Worcester Bosch Boiler D4 Error Code — Meaning & Causes (2026)
Primary flow temperature rising too fast, or the flow-to-safety-sensor temperature difference exceeds its limit. Below: what triggers it and what to check — this code carries no single repair to price. Reviewed July 2026.
What D4 means
Primary flow temperature rising too fast, or the flow-to-safety-sensor temperature difference exceeds its limit.
Likely causes, most common first
- Poor circulation through the primary circuit
- Closed valve or blocked heat exchanger
- Failed pump
Base: 3 published national repair-cost guides (retrieved 2026-07) · full source registry on the methodology page · Code meaning & causes · Reviewed July 2026
No single repair to price for this code
D4 doesn't point at one failed component, so we don't attach a price module — forcing a repair mapping here would be dishonest. If the code keeps returning, the underlying symptom (not the code) is what gets priced: browse the boiler repair costs for the problem you're actually seeing.
Codes are universal — your location changes the repair price, never the code's meaning. See the methodology for the exact localization formula and sources.
Reviewed by a Red Seal–certified appliance technician who also holds a Technical Safety BC gas ticket (Gas-C) and an ODS/halocarbon refrigerant ticket (CFC/HCFC/HFC).