Thermocouple
Common pilot and ignition part on gas water heaters.
Navien E006 Error Code — What It Means
E006 on a Navien tankless water heater or combi-boiler indicates the exhaust (flue) high-limit sensor detected temperatures above the safe threshold. The exhaust thermistor monitors flue gas temperature leaving the heat exchanger; if it reads above approximately 149°C (300°F), the unit shuts down to prevent damage to the PVC venting, the heat exchanger, or surrounding combustibles.
Common Causes
- Blocked or restricted flue/exhaust pipe — A partially blocked PVC exhaust cap (birds, debris, ice) forces flue gases to linger longer in the heat exchanger, raising exhaust temperature.
- Scaled or fouled heat exchanger — Mineral scale from hard water insulates the heat exchanger surfaces, causing combustion gases to remain hotter when they exit.
- Excessive gas pressure — High incoming gas pressure causes more intense combustion and elevated flue temperatures.
- Failed exhaust thermistor — A thermistor that drifts out of calibration may falsely report high exhaust temperature even when actual temperatures are normal.
Step-by-Step Fix
- Inspect the exhaust vent termination — Go outside and check the PVC exhaust cap. Clear any blockage, ice, or debris. For concentric vent systems, check both the exhaust and combustion air annulus.
- Check the exhaust thermistor — With power off, locate the exhaust temperature sensor (small probe near the flue outlet). Disconnect its connector and measure resistance with a multimeter. At room temperature (~25°C), most Navien exhaust thermistors read approximately 10 kΩ. Compare to the temperature-resistance chart in the service manual.
- Flush the heat exchanger — If the unit has been in service for more than 2–3 years in a hard water area, perform a descaling flush using white vinegar or Navien’s recommended descaling solution through the service valves.
- Check incoming gas pressure — Have a licensed gas technician measure manifold gas pressure at the unit. For natural gas, manifold pressure should be 3.5” WC; for propane, 10” WC (verify against unit nameplate).
- Clear E006 and test — After addressing the cause, reset the unit (press the Power button off and on, or hold Reset on NPE series units). Run a full demand cycle and confirm no E006 recurrence.
Parts Often Needed
| Part | Notes |
|---|---|
| Exhaust thermistor | Navien OEM; match to NPE or NCB model |
| Descaling kit (vinegar flush) | Use food-grade white vinegar; 1 gallon minimum |
| PVC vent cap (exhaust) | Replace if cracked or blocked by bird screen debris |
Need Replacement Parts?
Before ordering, match the water-heating part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
Gas pressure adjustment and heat exchanger descaling with chemical descalers should be performed by a licensed plumber or HVAC technician. Do not operate the unit with a confirmed blocked flue.