Chaffoteaux 402 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 402 fault code mean?
Fault code 402 appears on boilers fitted with the Chaffoteaux Expert Control interface (commonly found on the Urbia Green Evo series and similar models). The code indicates that the boiler's built-in GPRS modem — the module responsible for mobile-data connectivity and remote monitoring — has encountered a communication fault and is unable to operate correctly. This sits within the 4xx group of Expert Control codes, which covers modem and connectivity-related errors. Related codes in the same group include: 4 01 (Mdm-Bus communication error), 4 03 (SIM card error), 4 04 (Mdm-to-PCB communication error), 4 05 (Mdm In1 error), and 4 06 (Mdm In2 error). Because the GPRS modem is a connectivity add-on rather than part of the core heating circuit, your boiler will often continue to provide heating and hot water normally while this fault is active. That said, the fault should still be investigated — persistent communication errors can occasionally point to a wider PCB or control board issue.
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
- Failed or degraded GPRS modem module Common
The internal modem hardware within the Expert Control interface has failed outright or deteriorated over time. This is the most common root cause when signal and wiring have been ruled out.
- 2G/3G network no longer available in your area Common
The GPRS modem in older Expert Control units relies on 2G or 3G mobile networks. UK operators have progressively switched these off, which can permanently prevent the modem from connecting even when the hardware itself is perfectly functional.
- Poor or absent mobile signal at the installation location Sometimes
Basements, thick-walled properties, or remote locations may not provide enough cellular signal for the GPRS modem to connect reliably. The modem will log a fault if it cannot establish a link.
- Loose or damaged wiring between the modem and PCB Sometimes
Vibration, age, or poor original installation can cause connectors between the modem module and the main control board to work loose or corrode, interrupting the communication path.
- SIM card failure or displacement Sometimes
The embedded SIM card in the modem module may have failed or shifted. Note that a dedicated SIM card fault will typically show as code 4 03, but a deteriorating SIM can also trigger the broader 402 GPRS error.
- Firmware glitch or corrupted modem settings Rare
A software error within the modem firmware can cause it to stop reporting correctly without any underlying hardware failure. A power cycle or firmware update may resolve this.
How to fix it
- Power-cycle the boiler DIY safe
Switch the boiler off at the programmer or mains switch and leave it completely de-energised for 5–10 minutes. This allows the modem module to fully reset and can clear transient software faults. Power the boiler back on and check whether the 402 code returns.
- Reset the boiler using the control panel DIY safe
If the fault persists after the power cycle, use the boiler's built-in reset button (refer to your user manual for its location). Press once and wait for the boiler to restart. If the fault clears, monitor it over the next day or two. Do not repeatedly reset — if the fault returns more than twice, move on to the next steps.
- Check mobile signal strength at the boiler location DIY safe
Use your mobile phone to check the 2G or 3G signal in the room where the boiler is installed. A very weak signal (or no signal at all) in that location suggests the modem simply cannot connect. Also check whether your local network operator still provides 2G/3G service — if they have switched it off in your area, the modem module will need upgrading regardless of its condition.
- Have an engineer inspect internal wiring and connectors Gas Safe engineer
A Gas Safe registered engineer should remove the boiler casing and check the wiring harnesses and connector plugs between the GPRS modem module and the main PCB. Loose, corroded, or damaged connections are a straightforward fix but require internal access that homeowners must not attempt.
- Have the modem module and SIM assessed or replaced Gas Safe engineer
If wiring is intact and signal is adequate, an engineer should test the modem module itself and inspect the embedded SIM. A failed modem module will need to be replaced. If the current module uses only 2G/3G connectivity, the engineer may recommend an upgraded unit compatible with current UK network infrastructure.
- Consider a firmware update for the Expert Control interface Gas Safe engineer
If no hardware fault is found, an engineer familiar with the Expert Control system can check whether a firmware update is available for the modem or the control interface. Updated firmware can resolve known communication bugs without requiring any parts replacement.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the fault code 402 returns after a power cycle and basic signal checks, book a Gas Safe registered engineer to carry out a full diagnostic. Internal repairs, module replacement, and PCB-level work must never be attempted by an unqualified person. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- Expert Control GPRS modem module · from £120
- Communication wiring harness / connector set · from £25
- Expert Control interface PCB · 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Will my boiler still heat my home with a 402 fault showing?
In most cases, yes. The 402 code relates to the GPRS connectivity module rather than the core heating components such as the burner, gas valve, or pump. Your boiler should continue to produce heating and hot water normally. However, remote monitoring and any smart-control features that rely on the GPRS connection will be unavailable until the fault is resolved. It is still worth having an engineer investigate promptly, as a communication fault can occasionally be a symptom of a broader PCB issue.
Could the 2G/3G network switch-off be causing my Chaffoteaux 402 fault?
Yes, this is increasingly common. Older Expert Control modem modules communicate over 2G or 3G GPRS networks, and UK mobile operators have been progressively decommissioning these services. If your local network no longer supports 2G or 3G, the modem will be unable to connect regardless of its condition. An engineer familiar with the Expert Control system can advise on whether your modem module can be upgraded to a current-network-compatible version.
What is the difference between Chaffoteaux fault codes 402, 403, and 404?
All three sit in the 4xx group of Expert Control communication codes. Code 402 is a general GPRS modem error — the modem cannot communicate over the mobile network. Code 403 points specifically to a SIM card fault (the SIM may have failed or become dislodged). Code 404 indicates a communication breakdown between the modem module and the boiler's main PCB rather than an external network issue. If you see 403 or 404 instead of 402, mention the exact code to your engineer as it helps narrow down the diagnosis.
How much does it cost to fix a Chaffoteaux 402 fault?
For most households, the repair falls in the range of £120–£350. A straightforward wiring or connector fix sits at the lower end, while replacing the GPRS modem module (parts and labour) typically costs £150–£300. If the fault turns out to involve the main Expert Control PCB, costs can rise to £350–£500 or more — but this is less common with a 402 code and your engineer should confirm the diagnosis before ordering parts.