E33 = DHW sensor fault. Combi-only error. The thermistor mounted at the DHW plate exchanger outlet reads open or shorted. With no valid DHW temp signal, the combi can\'t safely modulate for DHW calls.
Symptoms
- E33 displayed when a hot tap opens
- Heating mode works normally; only DHW is affected
- Sometimes paired with intermittent cold water during a DHW call
Diagnostic sequence
- Locate the DHW sensor on the DHW outlet of the plate exchanger inside the cabinet
- Disconnect the sensor leads at the control board
- Measure resistance. Compare to NTC table (12kΩ at 77°F is the baseline)
- OL = failed open; near-0 = shorted; both = replace
- Test the wiring between sensor and control — broken wire or loose connector causes the same fault
Replacement
- Sensor: $25-50
- Access requires opening the combi front cover; isolate DHW shutoffs if removing sensor from a wet location
- Clip-on type vs well-mount type depending on platform
Common cause: condensation damage
The DHW sensor sits in a humid environment near the plate exchanger. Years of condensation can corrode the sensor leads and connector. After replacement, treat connector contacts with dielectric grease to prevent recurrence.
Bottom line
E33 = combi DHW sensor. Cheap fix. Almost always the sensor itself; occasionally the wiring or connector. Annual service should include sensor connector inspection.