NSS Diagnostics and Troubleshooting 2 — Questions and Answers
Question 1: A Navien NPE series tankless water heater displays error code E003. What does this indicate?
- Ignition failure (Correct answer)
- Exhaust blockage
- Inlet water temperature sensor fault
- Flow sensor malfunction
Correct answer: Ignition failure
Error code E003 on Navien NPE units indicates an ignition failure, meaning the burner did not ignite after the set number of attempts.
Question 2: When a Navien combi-boiler shows low water pressure (error E407), what is the FIRST corrective action?
- Replace the expansion tank
- Check and repressurize the heating system to 12–15 PSI (Correct answer)
- Inspect the pressure relief valve for leaks
- Clean the water filter strainer
Correct answer: Check and repressurize the heating system to 12–15 PSI
E407 indicates low system pressure; the first step is to repressurize the heating loop to the recommended 12–15 PSI range before further diagnosis.
Question 3: A technician finds a Navien NPN boiler short-cycling with no error code displayed. Which component should be inspected first?
- Burner orifice
- Thermistor wiring harness
- Differential pressure switch (Correct answer)
- DHW flow sensor
Correct answer: Differential pressure switch
A faulty or obstructed differential pressure switch can cause the unit to short-cycle by falsely signaling flue blockage or proving combustion air issues.
Question 4: What does a Navien error code E012 typically indicate on a tankless unit?
- Flame loss during operation
- Outgoing water overheating (Correct answer)
- Cold water sandwich effect
- Gas valve solenoid failure
Correct answer: Outgoing water overheating
E012 on Navien tankless units signals an outgoing water temperature overheat condition, triggering a safety shutdown.
Question 5: During a combustion analysis on a Navien unit, CO readings are elevated. Which adjustment should a technician make first?
- Increase the gas inlet pressure
- Adjust the air-to-fuel ratio via the gas valve offset (Correct answer)
- Replace the heat exchanger
- Clean the condensate trap
Correct answer: Adjust the air-to-fuel ratio via the gas valve offset
Elevated CO typically indicates a rich combustion condition; adjusting the gas valve offset corrects the air-to-fuel ratio before other interventions.
Question 6: A Navien NHB boiler is producing error E110 repeatedly. What does this fault code represent?
- DHW thermistor open circuit
- Exhaust high-limit trip (Correct answer)
- Low gas pressure lockout
- Condensate pump failure
Correct answer: Exhaust high-limit trip
E110 on Navien boilers indicates the exhaust high-limit sensor has tripped, usually due to blocked flue, recirculation, or a failed fan.
Question 7: When diagnosing a Navien NPE unit that fires briefly then shuts down with E016, what is the most likely cause?
- DHW inlet thermistor shorted
- False flame signal from the ionization rod (Correct answer)
- Blocked condensate line
- Low incoming water pressure
Correct answer: False flame signal from the ionization rod
E016 indicates a false flame detection; a dirty or improperly gapped ionization rod can send a phantom flame signal causing immediate shutdown.
NSS Diagnostics and Troubleshooting 2
A Navien NPE series tankless water heater displays error code E003.
What does this indicate?