Overview
A7 on a Worcester Bosch CDI Classic combi means the boiler has detected a fault with the domestic hot water NTC (negative temperature coefficient) sensor or its circuit. The NTC sensor reports the hot water temperature to the boiler control. If the control sees an open circuit, short circuit or an implausible resistance value it will flag A7 and may lock out or limit hot water control. Common root causes are a failed DHW (domestic hot water) NTC sensor, a damaged/loose connector or cable, or less commonly a related sensor or control electronics fault. Severity is medium: the boiler will usually still show as faulted for accurate hot water regulation and may default to a safe mode. This is not an emergency like a gas leak but it does require timely attention because incorrect temperature readings can cause loss of hot water or unsafe operation. Basic checks (reset, visible connector inspection) can be done by a homeowner, but any internal work, electrical testing or sensor replacement should be done by a Gas Safe qualified heating engineer. Opening the boiler casing or working on gas/electrical parts without the correct certification is unsafe and illegal in many jurisdictions.
Possible Cause: The hot water negative thermistor coefficient (NTC) sensor is defective.
Troubleshooting Steps
Safety precautions:
- Do NOT open the boiler casing or work on gas/electrical components unless you are a Gas Safe / licensed engineer. Internal repairs must be done by a qualified professional.
- If you smell gas or suspect a gas leak do not attempt any checks; leave the premises and call the gas emergency number immediately.
- Before any professional work, isolate electrical supply to the boiler and close gas supply if required by the engineer.
Initial checks a homeowner can do (safe, non-intrusive):
1. Note the exact error (A7) and any other codes or LEDs shown on the display.
2. Check the boiler display and manual for the recommended reset procedure. Perform a single reset (press the reset button or switch the boiler off then on) and see if the code clears. If it returns immediately or after a short time, further investigation is required.
3. Check that the boiler has normal mains power, water supply and adequate system pressure (shown on the boiler gauge). Low system pressure usually shows different codes but it is sensible to verify basic operating conditions.
4. Look at any visible wiring or connectors external to the boiler (e.g., sensor connectors if accessible without removing the cover) for obvious damage, corrosion or a loose plug. Do not force connectors or remove the casing.
Diagnostic and fix steps for a qualified heating engineer (do not attempt if not qualified):
1. Confirm fault history and repeat attempts to reset so you have a clear symptom log.
2. Isolate the electrical supply to the boiler before removing the casing. Follow safe isolation procedures.
3. Remove the boiler casing and visually inspect the DHW NTC sensor, its wiring loom and the PCB connector for corrosion, chafing, damaged insulation, or poor connections.
4. Unplug the relevant connector (commonly the 20‑pin connector on some models) and measure the resistance of the DHW NTC sensor at the cable side. On many Worcester models the engineer will measure between the designated NTC pins (often referenced as pins 3 and 4 on the cable side) and compare against the resistance values given in the boiler service manual (table of temperature vs resistance). If you do not have the manual follow the manufacturer's diagnostic chart.
5. Interpret measurements: an open circuit (infinite resistance) or a short (very low resistance) indicates a failed sensor or wiring fault. A resistance that does not match expected NTC values at ambient temperature indicates sensor degradation.
6. If wiring is intact but resistance is out of specification, replace the hot water NTC sensor. After replacement re-measure to confirm correct values and re-fit the sensor in the correct location and orientation.
7. If the sensor reads correctly, check continuity of the cable back to the PCB and for any intermittent fault with the connector. Wiggle tests under powered, controlled conditions can reveal intermittent breaks (only done by a qualified engineer).
8. If sensor and wiring are good, check related sensors and modules: flow temperature sensor, heat-exchanger sensor, flue gas thermostat or the boiler control (HCM/KIM/burner control unit) because wiring or control faults can report similar symptoms. The engineer may swap components or use manufacturer diagnostics to isolate a control board fault.
9. After repair or replacement, reassemble, restore power and run the boiler through domestic hot water demand tests and confirm the fault has cleared and temperatures are within expected ranges. Check for any other error codes and verify overall operation.
When to call a professional and final notes:
- If the A7 code does not clear after a reset, or if you see signs of damaged wiring or internal sensor fault, call a Gas Safe qualified heating engineer experienced with Worcester Bosch boilers.
- Do not attempt internal electrical tests, sensor replacement or gas-side work yourself. A qualified engineer will have the correct replacement NTC part, the service manual resistance references, manufacturer-level diagnostics and the legal qualifications to work safely on the boiler.
- The most common and straightforward fix is replacement of the defective DHW NTC sensor or repair of its connector/cable, but persistent or multiple faults may need deeper control board or module diagnosis by the manufacturer or authorised service agent.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch CDI Classic Combi Boiler.