Worcester Bosch A7 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch A7 fault code mean?
The A7 fault code indicates that your Worcester Bosch boiler has detected a problem with the domestic hot water (DHW) NTC thermistor — the temperature sensor responsible for monitoring the temperature of your hot water supply. When this sensor sends readings outside the expected range, or stops communicating altogether, the boiler flags it as defective. Depending on the severity, the boiler may continue producing hot water but with reduced temperature accuracy, or it may lock out entirely as a safety precaution. The fault and its indicator LED will typically flash on the display to alert you to the problem.
General guidance only — not a substitute for professional advice. 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 DHW NTC thermistor Common
The sensor itself has worn out, failed internally, or drifted so far out of calibration that the boiler can no longer trust its readings. NTC thermistors do degrade over time, particularly in hard-water areas where scale build-up can affect the sensor tip. This is by far the most common root cause of an A7 fault.
- Damaged, loose, or corroded wiring and connectors Sometimes
The wiring loom that connects the DHW NTC sensor to the PCB — including the 20-pin connector on the board — can develop loose pins, corrosion, or chafed insulation. A poor connection produces erratic resistance readings that the boiler interprets as a sensor fault, even if the sensor itself is perfectly healthy.
- Short circuit or open circuit in the sensor circuit Sometimes
Water ingress, incorrect installation, or physical damage to the sensor can cause the circuit to read either a dead short or an open break. Both conditions fall outside the boiler's acceptable sensor range and will trigger the A7 code.
- PCB (printed circuit board) fault Rare
In a minority of cases the PCB's own sensor-input circuitry is at fault rather than the sensor or its wiring. If an engineer replaces the sensor and renews all connections but the A7 returns, the control board should be investigated.
How to fix it
- Check the boiler display and note the exact behaviour DIY safe
Observe whether the boiler is fully locked out or whether it is still producing some hot water with a flashing A7 warning. This information is useful to relay to an engineer, as it helps distinguish between a soft warning and a full lockout.
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame or 'R' symbol) for 3–5 seconds, then release. Allow the boiler two to three minutes to restart and run through its ignition sequence. A single reset is reasonable to try — if the A7 code returns promptly, do not keep resetting, as this will not fix the underlying problem and can mask fault history.
- Do not attempt to open the boiler casing or touch internal components Gas Safe engineer
The DHW NTC sensor and its wiring are located inside the boiler casing. Removing panels and working on internal components without Gas Safe registration is unsafe, could invalidate your boiler warranty, and is a breach of gas safety regulations. Leave all internal investigation to a qualified engineer.
- Book a Gas Safe registered engineer to test the sensor and its wiring Gas Safe engineer
An engineer will use a multimeter to measure the resistance across the DHW NTC sensor — typically checking between the relevant pins on the 20-pin PCB connector — and compare the reading against the expected resistance-temperature curve. If resistance is out of range, the sensor is confirmed faulty. They will also check for loose pins, corrosion, and any wiring damage along the loom.
- Engineer replaces the DHW NTC sensor if confirmed faulty Gas Safe engineer
Genuine Worcester Bosch DHW NTC sensors are relatively affordable components. The engineer will fit a new sensor, reconnect the wiring securely, and clear the fault code before running the boiler through a full hot water demand cycle to confirm the repair has been successful.
- Engineer investigates PCB if sensor and wiring are both healthy Gas Safe engineer
If the sensor and all connections test within specification but the A7 code persists, the fault lies upstream in the PCB's sensor-input circuit. PCB replacement or repair is specialist work and should only be carried out by a Gas Safe engineer experienced with Worcester Bosch controls.
Parts you may need
- DHW NTC Thermistor Sensor (genuine Worcester Bosch) · from £35
- Wiring loom / harness (DHW sensor circuit) · from £45
- 20-pin PCB connector / terminal block · from £20
- PCB (printed circuit board) — if sensor circuit is faulty · 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–£280, depending on the underlying cause.
Frequently asked questions
Can I still use my boiler when the A7 fault code is showing?
It depends on whether the boiler has fully locked out or is continuing to run with a warning. Worcester Bosch's own guidance notes that in some A7 situations the boiler continues to produce hot water but with less precise temperature control. However, because the boiler has detected a sensor fault, it is not operating normally and you should arrange a repair promptly. If the boiler has locked out completely, you will have no hot water until it is fixed.
How much does it cost to fix an A7 fault on a Worcester Bosch boiler?
For a straightforward DHW NTC sensor swap carried out during normal working hours, most homeowners pay somewhere in the region of £100–£280 all-in, covering the call-out, an hour or so of labour, and the sensor itself. If the fault turns out to involve the PCB rather than the sensor, costs can rise significantly — a PCB can cost £200–£350 for the part alone, plus labour, so a PCB repair may total £350–£550 or more. Out-of-hours or emergency call-outs will also carry a premium.
Is the A7 fault a Gas Safe job, or can I replace the NTC sensor myself?
You should not attempt to replace the sensor yourself. While the sensor is not a gas component in the way a valve or burner is, it is located inside the boiler casing alongside live electrical connections and pressurised water circuits. Disturbing internal components without the appropriate qualifications can be dangerous, will almost certainly void your boiler warranty, and may contravene manufacturer guidelines. Always use a Gas Safe registered engineer.
My Worcester Bosch boiler keeps returning to A7 after the sensor has been replaced — what could cause this?
If the A7 fault returns shortly after a new sensor has been fitted, the most likely culprits are a wiring or connector issue that was not fully resolved during the repair — particularly corrosion or a loose pin on the 20-pin PCB connector — or, less commonly, a fault in the PCB's own sensor-input circuitry. Ask your engineer to re-check all connector pins and measure continuity along the full wiring loom. If the wiring is confirmed sound, the PCB should be investigated next.