Rinnai Tankless Error Code 03: What a Power Interruption During Bath Fill Means for East Cooper Homes
If your Rinnai tankless flashes Code 03 and the hot water won't start after a brief outage in Mount Pleasant, Daniel Island, or somewhere else in East Cooper — here's the short version.
Code 03 means power interruption during Bath Fill. The unit was mid-fill when the power dropped. When juice comes back, Rinnai holds the water off on purpose so a half-finished fill doesn't dump unexpectedly. Not a random glitch. Different from Code 65 (water flow control fault during bath fill) — 03 is specifically "power died while Bath Fill was running." Same bath-fill neighborhood. Different code.
I'm Christian with Avery's Plumbing. We work these units all over Charleston, and Code 03 shows up often enough after storms, breaker trips, or a quick power flicker that a plain-English walkthrough helps.
What the unit is doing
Bath Fill is the controller feature that fills a tub to a set volume (or time, depending on the model), then stops. If power cuts while that sequence is active, the board doesn't know how much water already went into the tub. So when power returns, it locks out flow and shows Code 03 instead of guessing.
Rinnai's own guidance is simple: turn off all hot-water fixtures, then press the ON/OFF button on the controller twice to reset the code. Fair. It's a safety hold — not something to ignore if it keeps coming back.
If you're comparing codes: we already covered Code 10 (air supply / exhaust blockage), Code 11 (no ignition), Code 12 (flame failure), Code 14 (thermal fuse / overheat), and Code 16 (over temperature warning) for East Cooper homes. Code 03 is about Bath Fill and power, not combustion or scale.
Common causes we see around here
Storm flicker / brief outage. Coastal thunderstorms around Charleston and East Cooper knock power for a few seconds all the time. If someone was running Bath Fill when the lights blinked, Code 03 is the unit saying it lost its place.
Breaker trips or GFCI resets. Garage and outdoor installs on Daniel Island and Mount Pleasant sometimes share a circuit with tools or a dehumidifier. A trip mid-fill looks the same to the board as a utility outage.
Bath Fill still armed when power hiccuped. Someone started the fill, walked away, and the sequence never finished cleanly. The code is doing what it's supposed to.
Repeated Code 03 after a clean reset. If you've reset properly three times and it comes straight back, Rinnai points at the control board needing service — not another button mash. That's pro territory.
Safe checks you can do yourself
Keep it simple. If you smell gas, leave the area and call your gas utility first — don't troubleshoot.
Turn off every hot tap and fixture. Tub, shower, sinks, laundry — all of them. The reset won't stick if something is still calling for hot water.
Press ON/OFF on the controller twice. That clears Code 03 on most Rinnai residential units. Watch the display go back to a normal temperature readout.
Open a hot tap and confirm flow. If hot water returns and the code stays gone, you were dealing with a one-time power interruption during Bath Fill.
Note the pattern. Lights flicker? Bath Fill running? Storm through Mount Pleasant or James Island? Write it down. If Code 03 returns after three clean resets, stop resetting and call.
Do not open the cabinet or keep forcing Bath Fill while the code is active. Gas appliances aren't where you get curious.
When to call a pro
Call if Code 03 comes back after you've turned off all hot fixtures and reset with ON/OFF twice (especially after three tries), if Bath Fill never completes cleanly, if you're under warranty, or if anything else on the controller looks wrong. A real diagnosis checks power at the unit, Bath Fill settings, and — when resets fail — the PC board per Rinnai's service steps.
Around Charleston, Daniel Island, Mount Pleasant, and the rest of East Cooper, brief storm outages and shared-circuit trips are the two we see most on Code 03. True board failures are less common, but they're not rare after years of hard coastal power events.
Soft next step
If your Rinnai is throwing Code 03 and the simple reset didn't stick — or you'd rather not guess — Avery's Plumbing can take a look. Call (843) 305-5555 or visit averysplumbing.com. We're local, we work these units every week, and we'll tell you straight what it needs.
Stay safe — and remember Code 03 is usually the unit being careful after a power blip mid-fill, not a mystery failure.
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