Worcester Bosch 1073 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 1073 fault code mean?
Fault code 1073 W appears on Worcester Bosch Greenstar 8000 series boilers and indicates that the flow temperature sensor (an NTC thermistor) has developed a short circuit. This sensor continuously monitors the temperature of water leaving the heat exchanger, feeding resistance-based readings to the PCB so the boiler can regulate heating output safely. When a short circuit occurs, the sensor sends an implausible signal — typically a falsely low resistance reading — causing the PCB to lock the boiler out rather than risk overheating. Related codes in the same family are worth knowing: 1074 W signals a loss of signal (open circuit) from the same flow sensor rather than a short; 1075 W indicates a short circuit on the separate heating-block temperature sensor; and 1076 W means the heating-block sensor has lost its signal entirely. If your display shows 1075 W or 1076 W, the fault is on a different sensor but the diagnostic and repair process is broadly similar to 1073 W.
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 short-circuited NTC flow sensor Common
The NTC thermistor itself is the most likely culprit. Over time the sensor's internal resistance can break down, causing it to short circuit and send the PCB a reading that is outside the expected range. The boiler shuts down as a safety precaution. Replacing the sensor usually resolves the fault.
- Loose, corroded, or disconnected sensor plug Common
The plug connecting the flow sensor to the wiring loom can work loose through vibration, corrode due to condensation inside the casing, or suffer from a pushed-back terminal pin. A poor connection can mimic a short circuit and trigger the same code without the sensor itself being faulty.
- Damaged wiring loom between sensor and PCB Sometimes
Chafed, pinched, or heat-damaged wiring between the NTC sensor and the PCB can create an unintended short along the cable run. This is less common than a sensor failure but should be checked before ordering parts.
- Moisture or water ingress on the sensor or connections Sometimes
Condensate or water leaks inside the boiler casing can track along cables and cause short circuits at connector points. If there are signs of internal dampness or a known leak, this should be investigated alongside the sensor check.
- PCB or control unit fault Rare
In a small number of cases the PCB misinterprets a healthy sensor as short-circuited, or the PCB input circuit has failed. This is usually confirmed only after the sensor and wiring have already been tested and found to be in good order.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (or follow the reset procedure in your Greenstar 8000 manual) for the required time. A single reset can clear a transient glitch. If the boiler fires up and runs normally, monitor it over the next few hours. If 1073 W returns, do not keep resetting — repeated resets without fixing the underlying fault can cause additional wear.
- Check system pressure is between 1.0 and 1.5 bar DIY safe
Look at the pressure gauge on the boiler. If it reads below 1 bar, top up using the filling loop until the needle sits in the green zone (typically 1.0–1.5 bar), then attempt one further reset. Low pressure alone will not cause 1073 W but resolving it rules out any compounding issues.
- Confirm the gas supply is on and other gas appliances work DIY safe
Check that a gas hob or another gas appliance in your home lights normally. If there is no gas supply, contact your gas supplier rather than an engineer, as this is a network issue rather than a boiler fault.
- Engineer to test the NTC flow sensor resistance Gas Safe engineer
A Gas Safe registered engineer will disconnect the sensor plug and measure the thermistor's resistance with a multimeter at a known ambient temperature. A short circuit will show near-zero resistance where several kilohms would be expected. This confirms or rules out sensor failure conclusively.
- Engineer to inspect the wiring loom and connector Gas Safe engineer
The engineer will check each terminal in the sensor plug for corrosion, pushed-back pins, and continuity along the cable run to the PCB. Damaged sections of loom can be repaired or replaced without needing a new PCB.
- Engineer to replace the NTC flow sensor if faulty Gas Safe engineer
If the sensor tests as short-circuited, the engineer will fit a new NTC thermistor — using a genuine Worcester Bosch part or an approved equivalent — refit the connector securely, and clear the fault code via the boiler's service menu before performing a test firing to confirm stable operation.
- Engineer to assess the PCB if the fault persists after sensor replacement Gas Safe engineer
Should the 1073 W code return after a confirmed sensor and wiring check, the PCB input circuit may be at fault. The engineer will advise whether repair or PCB replacement is the most cost-effective route given the age and condition of the boiler.
- Call a Gas Safe registered engineer if the fault has not cleared Gas Safe engineer
If the boiler is still displaying 1073 W after a single reset attempt, book a Gas Safe registered engineer. Internal sensor and wiring work must not be carried out by an unqualified person — doing so is unsafe and will invalidate your boiler warranty.
Parts you may need
- NTC flow temperature sensor (Greenstar 8000 compatible) · from £35
- Sensor connector plug and terminal kit · from £12
- Wiring loom section (flow sensor to PCB) · from £45
- PCB / control unit (Greenstar 8000) · from £280
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–£250, depending on the underlying cause.
Frequently asked questions
Can I fix Worcester Bosch fault code 1073 W myself?
The only safe DIY steps are a single boiler reset and checking system pressure is between 1.0 and 1.5 bar. Everything beyond that — removing the casing, testing or replacing the NTC sensor, and inspecting the wiring — must be done by a Gas Safe registered engineer. Working inside a boiler without the correct qualifications is dangerous, illegal for gas-related work, and will void your warranty.
What is the difference between fault codes 1073 W and 1074 W?
Both codes relate to the same flow temperature NTC sensor. 1073 W means the sensor is short-circuited — its resistance has collapsed to near zero, sending an implausibly high temperature reading to the PCB. 1074 W means the opposite: the sensor or its connection has gone open-circuit and the PCB is receiving no signal at all. The repair approach is similar for both, but the engineer's multimeter readings will look very different, so quoting the correct code when you book a call-out is helpful.
How much does it cost to repair a 1073 W fault on a Greenstar 8000?
For a straightforward NTC flow sensor replacement during a standard daytime appointment, most homeowners pay in the region of £100–£250 including callout, labour, and the part. If the wiring loom also needs attention the bill will sit toward the upper end of that range. A PCB replacement is a much rarer outcome and can cost £350–£500 or more — if your engineer suspects the PCB is at fault on a boiler that is already 10 or more years old, it is worth getting a replacement boiler quote for comparison.
Will 1073 W cause my heating and hot water to stop working completely?
In most cases, yes. The Greenstar 8000 treats a short-circuited flow sensor as a safety lockout because the boiler cannot confirm whether the primary circuit is overheating. Both central heating and hot water will be unavailable until the fault is resolved. Some boilers may allow a brief reset before the code reappears, but running the boiler repeatedly with an active sensor fault is not recommended.