Navien E008 Error Code — What It Means
E008 on your Navien tankless water heater indicates a fault in the outlet temperature sensor circuit, specifically a thermistor short. The outlet thermistor measures the temperature of hot water leaving the heat exchanger. When the control board detects a short in this sensor circuit, it throws E008 and shuts down heating to prevent unsafe operation.
This code falls into Navien’s category of sensor and internal faults, alongside E007 and E036. It is not related to water flow or pressure issues. The fault almost always involves the outlet thermistor itself or the wiring and connector that link it to the control board.
Common Causes
- Failed outlet thermistor The sensor element inside the thermistor has shorted internally, sending invalid readings to the control board.
- Damaged wiring harness or connector A pinched, frayed, or broken wire in the outlet thermistor circuit can mimic a sensor short and trigger E008.
- Moisture or corrosion at the sensor connector Water intrusion or corrosion at the thermistor plug can create a short circuit that appears as a failed sensor.
- Loose or corroded connector pins Poor contact at the thermistor connector can cause intermittent shorts or false fault readings.
Step-by-Step Fix
- Confirm the error code on the display panel and verify it reads E008, not E007, E036, or a flow-related fault.
- Power off the unit at the service disconnect or circuit breaker and allow a few minutes for capacitors to discharge before opening any panels.
- Locate the outlet thermistor on the hot water side of the heat exchanger and inspect the sensor body, wiring, and connector for visible damage, corrosion, moisture, or pinched wires.
- Disconnect the thermistor connector and use a multimeter to measure the resistance across the sensor leads, comparing the reading to the specification chart in your model’s service manual.
- Replace the outlet thermistor if the resistance reading is out of range or if the sensor shows a short (zero or near-zero resistance at room temperature).
- Repair or replace the wiring harness if the sensor tests good but the wiring or connector shows damage, corrosion, or poor contact.
- Clear the fault code by restoring power and running a hot water tap to verify the heater fires without error, then monitor for 24 hours to confirm the repair holds.
Parts Often Needed
| Part | Notes |
|---|---|
| Navien outlet thermistor / hot water outlet temperature sensor | Amazon | Verify the exact part number for your Navien model before ordering, as sensors vary by series. |
| Thermistor wiring harness or connector | Amazon | Order if the sensor is intact but the wiring or plug is damaged beyond repair. |
When to Call a Pro
If you are uncomfortable working inside a tankless water heater cabinet, measuring electrical resistance, or identifying the outlet thermistor circuit, contact a licensed plumber or Navien-certified technician. Also call a pro if replacing the sensor and harness does not clear E008, if the code returns intermittently, or if you see other fault codes alongside E008. Navien technical support can help your technician diagnose complex board or heat exchanger issues when you have the unit’s serial number ready.