Overview
Error 235 on a Worcester Bosch Greenstar I system/combi indicates that the boiler's heat control module (HCM) and the main control box (or burner control unit) have incompatible software or firmware versions. The HCM is the small electronic module that communicates configuration and identification information to the boiler’s control electronics. If the software versions do not match or the code plug data is not recognised, the boiler will flag the mismatch and may go into a fault or lockout state to protect components and ensure safe operation. This is not a typical mechanical fault like a frozen condensate pipe or low pressure; it is an electronic/software compatibility issue. Severity is medium to high because, while it is unlikely to create an immediate safety risk by itself, the boiler can be rendered partly or completely inoperative (loss of heating and/or hot water) until the software is corrected. Resolving the fault almost always requires specific service tools, firmware files or replacement modules and therefore should be handled by a qualified engineer. Homeowner checks are limited and must avoid opening the boiler or attempting module swaps unless they are a Gas Safe registered and competent technician.
Possible Cause: Incompatible heat control module (HCM) software versions.
Troubleshooting Steps
Safety precautions:
1. If you smell gas or suspect a gas leak, leave the house immediately, do not operate electrical switches, and call the gas emergency number. Do not attempt any internal repairs.
2. Do not open the boiler casing or attempt to remove internal electronic modules unless you are a qualified Gas Safe engineer. There are live electrical parts and gas components inside.
3. Isolate electrical power to the boiler at the fused spur only if instructed by an engineer or if you need to make a simple power-cycle. Repeated resetting/power-cycling is not advisable.
Initial homeowner checks you can safely do:
1. Note the exact error displayed (235) and any other flashing codes or messages on the panel. Take a clear photo of the display and the boiler’s nameplate (model and serial number) to pass to the engineer.
2. Check the boiler status using the quick info button (if available) to capture block codes and any other fault history.
3. Check basic system conditions: confirm the boiler has power, water pressure is around 1–1.5 bar when cold, and there is no visible water leak or frozen condensate pipe. These checks do not fix an HCM software mismatch but help rule out unrelated faults.
4. If the boiler is still in lockout and you want to attempt a single safe power-cycle: turn the boiler off at the fused spur, wait 30 seconds, then turn it back on. Do not keep resetting if the error returns immediately.
Specific diagnostic and fix steps (for a qualified engineer):
1. Confirm the fault: read the block cause code and service history using Worcester Bosch service tools or the boiler’s service menu. Verify that 235 corresponds to HCM/control box software mismatch on this platform.
2. Check HCM and control box hardware: inspect that the HCM is the correct part number for this boiler model and that the code plug/KIM (if fitted) is present and seated correctly. Check all connectors between the HCM and main PCB for secure fit and corrosion.
3. Check for recent part replacements or software updates recorded in the service history that could have caused the mismatch (e.g., an HCM fitted from a different generation or a control box firmware change).
4. Attempt software reconciliation: using authorised Worcester Bosch diagnostic software and the manufacturer’s update tool, update the control box firmware and/or the HCM to matching approved versions. Follow Worcester’s published procedure for firmware updates and version matching.
5. If software update is not possible or the HCM is incompatible by design, replace the incorrect HCM with the correct, compatible module supplied for this boiler model and series. If the main control board is at fault, replace or reprogram the main control unit as per manufacturer guidance.
6. After updating or replacing modules, clear the fault codes via the service menu, then run full commissioning checks: ignition, flame sensing, fan and pump tests, verify flow and return sensor readings, and check for stable operation in both heating and domestic hot water modes.
7. Record the work, firmware versions used, and test results on the service record and inform the owner of any follow-up requirements.
When to call a professional:
1. Error 235 requires a Worcester Bosch authorised or Gas Safe registered heating engineer. Do not attempt module swaps, firmware updates or PCB replacements unless you are qualified and have access to the correct diagnostic tools and manufacturer downloads.
2. Provide the engineer with the boiler model, serial number, exact error code (235), any other displayed block codes, recent service history, and photos you took. This helps them bring the correct replacement parts or software on their first visit.
Additional homeowner notes:
1. While waiting for a visit, avoid repeated resets. If the boiler is unusable, use temporary alternative heating or hot water solutions where safe.
2. If you detect any other signs of unsafe operation (strange smells, smoke, hot surfaces, persistent leaks) isolate the boiler and contact a professional immediately.
3. Keep receipts and service records; if the boiler is under warranty or covered by a service plan, inform the service provider before work begins.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch Greenstar I System / Combi Boiler.