E12 = supply temperature sensor fault. The supply (outlet) sensor reads open (disconnected/failed) or shorted. The control needs accurate supply temp to modulate the burner safely.
Sensor type
Lochinvar uses 12kΩ NTC (negative temperature coefficient) thermistors on most platforms. Resistance drops as temperature rises:
| Temperature | Expected resistance |
|---|---|
| 32°F (0°C) | 32,650 Ω |
| 77°F (25°C) | 12,000 Ω |
| 140°F (60°C) | 2,820 Ω |
| 180°F (82°C) | 1,520 Ω |
Compare to the resistance table in your service manual for exact values.
Diagnostic sequence
- Locate the supply sensor on the supply pipe at the boiler outlet
- Disconnect the sensor leads at the control board (sensor isolated)
- Set multimeter to Ω. Measure across the two sensor leads
- Compare to expected value for the current temperature
- OL (open) = sensor failed open; replace
- Near 0 Ω = sensor shorted; replace
- Out of spec for current temp = sensor drift; replace
Wiring fault
Sometimes E12 isn\'t the sensor — it\'s damaged wiring or a loose connector. With the sensor disconnected, the harness should read open. With the sensor connected at the sensor end only, you should read the sensor resistance through the harness. If readings differ, wiring is the fault.
Replacement
- Sensor: $25-50 (OEM Lochinvar part)
- Reuse the existing thermal grease or apply fresh on installation
- Mount with bracket clip flush to the pipe — air gap = wrong readings
- No drain required for clip-on sensors; well-mount types need pipe drain
Bottom line
E12 = NTC sensor test + replace. 12kΩ at 77°F is the baseline. Out of spec = replace. Always verify wiring before condemning the sensor.