Applies to: Vitocal 150-A / 151-A / 250-A / 252-A

System behaviour: Room heating only, no DHW heating.

Likely cause: Fault (short circuit) in the cylinder temperature sensor or its wiring.

Basic checks (owner): Confirm that hot water demand is present and that no other lockout code is active. Power-cycle the unit once to see if the message reappears.

Installer actions: Measure resistance of the cylinder sensor (NTC 10k) and inspect wiring/connectors. Replace sensor and/or repair wiring as required.

System behaviour: Controller assumes 0 °C outside when calculating flow temperature.

Likely cause: Broken or disconnected outside temperature sensor wiring.

Basic checks (owner): Visually check the outdoor sensor if accessible (damaged cable, chewed, pulled out). Note any unusual flow temperature behaviour.

Installer actions: Check sensor resistance and continuity back to the control board, remake connections, and replace the sensor if readings are out of range.

System behaviour: Controller again assumes 0 °C outside.

Likely cause: Short circuit in the outdoor sensor or cable.

Basic checks (owner): As for F.13 – visual inspection only.

Installer actions: Isolate the sensor, measure resistance, check for moisture or crushed cable, and replace sensor/cable as required.

System behaviour: Refrigerant circuit switched off.

Likely cause: Open circuit at the air inlet temperature sensor.

Basic checks (owner): None – requires engineer investigation. You can safely note the conditions when the fault appears.

Installer actions: Measure sensor resistance at the controller terminals, check harness and plugs, and replace the sensor if resistance is not plausible.

System behaviour: Refrigerant circuit switched off.

Likely cause: Short circuit at the air inlet temperature sensor or wiring.

Basic checks (owner): As for F.33 – engineer required.

Installer actions: As for F.33 but looking for shorts to other conductors or earth; replace sensor/harness if necessary.

System behaviour: Heat pump switched off.

Likely cause: Heating system pressure has dropped below the safe minimum, or the pressure sensor/expansion vessel is faulty.

Basic checks (owner): Check the pressure gauge. If it is low and you have been shown how, top up the system via the filling loop and bleed radiators. If the fault recurs frequently, contact your installer.

Installer actions: Verify pressure sensor readings, expansion vessel pre-charge, and programmed set-pressure/range. Investigate leaks if frequent top-ups are needed.

System behaviour: Refrigerant circuit can only run when outside temperature is above approx. 5 °C. [oai_citation:1‡viessmanndirect.co.uk](https://viessmanndirect.co.uk/files//1041ad9a-7e00-4a2d-9026-ae8a00ced6e2/Service%20Intructions.pdf?srsltid=AfmBOoqGPFmyeJXYagVL-aLS7nhabMstRD074uzxSfFJUusy_WDu7Bs2&utm_source=chatgpt.com)

Likely cause: Short-circuited oil sump temperature sensor.

Basic checks (owner): Note ambient temperature when the fault occurs; report to installer.

Installer actions: Measure sensor resistance at the designated terminals on the refrigerant circuit controller and replace if out of range; check wiring.

System behaviour: Heat pump switched off. [oai_citation:2‡viessmanndirect.co.uk](https://viessmanndirect.co.uk/files//1041ad9a-7e00-4a2d-9026-ae8a00ced6e2/Service%20Intructions.pdf?srsltid=AfmBOoqGPFmyeJXYagVL-aLS7nhabMstRD074uzxSfFJUusy_WDu7Bs2&utm_source=chatgpt.com)

Likely cause: Flow sensor lead break or internal failure.

Basic checks (owner): Ensure all valves are open and strainers recently cleaned; then call installer.

Installer actions: Verify flow rate mechanically, check flow sensor wiring and signal, replace sensor if no signal with confirmed flow.

System behaviour: Refrigerant circuit will not start. [oai_citation:3‡viessmanndirect.co.uk](https://viessmanndirect.co.uk/files//1041ad9a-7e00-4a2d-9026-ae8a00ced6e2/Service%20Intructions.pdf?srsltid=AfmBOoqGPFmyeJXYagVL-aLS7nhabMstRD074uzxSfFJUusy_WDu7Bs2&utm_source=chatgpt.com)

Likely cause: Inverter safety trip or repeated protective shutdown.

Basic checks (owner): Switch the unit off at the isolator for a few minutes and back on. If the code returns, stop trying to reset and call your installer.

Installer actions: Check inverter fault history, compressor current, and refrigerant circuit conditions; follow Viessmann diagnostic instructions.

System behaviour: Refrigerant circuit off. [oai_citation:4‡viessmanndirect.co.uk](https://viessmanndirect.co.uk/files//1041ad9a-7e00-4a2d-9026-ae8a00ced6e2/Service%20Intructions.pdf?srsltid=AfmBOoqGPFmyeJXYagVL-aLS7nhabMstRD074uzxSfFJUusy_WDu7Bs2&utm_source=chatgpt.com)

Likely cause: Defective or disconnected oil sump heater, or wiring fault.

Basic checks (owner): None – engineer only.

Installer actions: Check power at the appropriate terminals on the controller, continuity of the heater element, and replace heater if faulty.