Protherm F.90 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.90 fault code mean?
The F.90 code appears on Protherm boilers when the control board cannot detect the APC (Accessory/Peripheral Control) module. On the boiler display, the letter 'F' will flash alongside the number '90'. This is essentially a communication loss between the main PCB and the add-on APC module — a component used in certain Protherm models to enable advanced control features or cascade system functionality. It is worth noting that the closely related code F.91 is a distinct fault: where F.90 means the module cannot be found at all (not connected or not communicating), F.91 indicates the module has been detected but has an internal failure. If your display shows F.91, the module itself is likely faulty rather than simply disconnected.
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 plug at the APC module Common
The most frequent trigger for F.90 is a wiring connector that has worked loose from the APC module. Vibration from normal boiler operation, or a nudge during a previous service visit, can cause the plug to partially or fully disengage, breaking the communication signal to the PCB.
- Corroded or damaged connector pins Sometimes
Moisture ingress around the connector or age-related corrosion can degrade the electrical contacts between the APC module wiring harness and the control board. Even a partially corroded pin can be enough to interrupt the signal entirely.
- Damaged wiring loom between module and PCB Sometimes
A break, short circuit, or chafed section of the cable running between the APC module and the main control board can prevent any signal from reaching the PCB. This is more likely in older installations or where cabling has been routed close to a heat source.
- Faulty or failed APC module Rare
If the wiring and connections are all intact and secure, the APC module itself may have developed an internal fault that prevents it from communicating. In this scenario the module will need replacing rather than reconnecting.
- Main PCB fault Rare
In uncommon cases where a replacement APC module and confirmed good wiring still result in an F.90 code, the main control board itself may have failed and is no longer able to receive or interpret the module's signal.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button as described in your boiler's user manual. A transient communication glitch can occasionally trigger F.90 and a single reset may clear it. If the code returns immediately or after a short time, do not keep resetting — move on to the next step and contact an engineer.
- Check that the boiler's gas supply is on DIY safe
Confirm your gas meter isolation valve is in the open (in-line) position and that other gas appliances in your home are working. An interrupted supply is unlikely to cause F.90 specifically, but it is always worth ruling out a wider gas issue before calling an engineer.
- Visually inspect accessible wiring around the APC module (do not remove covers) DIY safe
Without removing any boiler casing panels, check whether any wiring visible near external connections or accessible ports looks obviously damaged, kinked, or unplugged. Do not attempt to open the boiler casing or handle internal components — this must be done by a qualified engineer.
- Have a Gas Safe engineer inspect the APC module connections and wiring harness Gas Safe engineer
An engineer will safely remove the boiler casing and physically check the wiring plug seating at the APC module, inspect each connector pin for corrosion or damage, and reseat or replace the harness as needed. This is the most likely fix if the fault has appeared suddenly.
- Have the APC module tested and replaced if necessary Gas Safe engineer
If the wiring is confirmed to be in good condition, the engineer will test the APC module itself. A module that has developed an internal fault will need to be replaced with a compatible Protherm part. The engineer should verify the fault clears once the new module is fitted.
- Have the main PCB inspected if the fault persists after module replacement Gas Safe engineer
Should F.90 remain after a new APC module is installed and all wiring checked, the main control board may be the source of the problem. PCB diagnosis and replacement must be carried out by a Gas Safe registered engineer and, depending on the board's availability, may require ordering a specific Protherm-compatible part.
- Contact a Gas Safe registered engineer Gas Safe engineer
If you have not already done so, arrange a visit from a Gas Safe registered engineer to diagnose and resolve this fault. You can verify any engineer's registration at gassaferegister.co.uk. Attempting internal boiler repairs yourself is unsafe and illegal for gas appliances in the UK.
Parts you may need
- Protherm APC module (model-specific) · from £120
- Wiring harness / connector loom · from £35
- Protherm main PCB (control 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 £100–£350, depending on the underlying cause.
Frequently asked questions
Is the Protherm F.90 fault dangerous?
F.90 is a communication or connectivity fault rather than a gas or combustion safety issue, so it does not present an immediate danger in the way that, say, a carbon monoxide-related fault would. However, it will typically prevent the boiler from operating normally, and you should still have it investigated promptly by a Gas Safe registered engineer rather than leaving it unattended.
What is the difference between Protherm F.90 and F.91?
F.90 means the boiler's control board cannot detect the APC module at all — it is essentially absent from the boiler's communication network, most often due to a loose connector or broken wire. F.91, by contrast, means the module has been found but has reported an internal fault or failure. The distinction matters because F.90 is more likely to be resolved by reconnecting wiring, whereas F.91 more commonly requires the module itself to be replaced.
Can I fix Protherm F.90 myself?
The only safe DIY actions are resetting the boiler (once or twice at most) and doing a visual check for any obviously disconnected wiring visible without removing casing panels. Any work that involves opening the boiler, handling internal wiring, or replacing electronic components must be carried out by a Gas Safe registered engineer. Attempting internal repairs yourself is both unsafe and a legal requirement in the UK.
How much will it cost to fix a Protherm F.90 fault?
If the fault turns out to be a loose wiring plug that simply needs reseating, you are likely to pay £100–£180 covering the engineer's call-out and a short labour time. If the APC module needs replacing, expect a total of around £200–£350 including parts and labour. In the less common scenario where the main PCB is also faulty, costs can rise above £400; if you are facing that level of expenditure on an older boiler it is worth getting a quote for a replacement boiler as well.