NSS Diagnostics and Troubleshooting 4 — Questions and Answers
Question 1: A technician is diagnosing a Navien NPN-199 that displays E407 even after repressurizing the system. What should be checked next?
- Verify the expansion tank pre-charge pressure and condition (Correct answer)
- Replace the gas valve assembly
- Clean the heat exchanger waterways
- Inspect the ignition electrode gap
Correct answer: Verify the expansion tank pre-charge pressure and condition
If repressurizing does not resolve E407, a waterlogged or failed expansion tank cannot maintain system pressure, causing repeated pressure drops and the same error.
Question 2: What is the correct method to clear a hard lockout on a Navien NPE tankless water heater?
- Cycle the DHW flow sensor three times
- Press and hold the power button for 3 seconds or cycle power at the breaker (Correct answer)
- Adjust the temperature setpoint up then down
- Disconnect and reconnect the gas supply line
Correct answer: Press and hold the power button for 3 seconds or cycle power at the breaker
Navien hard lockouts are cleared by pressing the power button for 3 seconds or cycling the unit's circuit breaker after addressing the root cause.
Question 3: A Navien unit is installed in a direct-vent configuration but is drawing combustion air from the room. What error is most likely to result?
- E407 — low water pressure
- E012 — overheating
- E003 — ignition failure due to oxygen-depleted combustion air (Correct answer)
- E110 — exhaust high limit
Correct answer: E003 — ignition failure due to oxygen-depleted combustion air
If the combustion air pipe is disconnected or improperly terminated, the unit may draw oxygen-depleted room air, causing unreliable ignition and E003 lockouts.
Question 4: When checking a Navien unit's gas valve, a technician measures 0V at the valve solenoid terminals during a call for heat. What does this indicate?
- The gas valve solenoid coil has failed open
- The PCB is not sending the activation signal to the valve (Correct answer)
- The gas valve is operating normally in standby
- The inlet gas pressure is too high
Correct answer: The PCB is not sending the activation signal to the valve
Zero voltage at the solenoid terminals during a heat call indicates the control board is not energizing the valve, pointing to a PCB or wiring fault rather than a valve failure.
Question 5: A Navien NHB combi-boiler is losing DHW temperature during simultaneous space heating demand. What is the most appropriate diagnostic step?
- Increase the gas orifice size
- Check the diverter valve operation and priority switching logic in the installer settings (Correct answer)
- Clean the condensate trap
- Replace the expansion tank
Correct answer: Check the diverter valve operation and priority switching logic in the installer settings
Simultaneous DHW and heating demand is managed by the diverter valve and priority logic; improper settings or a sluggish valve will cause DHW temperature to drop.
Question 6: After replacing a Navien flow sensor, the unit still shows no DHW activation. What is the next diagnostic step?
- Verify minimum flow rate is being met by opening additional fixtures (Correct answer)
- Replace the PCB immediately
- Check the gas supply shutoff valve position
- Test the ignition electrode resistance
Correct answer: Verify minimum flow rate is being met by opening additional fixtures
Navien units require a minimum flow rate (typically 0.5 GPM) to activate; if minimum flow is not achieved, the unit will not fire regardless of sensor condition.
Question 7: A Navien unit is installed at high altitude (above 4,500 ft). A technician notices the unit is sooting and producing high CO. What adjustment is required?
- Increase incoming gas pressure to compensate for altitude
- Apply the high-altitude derate kit and adjust the gas valve offset for reduced air density (Correct answer)
- Replace the burner with a high-altitude model
- Increase the fan speed via the service menu only
Correct answer: Apply the high-altitude derate kit and adjust the gas valve offset for reduced air density
At high altitude, reduced air density requires a high-altitude derate kit and gas valve offset adjustment to maintain proper air-to-fuel ratio and prevent rich combustion.
NSS Diagnostics and Troubleshooting 4
A technician is diagnosing a Navien NPN-199 that displays E407 even after repressurizing the system.
What should be checked next?