E4 = inlet temperature sensor fault. Same as E3 diagnostic but on the inlet side rather than outlet.
What it does
Inlet sensor lets the Tempra know how cold the incoming water is. Combined with the outlet sensor reading, the control calculates required power output. If inlet sensor fails, the control can\'t determine temperature rise needed and locks out.
Diagnostic sequence
- Power off at all breakers; verify with voltage tester
- Open the unit per service manual
- Locate inlet sensor at the cold inlet pipe
- Disconnect sensor leads at control board
- Multimeter on Ω; measure across leads
- Compare to NTC table (typically 10kΩ at 77°F baseline)
- OL = replace; near 0 = replace; out of spec = replace
Replacement
- Stiebel inlet sensor: $25-50
- Often the same part number as outlet sensor — same NTC characteristics
- Within 3-year parts warranty, free through Stiebel tech support
Common cause: water leak damage
The inlet sensor is in a wet environment. Sensor or connector corrosion from condensation or minor leaks is common after 5-8 years. After replacement, dielectric grease on the connector contacts extends sensor life.
Reset
- Replace sensor
- Power cycle: all breakers off 30 seconds, then on
- Verify Flow Control behavior at hot taps
Bottom line
E4 = inlet NTC sensor. $25-50 part. Same procedure as E3 but on the inlet side. Stiebel sensor diagnostic always starts with the resistance table.