278Error - Navien NFC-175 Service Manual

5.2.14 278Error

Error conditions and Check Items
Error
E278
System supply temperature sensor open
Check items
Check method
Fault
Defective sensor
Possible Issues
Version 1.0
If an error (open: MΩ) in the system supply temperature sensor is detected, the system displays
the 278E error on the front panel. If this occurs, the boiler initiates shutdown.
1. Check if the system supply temperature sensor connector is wet due to any reason,
including leakage.
2. Replace the defected system supply temperature sensor.
3. Check circulation pump operating status and for proper flow through the space heating
line.
4. Check the voltage on the PCB to verify proper power to the pump. If there is no voltage,
bleed out air from the system before resetting the unit. If issue persists, replace the pump.
Is the system supply
temperature sensor
normal?
Yes
Check connector
(#7 and #8 of CNT1)
Possible Causes
Defective temperature Check if the temperature sensor is open and if the connector is connected
sensor connector properly.
Check the resistance of the temperature sensor. (Defective if it is 197kΩ or higher)
Temperature sensor Replace the temperature sensor if the resistance value is abnormal.
●
Check the temperature displayed on the front panel.
●
Defective circulation
Check power supply to the circulation pump(AC 120V)
pump(external)
Defective PCB If the issues continue despite checking the items above, replace the PCB.
Incorrect setting Ensure SH Control Method is set for the correct mode (refer to page 37).
Description
No
Replace system supply thermistor
Resistance between #7 and #8 of CNT1
is from 0.437kΩ to 176kΩ at normal
condition. (See page 50 for Wiring
Diagram)
Before measuring resistance of the
thermistor, detach the connector
from the PCB.
Check method
NFC Service Manual 99