03 Moderate Moderate DIY

Rinnai Tankless Water Heater 03 — Loss of Communication / EZConnect Issue

Rinnai Tankless Water Heater · Error code 03

What 03 Means

Communication fault between two units linked with EZConnect/MSA or between the main board and remote.

Symptoms

  • Code 03 with multi-unit setup
  • One unit fires, other doesn't
  • Remote dropping out

Common Causes

1
EZConnect cable loose or damaged

Communication harness between units unplugged or pinched.

2
MSA controller fault

Manifold System Adapter failed.

3
Bad remote wiring

MC-91 controller wire shorted or open.

4
PCB communication chip failure

Less common; replace board.

How to Fix It

  1. Power down both units at the breaker.
  2. Reseat the EZConnect / MSA communication cables on both ends.
  3. Inspect for rodent damage or pinched wires.
  4. Swap remote wires to isolate which unit is faulting.
  5. If wiring is good, replace the MSA or PCB.

More Rinnai Tankless Water Heater Codes

02 02 — Power Interruption / Memory Loss 10 10 — Air Supply or Exhaust Blockage 11 11 — No Ignition 12 12 — Flame Failure 14 14 — Thermal Fuse / Overheat Cutoff 16 16 — Outgoing Water Over-Temperature

All Rinnai codes → All tankless water heater codes →

Verified against manufacturer documentation: rinnai.us

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.