or damaged connector pins Sometimes
Moisture ingress, age, or dust accumulation inside the unit can cause the connector pins between the MMI and ESYS to corrode or oxidise. This increases electrical resistance at the joint and eventually kills the data link entirely, even if the plug appears seated correctly.
- Faulty limit switch disrupting the communication pathway Sometimes
Johnson & Starley warm air units use limit switches as safety cut-outs. A limit switch that has failed open-circuit can interrupt signals within the control circuit, sometimes manifesting as a communication fault rather than a straightforward overheat code.
- ESYS control board failure Sometimes
If the ESYS module itself has developed a fault — due to a power surge, component failure, or age-related degradation — it may be unable to maintain communication with the MMI regardless of wiring condition. Burnt components or failed solder joints on the board are typical findings in these cases.
- MMI panel failure Rare
Less commonly, the fault originates at the MMI end rather than the ESYS board. A failed display panel or internal MMI component can prevent it from sending or acknowledging communication signals, producing the same code 99 lockout.
How to fix it
- Check that the boiler's power supply is on and stable DIY safe
Confirm the unit is switched on at the spur or isolator and that no fuses or circuit breakers have tripped. A momentary power interruption can sometimes cause a spurious communication fault. If everything looks normal, attempt a single boiler reset using the reset button or procedure described in your user manual. Do not reset more than two or three times — repeated resets without a lasting fix will not clear code 99 and may mask a worsening fault.
- Visually inspect the external wiring and casing for obvious damage DIY safe
With the boiler powered off at the isolator, take a look at any visible cables running to the control panel for obvious signs of chafing, pinching, or rodent damage. Do not open the boiler casing or touch internal components — this is purely an external visual check. Report anything unusual to your engineer.
- Contact a Gas Safe registered engineer to inspect internal wiring and connectors Gas Safe engineer
An engineer will open the unit and systematically check every connector in the communication loom between the MMI and the ESYS board. Loose or corroded plugs will be reseated or replaced. This is the most common fix and often resolves code 99 without the need for new parts.
- Engineer to test limit switch continuity Gas Safe engineer
Using a multimeter, the engineer will check whether the limit switch or switches within the unit are reading correctly. A switch that has failed open will be identified here and replaced if necessary, as it can interfere with the control circuit even when the primary wiring appears intact.
- Engineer to inspect and test the ESYS control board Gas Safe engineer
If wiring and limit switches check out, the engineer will examine the ESYS board for signs of heat damage, corrosion, or failed components. Voltage and continuity tests will determine whether the board is salvageable or needs replacing. The MMI panel may also be tested at this stage to rule out a fault at that end.
- Replace the ESYS module or MMI panel if found to be faulty Gas Safe engineer
Johnson & Starley maintain a broad spares programme covering both current and older warm air models, so replacement boards are generally available. The engineer will source the correct part, fit it, and verify that communication between the two modules is fully restored before leaving.
Parts you may need
- ESYS electronic control board · from £280
- MMI control panel assembly · from £180
- Wiring harness / connector loom · from £45
- Limit switch (high temperature safety) · 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 £120–£400, depending on the underlying cause.
Frequently asked questions
Can I reset a Johnson & Starley fault code 99 myself?
You can try a single boiler reset using the button or procedure in your user manual, and it is worth doing once in case the code was triggered by a temporary power blip. However, code 99 is a communication fault between two internal electronic modules, so a reset will not fix an underlying wiring or board problem. If the fault returns after one or two resets, stop and call a Gas Safe engineer — repeated resets without a fix will not help and could mask a developing fault.
How much does it cost to fix fault code 99 on a Johnson & Starley warm air boiler?
If the cause turns out to be a loose or corroded connector — the most common scenario — labour to diagnose and reseat the wiring typically costs in the region of £120–£200 including a call-out. If a limit switch needs replacing, add roughly £25–£50 for the part. A full ESYS board replacement is a larger job; parts and labour combined typically fall in the £300–£500 range for most models, though older or discontinued units where parts are harder to source can occasionally push beyond that. As a rule of thumb, if repair costs approach half the price of a new warm air unit, a replacement may be the more economical long-term choice.
Why does my Johnson & Starley boiler keep showing error code 99 after being reset?
A fault that returns repeatedly after resetting almost always points to a persistent hardware problem rather than a one-off glitch. With code 99, the most likely culprits are a connector that is making intermittent contact, a corroded pin that looks fine but fails under heat, or a control board that is beginning to fail. Each reset may temporarily restore communication, but the underlying fault will keep triggering the lockout. An engineer needs to run proper continuity and voltage tests to find the exact point of failure.
Is fault code 99 dangerous? Can I keep using the boiler?
Code 99 itself is a communication error rather than a gas or carbon monoxide hazard, but because the boiler shuts down when it appears, you will have no heating or hot air output. More importantly, you should not attempt to bypass or override the lockout — the boiler's safety logic is working as intended by refusing to fire when its control systems cannot communicate properly. Leave the unit off, keep the property ventilated normally, and arrange an engineer visit promptly, especially during cold weather.