Every Rinnai Error Code and How to Clear It

If your Rinnai tankless water heater quits mid-shower, it rarely dies quietly. The unit runs a constant self-check, and the moment it detects a fault it shuts down and flashes a two-digit code that names the exact problem.

That code is your shortcut. Match it against the list below and you will know in seconds whether you are dealing with a five-minute filter cleaning, a simple reset, or a job for a licensed tech.

Here we walk through every common Rinnai code, where the controller hides it, and the exact steps to clear each one, plus how to stop the scale and airflow faults from coming back.

Where Will I Find Rinnai Error Codes?

You will find Rinnai error codes on the temperature controller panel (MC-91, MCC-91, MC-502) or on the clock display (MC-100, BC-100). Rinnai tankless water heaters have a built-in self-diagnostic system that monitors performance constantly. If the system detects an issue, it automatically shuts down to prevent damage and flashes a two-digit code.

When a code appears and the water runs cold, you can often fix it with a few troubleshooting steps. However, because these units involve gas and electricity, complex internal issues should be handled by a qualified technician. According to Rinnai, tankless units carry 1 year of labor coverage standard, extendable to 5 years if you register the unit within 90 days of purchase ⁽¹⁾.

Rinnai Tankless Error Codes

Below is a comprehensive list of Rinnai error codes. Use this guide to identify the problem and determine if it is a quick DIY fix or a job for a pro.

Error Code 02

What It’s Telling You

• The freeze protection mode is active and has stopped the water heater from operating.

The Solution

1. Call for service: This usually requires professional intervention. Contact the support team at 1-800-621-9419.

Error Code 03

What It’s Telling You

• The power supply was interrupted during the automatic bath fill function. If this happens repeatedly after power is restored, the fill function may be disabled.

The Solution

Follow these steps to reset the function:

1. Close the valves: Turn off all hot water faucets in your home.
2. Reset the unit: Locate the On/Off button on the controller and press it twice.
3. Check the PC board: If the code returns, the PC board may be faulty and require replacement.

Error Code 05

What It’s Telling You

• The bypass servo is malfunctioning. This only applies if your heater has bypass valves installed.

The Solution

Try these steps to clear the servo error:

1. Cycle the power: Turn the heater’s power supply off and then back on.
2. Test the water: Open a hot water tap to see if the heater fires up.
3. Replace the valve: If the code persists, the bypass valve is likely defective and needs replacement.

Error Code 07

What It’s Telling You

• The water flow rate has dropped below 2.1 GPM (Gallons Per Minute).

The Solution

Check the following components to restore proper flow:

1. Verify power: Ensure the heater is receiving power.
2. Check water pressure: Confirm the water supply is on and the pressure is at least 30 psi.
3. Inspect wiring: Look for loose or damaged wires in the pump harness.
4. Clean the filter: Remove the water inlet filter, clean out any debris, and reinstall it.
5. Flush the system: Refer to your manual to flush the heat exchanger and remove mineral buildup.

Error Code 10

What It’s Telling You

• There is a blockage in the air supply or exhaust vent.

The Solution

Inspect your venting system for these issues:

1. Check the vents: Ensure you are using manufacturer-approved venting materials and that there are no bird nests or debris blocking the pipe.
2. Verify clearances: Check that the vent installation meets proper clearance limits.
3. Check installation: Incorrectly installed vent pipes can restrict airflow and trigger this code.

Error Code 11

What It’s Telling You

• Ignition failure. The unit is trying to fire up but cannot establish a flame.

The Solution

First, ensure your gas is turned on and the tank is not empty. If gas is present, check the following:

1. Listen for the igniter: Verify that the unit is generating a spark when you turn on the water.
2. Check wiring: Look for loose connections on the igniter wiring harness.
3. Verify grounding: Ensure the heater is properly grounded.
4. Check dip switches: Confirm the dip switches on the circuit board are set correctly for your gas type.
5. Inspect gas valves: Make sure the gas solenoid valves are functioning and not stuck open.
6. Clean the burner: Remove any dirt or debris clogging the burner assembly.

Error Code 12

What It’s Telling You

• Flame failure. The unit lit initially but the flame went out unexpectedly.

The Solution

Assuming proper gas pressure and supply, investigate these components:

1. Inspect PC board wiring: Tighten any loose connections.
2. Check power consistency: Ensure there are no voltage drops in the power supply.
3. Clean the flame rod: Remove the flame rod and gently clean off any carbon buildup with sandpaper.
4. Check the gas valve: Verify the solenoid valve is not short-circuiting.
5. Clear debris: Clean the burner to ensure proper airflow and combustion.

Error Code 13

What It’s Telling You

• Improper combustion is occurring inside the unit.

The Solution

This is often related to airflow or gas pressure:

1. Check the venting: Ensure the venting system is installed according to the manual.
2. Verify dip switches: Confirm switches are set for the correct vent length and gas type.
3. Check gas supply: Ensure gas is flowing freely to the unit.
4. Adjust pressure: Have a professional verify that low and high fire pressures are set to specification.

Error Code 14

What It’s Telling You

• The thermal fuse has tripped, usually due to excessive heat.

The Solution

You need to find out why the unit got too hot:

1. Check for restrictions: Look for blockages in the venting, air intake, or water flow.
2. Inspect the heat exchanger: Cracks or blockages in the heat exchanger can cause this fault.

Error Code 16

What It’s Telling You

• Overheating warning. The output water temperature is dangerously high.

The Solution

This is often caused by low water flow or restricted airflow:

1. Clear obstructions: Check the exhaust pipe and combustion chamber for blockages.
2. Test water flow: Ensure the flow rate is high enough to prevent “short cycling” (rapid on/off switching).

Error Code 19

What It’s Telling You

• Electrical grounding fault.

The Solution

This is an electrical safety issue:

1. Test for shorts: Inspect all internal components for electrical short circuits ⁽²⁾. You may need an electrician to verify the home’s grounding.

Error Code 25

What It’s Telling You

• The condensate trap is full or blocked.

The Solution

Condensing units must drain water properly to function:

1. Clear the drain: Check the condensate drain line for clogs or kinks.
2. Check the air gap: Ensure there is a proper gap where the drain line exits the unit.
3. Inspect connections: Look for loose wires connecting the condensate sensor to the PC board.
4. Replace the sensor: If the trap is empty but the code remains, the sensor is likely faulty.

Error Code 30

What It’s Telling You

• The sensor has detected flammable vapors near the unit.

The Solution

Once the area is deemed safe:

1. Inspect the sensor: Check the flammable vapor sensor for damage or loose wiring.
2. Replace the sensor: If the code persists after the air is clear and the unit is reset, the sensor must be replaced.

Error Code 31

What It’s Telling You

• Burner sensor (thermocouple) failure.

The Solution

1. Check settings: Verify manifold pressures and dip switch settings.
2. Test voltage: Use a multimeter to ensure the thermocouple is reading the correct milli-volts.
3. Replace the part: If readings are out of range, replace the burner sensor.

Error Code 32

What It’s Telling You

• Outgoing water temperature sensor (thermistor) fault.

The Solution

1. Check wiring: Ensure the sensor is plugged in securely.
2. Measure resistance: Check the ohms reading against the manual specifications.
3. Clean the sensor: Remove any mineral scale buildup.
4. Replace the thermistor: If cleaning fails, install a new sensor.

Error Code 33

What It’s Telling You

• Heat exchanger temperature sensor fault.

The Solution

1. Inspect connections: Look for loose or broken wires.
2. Test resistance: Verify the sensor is functioning within the correct range.
3. Descale: Clean off any scale deposits.
4. Replace: Install a new heat exchange sensor if the error continues.

Error Code 34

What It’s Telling You

• Combustion air temperature sensor fault.

The Solution

1. Check wiring: Secure any loose connections.
2. Test resistance: Verify the sensor readings.
3. Clean the sensor: Remove dust or debris.
4. Check the fan: Ensure the fan blade is tight and in good condition.
5. Replace: Install a new air temperature sensor if needed.

Error Code 35

What It’s Telling You

• Tank temperature sensor fault.

The Solution

1. Inspect wiring: Check for damage to the sensor wires.
2. Test resistance: Confirm the sensor is reading correctly.
3. Clean: Wipe away any debris on the sensor.
4. Replace: Install a new tank temperature sensor.

Error Code 52

What It’s Telling You

• Modulating solenoid valve (POV) signal fault.

The Solution

1. Check wiring: Look for broken wires leading to the valve.
2. Measure resistance: Verify the valve’s resistance is within spec.
3. Check voltage: Ensure the proper voltage reaches the valve during ignition.
4. Replace the board: If the valve is fine but voltage is wrong, the PC board needs replacing.

Error Code 58

What It’s Telling You

• Secondary heat exchanger fault. This is specific to condensing models.

The Solution

1. Check connections: Ensure the sensor wiring is intact.
2. Flush the unit: Scale buildup is a common culprit; perform a flush.
3. Water treatment: If you have hard water, install a softener to prevent future errors.

Error Code 61

What It’s Telling You

• Combustion fan motor failure.

The Solution

1. Inspect wiring: Check the fan harness for loose connections.
2. Spin the fan: Ensure the blower turns freely and isn’t seized.
3. Check for blockage: Remove any debris from the fan housing.
4. Test resistance: If motor windings show bad resistance, replace the fan.

Error Code 65

What It’s Telling You

• Water flow control fault. The bath fill valve failed to close completely.

The Solution

1. Stop the water: Turn off the main water supply to the unit immediately.
2. Call a pro: This valve is internal and typically requires professional service to replace.

Error Code 70

What It’s Telling You

• PC board (Computer Board) fault.

The Solution

1. Inspect connections: Ensure all plugs on the board are seated correctly.
2. Check dip switches: Verify they are set to the correct position.
3. Replace the board: If connections are good, the PC board itself is dead and needs replacement.

Error Code 71

What It’s Telling You

• Solenoid valve circuit fault.

The Solution

1. Check wiring: Inspect connections on solenoids SV0 through SV4.
2. Check for moisture: Look for water in the plastic pressure tubes. If found, the gas valve may need replacing.
3. Test resistance: Measure the ohms on the solenoids.
4. Replace parts: You may need a new gas valve or PC board.

Error Code 72

What It’s Telling You

• Flame sensing device fault.

The Solution

1. Check wiring: Ensure the flame rod wire is secure.
2. Clean the rod: Remove the flame rod and clean off carbon deposits. Ensure it actually touches the flame.
3. Check for debris: Debris on the burner can cause a short.
4. Replace parts: You may need a new flame rod, gasket, or PC board.

Error Code 73

What It’s Telling You

• Burner thermocouple fault.

The Solution

1. Inspect wiring: Check for broken or loose thermocouple wires.
2. Verify settings: Ensure dip switches match the gas type.
3. Replace parts: Install a new thermocouple or PC board.

Error Code 79

What It’s Telling You

• Water leak detected inside the unit.

The Solution

1. Inspect for leaks: Check the heat exchanger and piping for water.
2. Dry the unit: If no active leak is found, dry the leak sensor and the bottom of the cabinet.
3. Reset: Restart the unit. If the code persists, disconnect the sensor. If the unit runs without the sensor, replace the sensor.

Error Code FE

What It’s Telling You

• Flammable vapors detected.

The Solution

1. Check the sensor: Ensure the vapor sensor wires are intact.
2. Ventilate: Clear the area of fumes, then restore power and gas.
3. Test operation: Open a hot water tap.
4. Replace parts: If the code remains despite clean air, replace the sensor or control board.

Error Code FF

What It’s Telling You

• Maintenance indicator. This often appears after parts have been replaced or serviced.

The Solution

1. Check history: This code usually marks the spot where a technician worked on the unit. You can view the error history by pressing and holding the On/Off button and using the arrow keys to scroll through the last nine codes.

Error Code LC

What It’s Telling You

• Limescale buildup warning. The heat exchanger is clogging up.

The Solution

1. Flush the heater: According to Rinnai, circulate 4 gallons of undiluted white vinegar through the unit to dissolve the scale, flush the system at least once a year (twice in hard-water homes), and skip chemical descalers, since your drinking and bathing water runs through the heater ⁽⁴⁾.
2. Temporary reset: You can reset the LC code by pressing the On/Off button five times. This gives you about 70 hours of use before the unit locks out again, giving you time to buy flushing supplies.

FAQs