Rinnai Tankless Error Code 51: What an Inlet Water Temperature Sensor Fault Means for East Cooper Homes
If your Rinnai tankless flashes Code 51 in Mount Pleasant, Daniel Island, or somewhere else in East Cooper, the heater is telling you it cannot trust the temperature of the cold water coming into the unit.
Code 51 means the inlet water temperature thermistor — the sensor on the cold-water side — is open, disconnected, shorted, scaled up, or has a wiring problem. Unlike a hard lockout, Rinnai still lets the heater run when this sensor is open or unplugged. The display usually alternates between your set temperature and Code 51. That does not mean everything is fine. The board is guessing at incoming water temp, so it cannot modulate gas and flow as accurately as it should.
I'm Christian with Avery & Sons Plumbing + Tankless. We see Code 51 around Charleston often enough that neighbors deserve a plain walkthrough — what it means, what you can safely check, and when to call.
What the unit is doing
Your tankless reads cold water in, heats it on demand, and sends hot water out. The inlet thermistor tells the control board how warm or cool that incoming water is. Ground water and summer mains around Charleston are not the same temperature, and the board uses that reading to set burner output.
Code 51 sits in the same family as Code 32 (outgoing water temperature sensor), Code 33 (heat exchanger outgoing temperature sensor), and Code 34 (combustion air temperature sensor). Different probe, same idea: bad thermometer, shaky control. The difference is Rinnai treats a failed inlet sensor as a soft fault — the unit keeps firing instead of locking out — so people sometimes ignore it for months.
Common causes we see around here
Failed or scaled thermistor. The sensor opens, shorts, or drifts. Hard-water scale coats the probe tip so resistance readings no longer match real water temperature. Same Charleston / East Cooper water story we see with Codes 14, 16, 32, 33, and LC.
Loose, chewed, or heat-damaged wiring. A harness can crack near the inlet, get nicked during a filter clean, or pick up moisture after a coastal storm. The board sees an open circuit and flashes Code 51 while still allowing hot water.
Disconnected plug after service. After a flush, valve swap, or DIY filter clean, the inlet sensor connector sometimes does not seat fully. The unit runs, the code stays.
Leaving it because "it still works." Because Code 51 does not shut you down, neighbors leave it alone until showers go lukewarm, scalding, or inconsistent. By then the board has been flying half-blind for a while.
Safe checks you can do yourself
Keep it simple. If you smell gas, leave and call your gas utility — do not troubleshoot.
Note what the display shows. Code 51 alone, or alternating with the set temperature? Write it down before you reset anything.
Clean the inlet water filter. Shut the cold isolation valve, relieve pressure at a faucet, pull the inlet screen, and rinse debris. A clogged filter is not Code 51 by itself, but restricted flow and dirty water both stress inlet-side parts — and cleaning the screen is a safe homeowner step.
Look for obvious wire damage near the cold inlet if you can see the harness without removing the front cover. Do not pull plugs or drain the heater unless you know how to reseat the O-ring and pressure-test afterward.
Try one clean power reset. Turn the unit off at the controller, kill the breaker for 30 seconds, restore power, and see if Code 51 returns on the next hot-water call. If it comes right back, stop resetting.
Do not yank the inlet sensor unless you are a licensed tech. Pulling that probe means isolating water, draining, ohm readings, and putting the seal back correctly so you do not create a leak.
Do not ignore a flashing Code 51 because the showers still work. "Still running" is not the same as "running right."
When to call a pro
Call if Code 51 will not clear after a filter clean and one reset, if water temperature feels off, if you also see Codes 32, 33, 14, 16, or LC, or if the unit short-cycles.
A real service call will ohm the inlet thermistor, inspect the harness back to the board, clean scale off the probe, and replace the sensor if the readings are wrong. Around Charleston, Daniel Island, Mount Pleasant, and the rest of East Cooper, we see Code 51 most often from a scaled or open inlet probe — the soft fault that quietly ruins temperature control until someone finally looks.
Soft next step
If your Rinnai is throwing Code 51, clean the inlet screen, try one reset, and get eyes on it if the code sticks. Avery & Sons Plumbing + Tankless can help. Call (843) 305-5555 or visit averysplumbing.com. We're local, we work these heaters every week, and we'll tell you what it needs without the scare tactics.
Stay comfortable — Code 51 is the heater asking for a trustworthy inlet-temp reading, not a mystery you have to live with.
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