Overview
D4 213 on a Worcester Bosch GB162 means the boiler has detected the primary flow temperature (or the safety sensor temperature) is rising too quickly and has locked out to protect the appliance. In plain terms the boiler’s temperature sensors or monitoring logic have seen an unexpected, rapid increase in the flow or safety temperature and the controller has stopped the burner to avoid overheating, boiling or damage to the heat exchanger. This can be caused by several things: a faulty or shorted temperature sensor (NTC), an airlock or very low water content in the primary circuit, a stuck or failed pump causing poor circulation so local temperatures spike, scale or blockage in the heat exchanger, or wiring/PCB faults. Sometimes it will be a transient condition cleared by a reset, but it can also signal a real circulation or sensor failure that needs attention. Severity is moderate-to-high: the boiler has locked out to prevent damage, so you should not ignore it. Basic checks can be done by a homeowner, but diagnosis and repairs that involve gas, internal wiring, sensors, the heat exchanger or pump should be carried out by a Gas Safe registered engineer. If the fault is due to low pressure or trapped air in the system you can often clear it by repressurising and bleeding radiators, but if the error returns, there is likely a failing component (pump, sensor, heat exchanger fault or wiring) that requires a professional inspection and repair.
Possible Cause: The actual temperature recorded by the flow temperature sensor or return temperature sensor is rising too quickly.
Troubleshooting Steps
Safety precautions:
1. If you smell gas or suspect a gas leak, leave the property immediately and call the gas emergency number. Do not attempt any checks.
2. Before touching the boiler or opening any covers, isolate electrical power at the fused spur or mains switch. Do not open sealed or gas-containing panels; only a Gas Safe engineer should work on gas-carrying components.
3. Wear gloves and eye protection if you are bleeding radiators or working near hot pipework. Be cautious around hot surfaces if the boiler has been running recently.
Initial homeowner checks (safe, non-intrusive):
1. Record the exact error code and any cause code, and take a photo of the display for the engineer.
2. Try a controlled reset: follow the GB162 reset method (press the reset button or hold the dial per the manual) and observe whether the fault clears and returns. Do not repeatedly reset more than 2–3 times.
3. Check the system pressure on the boiler gauge. If the pressure is below about 1.0 bar (typically recommended 1.0–1.5 bar cold), the system may have reduced water content causing rapid temperature changes. If you are comfortable doing so, repressurise the system using the filling loop until the gauge reads about 1.0–1.5 bar. Close the filling loop and recheck for leaks.
4. Bleed a couple of radiators (starting with the highest in the house) to remove trapped air and then re-check pressure and the fault. Trapped air can cause poor circulation and local overheating.
5. Note whether the heating pump is running (you may hear it hum or feel vibration on the pump body). If the pump is silent while the boiler is trying to fire, that indicates a circulation problem.
6. Check for visible leaks under the boiler or on pipework. If pressure repeatedly drops after repressurising, do not continue topping up; call an engineer.
Specific diagnostic and next steps (if basic checks do not clear the fault):
1. If the fault returns after pressure and bleeding checks, do not attempt internal repairs. The likely causes now are: a faulty flow or safety NTC sensor, a stuck or failed pump, a heat exchanger blockage/scale, or wiring/PCB issues.
2. Prepare information for the engineer: record when the fault happened, whether it happens on hot-water demand, central heating demand, or both, what the boiler pressure was, any noises, and whether a reset temporarily clears it.
3. The qualified engineer will isolate gas and power and then: check sensor resistances and compare to manufacturer values; inspect connectors and wiring for corrosion, water ingress or damage; check pump operation, speed settings and impeller (free the pump or replace if seized); check flow/return temperatures with a thermometer to confirm differential; inspect heat exchanger for scale or blockages and measure for overheating conditions; check the safety temperature limiter and flue thermostat; and test the PCB and related components.
4. If a sensor is faulty, the engineer will replace the flow or safety NTC. If the pump is seized or failing they will repair or replace it. If there is scale or a blocked heat exchanger that is causing poor circulation heating, the engineer will advise on descaling or replacement depending on severity. If wiring or control board faults are found they will be repaired or replaced by the engineer.
When to call a Gas Safe engineer and what to tell them:
1. Call a Gas Safe registered heating engineer whenever the error persists after you have checked pressure and bled radiators, or immediately if you see leaks, hear unusual noises (grinding/seizing pump), or if the boiler locks out repeatedly.
2. Give the engineer the exact boiler model (Worcester Bosch GB162), the error code D4 213, what you have already tried (reset, repressurised to X bar, bled radiators), and whether the fault is on hot water, central heating or both.
3. Do not attempt to replace sensors, pump or internal components yourself unless you are a qualified Gas Safe engineer. These components involve gas and live electrical connections and require correct commissioning and safety checks.
Important notes and warnings:
1. A temporary reset may allow the boiler to run but does not fix the underlying cause. Repeated resets without diagnosis can risk damage.
2. If pressure drops after repressurising, there is a leak — stop adding water and call the engineer.
3. Do not bypass any safety devices or attempt to defeat the lockout; these exist to protect the appliance and your home.
Summary:
1. Do basic safe checks first (record code, try one reset, check and top up pressure to ~1.0–1.5 bar, bleed radiators, listen for pump).
2. If the fault returns, stop and call a Gas Safe engineer to diagnose sensors, pump, heat exchanger and wiring.
3. Provide the engineer with the exact error D4 213 and the steps you have taken — this speeds up diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.