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Navien tankless water heater code E736: Cascade communication fault

Tankless water heater, Canada and United States

In a cascade of linked water heaters, the heaters are not communicating with each other normally.

This needs a licensed gas technician. There is nothing safe to check yourself. Leave the covers on and book a professional.

What causes it

  1. A communication fault between the linked heaters that needs a technician. Navien lists no owner fix for this code.

    Book a licensed gas technician

How to read this code on your unit

Booking a licensed gas technician

Contact Navien before booking a repairYou can do this

  1. Have the model number, serial number, purchase date, where the heater is installed and the error code ready.
  2. Call Navien Technical Support on 1-800-519-8794, or contact the company that installed the heater.
  3. For a repair under warranty, the installer or service provider must contact Navien before starting the work.

Book a licensed gas technicianlicensed gas technician

  1. Turn the unit off and leave the covers on. Gas components must only be serviced by a licensed technician.
  2. Note the code, how often it appears, and the model and serial numbers from the rating label.
  3. Book a licensed gas technician and give them those details.

Models that show this code

21 models in 6 ranges
RangeModels
NPE-A (first generation)NPE-180A, NPE-210A, NPE-240A
NPE-S (first generation)NPE-150S, NPE-180S, NPE-210S, NPE-240S
NPE-A2NPE-180A2, NPE-210A2, NPE-240A2
NPE-S2NPE-180S2, NPE-210S2, NPE-240S2, NPE-150S2
NPN-ENPN-120E, NPN-160E, NPN-180E, NPN-199E
NPN-UNPN-160U, NPN-180U, NPN-199U

Where this comes from

Checked against the manufacturer's manual on 9 October 2026.

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Other Navien tankless water heater codes

Code Meaning What you can do
E445 Bypass mixing valve fault Call a pro
E515 Circuit board fault Safe checks first, then a pro
E517 DIP switch setting error Call a pro
E593 Front panel button fault Call a pro
E594 Memory chip (EEPROM) fault Call a pro
E615 Input and memory fault Call a pro
E740 Outdoor temperature sensor fault Call a pro
E760 Flushing or service reminder Call a pro
E762 Vent pipe not fully inserted Call a pro
E773 Wrong gas type detected or gas connection fault (depends on model) Call a pro
E782 Main panel communication fault Call a pro
E786 Panel connection fault Call a pro