Worcester Bosch [A5] fault codewhat it means and how to fix it

A5 belongs to Worcester boilers that store hot water and watch its temperature with a sensor of their own: chiefly the Greenstar Highflow CDi, whose internal heat bank carries a tank sensor. The CDi Classic engineer's booklet carries the same A5 procedure for a tank or heat bank sensor, though the wall-hung CDi Classic tables themselves do not list the code, because those boilers leave cylinder control to an external thermostat. When the sensor reads outside its range the board flags A5.

Worcester's service booklet is careful about the consequence — the appliance is still available for heating demands — and the Highflow manual names the part plainly as tank NTC defect. What you notice is on the hot water side: the boiler may stop reheating the store because it believes it is already hot, or run for too long because it believes it is cold, and on a Highflow the tap may go lukewarm at the wrong moment.

The sensor itself is a small thermistor whose resistance falls as it warms, and Worcester publish the range an engineer should measure across it, 33.5 kilohm cold down to about 500 ohms hot. Corrosion on its plug is checked first, then the resistance at the board connector, then at the sensor itself, and the answer to that sequence decides between a lead, a sensor and — rarely — the board. It is an information fault on the boiler's own scale, but a house without reliable hot water does not experience it as minor, so it is worth having fixed rather than lived with.