Navien Error Code 127 (E127): Air Pressure Sensor (APS) Fault

What the code means

Navien's NPN User's Information Manual (NPN-160U/180U/199U and NPN-120E/160E/180E/199E) lists E127 as "Abnormal operation: Air volume Open/Short/Default value/No answer," with the action "Contact Technical Support at 1-800-519-8794." Navien's NFB-C commercial boiler manual lists the same code number as "Abnormal operation: APS," which confirms that the "air volume" signal comes from the air pressure sensor. The APS is one of the safety devices Navien lists for the NPN, and the NFB-C manual shows both a target APS voltage and the current APS voltage in its information menu, so the board expects a specific voltage from it. In plain terms, E127 is a sensor-signal fault: the reading is missing, impossible (open or shorted), or never moved from its starting value. That's different from E110, which Navien lists as abnormal air pressure on the NPN (the sensor works but reports too little airflow, usually a blocked vent), and from E157, abnormal air volume control, which is also its own code in the NPN chart.

Likely causes

• A failed air pressure sensor (APS) that reads open or shorted
• A loose, unplugged or damaged wire or connector between the APS and the main control board (PCB)
• A cracked, kinked, disconnected or water-filled pressure tube going to the APS, which can leave the sensor stuck at its default reading (a technician check; Navien's manuals don't spell out the tubing as a cause)
• A combustion fan problem. Navien's manuals don't list this for E127, but the APS reads the air the fan moves, and one NPN-160U owner on JustAnswer reported Navien Tech Support sending both an air pressure switch and a new fan assembly for code 127.
• Less often, a faulty main control board (PCB) that can't read the sensor

Safe DIY checks

1. Reset once. Navien's NPN manual says to press the Reset button on the front panel; on outdoor models, press the OK button once so "RST" shows on the display.
2. Check outside that the vent and air intake terminations aren't blocked by snow, ice, leaves or a nest. That's Navien's step for the related E110 code, and it's safe to look without taking anything apart.
3. If E127 comes back after a reset, stop resetting. Navien's chart gives no owner fix for E127 and says to contact Technical Support at 1-800-519-8794.
4. Have the model number, serial number and error code ready when you call, and ask whether the unit is still under warranty before paying for parts.
5. If you smell gas or a CO alarm goes off, leave the house and call your gas company or 911 from outside.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• E012 / 012 / 012E Navien Tankless Water Heater E012 / 012 / 012E
• E109 / 109 / 109E Navien Tankless Water Heater E109 / 109 / 109E
• 10 Bradford White Tankless Water Heater 10
• 20 Bradford White Tankless Water Heater 20
• 10 Noritz Tankless Water Heater 10

All Navien codes →

Sources

1. Navien NPN User's Information Manual, 3.4 Resetting the Water Heater and 5.2 Understanding Error Codes (E127 Abnormal operation: Air volume Open/Short/Default value/No answer; E110; E157)
2. Navien NPN Installation & Operation Manual (APS listed under safety devices, component diagram and parts list)
3. Navien NFB-C Installation & Operation Manual, 11.7 Understanding Error Codes (E127 Abnormal operation: APS) and information menu (Target APS / Current APS voltage)
4. JustAnswer: Navien NPN-160U error code 127 (owner report of Navien Tech Support sending an air pressure switch and fan assembly)