Rinnai Tankless Error Code Troubleshooting

Troubleshooting your Rinnai tankless water heater is often a matter of reading the error code the unit provides when a hot water issue comes up and performing basic troubleshooting steps for that error code.

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Since Rinnai water heaters are one of the most advanced tankless water heaters available, we recommend to contact a service technician that’s authorized to work on Rinnai water heaters.

Each Rinnai unit uses a temperature controller (either MC-91-1 or MC-91-2) on the front panel to display various configuration options for installation as well as error codes for when problems arise. The previous nine error codes are stored in memory. To access these codes, hold the ON/OFF button for two seconds and simultaneously press the UP arrow.

Have a different tankless water heater brand that needs troubleshooting?

Note: While some error codes can be easy do-it-yourself fixes with a few simple tools, others are more complex and require the help of a professional plumber. Since you are dealing with natural gas and advanced technology, it’s often best to at least get a free quote from a local plumber.

Rinnai Tankless Water Heater Error Codes

Error Code 02
What it Means: Water heater will not work during freeze protection mode.

How to Fix: Place a service call.

Error Code 03
What it Means: Power interruption during Bath Fill (Water will not flow when power returns).

How to Fix: Turn off all the hot water fixtures in the house. Press the ON/OFF button twice to reset the unit and code. If code reappears, P.C. board will likely need replacing.

Error Code 05
What it Means: Bypass servo fault (on all water heaters with bypass valves).

How to Fix: Turn off power supply to the unit and then turn back on. Open a hot water tap to fire up the unit. If error code “05” reappears, bypass valve will need replacing.

Error Code 07
What it Means: Circulation flow rate has dropped below 2.1 gpm.

How to Fix: Confirm unit has power. Confirm water supply is on and you have at least 30 psi coming in. Check for loose, damaged, or broken connections in the pump wiring harness. Drain tank and confirm the inlet water filter isn’t clogged. Flush the heat exchanger.

Error Code 10
What it Means: Air supply or exhaust blockage.

How to Fix: Confirm that recommended Rinnai venting materials are used. Make sure there aren’t any obstructions in the venting system. Confirm vent length and diameter is within proper limits and clearances. There is a 95% chance the issue is with improper vent pipe installation.

Error Code 11
What it Means: No ignition (heater not turning on).

How to Fix: Assuming gas type and pressure are correct and there is no air in the gas line or other problem with the gas supply, confirm that: igniter generates spark, wiring connections are ok, unit is properly grounded, dip switches are set correctly, gas solenoid valves have no open or short circuits, burners are seated correctly, and no debris or dirt are clogging the burners.

Error Code 12
What it Means: Flame failure.

How to Fix: Assuming gas type and pressure are correct and there is no air in the gas line or other problem with the gas supply, confirm that: there are no loose connections at the PC board, power supply or flame rod, gas solenoid valve doesn’t have a short circuit, there is no obstruction in the venting system and gases are correctly venting out. Clean flame rod from scale buildup and burners from debris and dirt.

Error Code 13
What it Means: Combustion abnormality.

How to Fix: Confirm 4″ B-Vent venting materials were properly installed. Confirm proper installation of rest of vent system (clearance, materials, joints sealed, properly seated sections). Confirm dip switches properly set according to altitude. Confirm min/max inlet gas supply. Confirm unit’s low/high fire pressures properly set.

Error Code 14
What it Means: Thermal fuse fault or overheating.

How to Fix: Confirm there are no air flow obstructions in vent system or around the unit. Confirm no obstructions in the water flow system as there needs to be enough incoming cold water going through the heat exchanger. Confirm heat exchanger is in properly working condition.

Error Code 16
What it Means: Over temperature warning (safety shutdown because unit is too hot).

How to Fix: Check for low flow in the water circulation system which causes short cycling. Confirm there are no obstructions in the combustion chamber or exhaust piping. Confirm no blockage in heat exchanger fins. Clean out heat exchanger fins or replace heat exchanger.

Error Code 19
What it Means: Electrical grounding fault.

How to Fix: Check/test all water heater components for an electrical short.

Error Code 25
What it Means: Condensate trap is full.

How to Fix: Check condensate drain line and trap for blockage as well as the tank inside the cabinet. Confirm the condensate line has an air gap between unit and drain line leaving the unit. Check for loose, broken, or damaged connections from sensor to the PC board. If tank is correctly draining but error code 25 persists, replace the condensate tank and sensor.

Error Code 30
What it Means: Flammable vapors detected.

How to Fix: IMPORTANT: Leave the area the water heater is installed in until safety personnel have identified the area as safe and all flammable vapors have been eliminated.

Inspect flammable vapor sensor to confirm it’s not damaged and check wiring harness for loose, damaged, or broken connections. Once 100% sure no flammable vapors are present, turn power and gas back on, and open a hot water tap to fire the unit back up. If code reappears, replace the flammable vapor sensor.

Error Code 31
What it Means: Burner sensor abnormality (thermocouple).

How to Fix: Confirm dip switches and low/high fire manifold pressures are set correctly and thermocouple milli-volt range is within specs. If everything looks good, replace thermocouple.

Error Code 32
What it Means: Outgoing water temperature sensor abnormality (thermistor).

How to Fix: Confirm no loose, damaged, or broken connections on the thermistor. Check resistance readings to make sure they’re within specs. Check for scale build up on thermistor. Replace thermistor.

Error Code 33
What it Means: Heat exchanger outgoing sensor abnormality (thermistor).

How to Fix: Confirm no loose, damaged, or broken connections on the thermistor. Check resistance readings to make sure they’re within specs. Check for scale build up on thermistor. Replace thermistor.

Error Code 34
What it Means: Combustion air temperature sensor failure.

How to Fix: Confirm no loose, damaged, or broken connections on the thermistor. Check resistance readings to make sure they’re within specs. Check for scale build up on thermistor. Replace thermistor.

Error Code 35
What it Means: Tank temperature sensor abnormality.

How to Fix: Confirm no loose, damaged, or broken connections on the tank temperature sensor. Check resistance readings to make sure they’re within specs. Check for scale build up on sensor. Replace sensor.

Error Code 38
What it Means: CO sensor abnormality (RH-180 only).

How to Fix: Confirm 4″ B-Vent venting materials were properly installed. Confirm proper installation of rest of vent system (clearance, materials, joints sealed, properly seated sections). Confirm dip switches properly set according to altitude. Confirm min/max inlet gas supply. Confirm unit’s low/high fire pressures properly set. Replace CO sensor and PC board.

Error Code 41
What it Means: Freeze protection sensor abnormality (external units).

How to Fix: Confirm no loose, damaged, or broken connections on the freeze protection sensor/thermistor. Confirm sensor is open to outside air and not covered with insulation. Check resistance readings to make sure they’re within specs.

Error Code 51
What it Means: Inlet water temperature sensor abnormality.

How to Fix: Confirm no loose, damaged, or broken connections on the sensor wiring harness. Check for scale buildup on sensor and clean if needed. Check resistance readings to make sure they’re within specs.

Error Code 52
What it Means: Modulating solenoid valve signal abnormality (POV).

How to Fix: Confirm no loose, damaged, or broken connections on the POV valve. Check resistance readings to make sure they’re within specs. Check voltage to the POV valve while unit is attempting to fire. If you get proper voltage and error code remains, replace the PC board.

Error Code 57
What it Means: Burner overheat bi-metal switch fault.

How to Fix: Confirm no loose, damaged, or broken connections on the POV valve. Check resistance readings to make sure they’re within specs. Check voltage to the POV valve while unit is attempting to fire. If you get proper voltage and error code remains, replace the PC board.

Error Code 58
What it Means: Secondary heat exchanger sensor fault.

How to Fix: Code appears when one of the two bi-metals opens at the rear of the secondary heat exchanger. Confirm no loose, damaged, or broken connections at the sensor wiring harness. Check for scale build-up on the heat exchanger and flush if necessary. Treat hard water to prevent scale build-up in future.

Error Code 61
What it Means: Combustion fan motor fault.

How to Fix: Confirm no loose, damaged, or broken connections on fan motor wiring harness. Remove fan motor and inspect blower wheel to make sure it turn freely. Check fan housing for obstructions. Measure fan motor winding resistance. If windings open, replace motor. If windings within specs, replace PC board.

Error Code 63
What it Means: Circulation flow rate dropped below 1.3 gpm.

How to Fix: Use temperature controller to display flow rate (hold UP for 3 seconds and press the ON/OFF button). If flow rate is under 1.3 gpm, bleed air from tank and pump, clean inlet water filter, confirm your incoming flow rate is at least 6 gallons, and verify you have 30-50 PSI of inlet water pressure.

Error Code 65
What it Means: Water flow control abnormality.

How to Fix: Confirm no loose, damaged, or broken connections on the water flow servo. Turn off power to the unit and then turn back on to see if code disappears. If code reappears, place a service call.

Error Code 70
What it Means: PC board fault.

How to Fix: Confirm no loose, damaged, or broken connections on the PC board. Confirm dip switches set correctly. Replace PC board.

Error Code 71
What it Means: Solenoid valve circuit fault.

How to Fix: Confirm no loose, damaged, or broken connections on wiring harnesses for SV0, SV1, SV2, SV3, and SV4 gas valve solenoids. Check for moisture in plastic tubing on internal units and if moisture found, replace gas valve. Measure voltage of SV0 and SV1 and confirm they’re within specs. Measure resistance. Replace gas control valve. Replace PC board.

Error Code 72
What it Means: Flame sensing device fault.

How to Fix: Confirm no loose, damaged, or broken connections on flame rods. Confirm flame rod is touching flame when firing. Remove flame rod to inspect for carbon build-up and if necessary, clean flame rod and replace flame rod gasket. Check inside burner for debris which could be causing a short circuit. Replace flame rod, flame rod gasket, and PC board.

Error Code 73
What it Means: Burner thermocouple fault.

How to Fix: Confirm no loose, damaged, or broken connections on the burner thermocouple. Confirm dip switch settings are correct. Replace burner thermocouple. Replace PC board.

Error Code 79
What it Means: Water leakage detected.

How to Fix: Confirm no loose, damaged, or broken connections on the leak detector. Check heat exchanger and all piping for leaks. If no leaks found, wipe out any water inside cabinet, dry off leak sensor, and restart unit. If error code reappears, unplug sensor and fire up the water heater. If unit operates without the sensor, replace sensor and screw.

Error Code FE
What it Means: Flammable vapors detected.

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How to Fix: IMPORTANT: Leave the area the water heater is installed in until safety personnel have identified the area as safe and all flammable vapors have been eliminated.

Inspect flammable vapor sensor to confirm it’s not damaged and check wiring harness for loose, damaged, or broken connections. Once 100% sure no flammable vapors are present, turn power and gas back on, and open a hot water tap to fire the unit back up. If code reappears, replace the flammable vapor sensor and/or PC board.

Error Code FF
What it Means: Maintenance has been performed.

How to Fix: This code indicates that a service technician has recently performed maintenance or repair on the unit. This tells the system any error codes which appear happened after repairs were made. To enter the FF code, simultaneously press UP, DOWN, and ON/OFF.

Error Code LC0 (through LC9)
What it Means: Lime scale build-up.

How to Fix: Flush the heat exchanger as per the recommended flushing procedure found in the owner’s manual. To temporarily reset the code, press the ON/OFF button 5 times. This will allow the unit to operate for 70 hours after which another LC code will lock the unit from operating.

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