E3 = outlet temperature sensor fault. The NTC thermistor that monitors output water temperature reads open or shorted. Without valid outlet temp, the control can\'t modulate element power safely.
What it does
Outlet sensor closes the loop on Tempra\'s Flow Control logic. The control reads inlet temp and outlet temp continuously; the difference, multiplied by flow, equals heat output. If outlet sensor fails, the control loses Flow Control capability and locks out.
Diagnostic sequence
- Power off at all breakers. Verify with voltage tester
- Open the unit per the service manual
- Locate outlet sensor at the hot outlet pipe inside the cabinet
- Disconnect sensor leads at the control board
- Multimeter on Ω; measure across the sensor leads
- Compare to NTC table — 10kΩ at 77°F is the baseline for many Stiebel sensors
- OL = open, replace. Near 0 = shorted, replace
Wiring check before replacing
Sometimes E3 isn\'t the sensor — it\'s damaged wiring between sensor and control board:
- Disconnect at both ends
- Test continuity through harness
- If harness is open or shorted, repair wiring
- If harness is good and sensor is bad per resistance test, replace sensor
Replacement
- Stiebel outlet sensor: $25-50
- Order through Stiebel tech support — warranty covers in first 3 years (parts) on Tempra Plus
- Replace with same thermal grease application
- Verify sensor seats firmly against pipe — air gap = wrong readings
Reset
- Replace sensor
- Power cycle: all breakers off 30 seconds, then on
- Open hot tap; verify operation and steady setpoint maintenance
Bottom line
E3 = outlet NTC sensor. $25-50 part; 30-minute service. Common Tempra Plus issue after 5+ years. Stiebel tech support helps with diagnostic if multimeter readings are ambiguous.