Rheem Tankless Water Heater Error Codes Explained

Your Rheem tankless heater flashes a code and the hot water quits. That code is not bad news, it is the fastest clue you have. It names the exact fault instead of leaving you to guess.

The catch is that a two-digit number on the display means nothing without the key. Match it to the right cause and you often clear the problem in minutes with a filter rinse or a reset.

Below is every Rheem tankless error code, what each one means, and the step-by-step fix, so you can get the hot water back or know when to call a pro.

Rheem Error Code Breakdown

Rheem water heater error codes are straightforward. You will usually see them flashing on the digital screen on the front of the unit.

Some models connect to Wi-Fi. In this case, you might need to check the Rheem EcoNet app on your phone for the specific alert. Check your owner’s manual to confirm where your specific model displays its status.

Once you have the code, find it in the list below to start troubleshooting.

Error Code: No Code

The Problem

• Flow rate or low activation: Water flows through the heater, but it stays cold. The screen displays a temperature but no error code.

The Solution

1. Increase demand: Open the hot water tap fully to increase the flow rate. Rheem’s Professional Prestige series needs 0.26 GPM of flow before it registers demand and about 0.40 GPM to fire the burner ⁽¹⁾.
2. Check plumbing lines: Ensure the hot and cold water lines are not crossed.
3. Clean the filter: Turn off the water, remove the inlet water filter, and rinse it under running water to remove debris.
4. Inspect the flow sensor: Check the sensor for debris. It might be stuck.

Error Code P1

The Problem

• Low flow: The water flow rate is too low to maintain consistent operation. The unit may still run, but performance will suffer.

The Solution

1. Check the aerators: Clean the aerators on your faucets and showerheads.
2. Clean the inlet filter: Debris in the cold water inlet filter often causes this restriction.
3. Adjust settings: You may need to lower the temperature setting to allow for a higher flow rate.

Error Code 1L

The Problem

• Scale build-up: The system detects limescale deposits in the heat exchanger. This is common in areas with hard water.

The Solution

1. Flush the heat exchanger: Perform a vinegar flush to remove mineral deposits. Refer to your manual for the specific procedure.
2. Check altitude: Verify the unit is configured for the correct elevation if you live in a high-altitude area.

Error Code 05

The Problem

• Airflow obstruction: The exhaust vent or air intake is blocked.
• Improper venting: The vent pipes are not connected correctly to the terminal.

The Solution

1. Inspect the vents: Go outside and ensure leaves, snow, or nests are not blocking the exhaust or intake.
2. Check connections: Ensure all vent pipe connections are sealed and secure.
3. Listen to the fan: If the venting is clear, the fan motor might be failing.

Error Code 10

The Problem

• Air supply/exhaust blockage: The blower motor cannot move enough air.

The Solution

1. Clear obstructions: Remove any debris from the venting system, blower motor, and air intake.
2. Test the motor: If the airways are clear, the blower motor itself may be defective.

Error Code 11

The Problem

• Ignition failure: The unit attempts to fire up but fails to light the gas.

The Solution

1. Verify gas supply: Ensure the gas valve is in the fully “On” position.
2. Check gas type: Confirm you are using the correct fuel (Natural Gas vs. Propane).
3. Purge the line: If the unit is new or rarely used, air might be in the gas line. Turn the hot water on and off repeatedly to purge it.
4. Inspect igniter: Check the igniter rod wiring for breaks or loose connections.
5. Check grounding: Ensure the unit is properly grounded electrically.

Error Code 12

The Problem

• Flame failure: The unit lights momentarily but the flame extinguishes unexpectedly.

The Solution

1. Check gas pressure: Low gas pressure will cause the flame to die. You may need a manometer to test this.
2. Inspect the flame rod: Remove the front cover and look for carbon build-up on the flame rod. Clean it gently with fine sandpaper.
3. Check electrical grounding: The flame sensor relies on a good ground connection to work.
4. Look for loose wires: Secure any loose connections on the PC board or gas valves.

Error Code 13

The Problem

• Improper combustion: The flame rod detects an abnormal burn, likely due to a lack of oxygen.

The Solution

1. Check venting length: Ensure your exhaust pipe does not exceed the maximum allowable length for the unit.
2. Remove blockages: Clear any spiderwebs or debris from the intake and exhaust.
3. Verify altitude settings: High-altitude installations require specific DIP switch settings.

Error Code 14

The Problem

• Overheat limiter (OHL) tripped: The unit is getting too hot.
• Flue pipe overheating: Common in condensing units.

The Solution

1. Inspect the heat exchanger: Look for cracks or heavy scale build-up restricting water flow.
2. Check the OHL wrap: Verify the thermal fuse wrap around the heat exchanger is intact. If it is broken, the heat exchanger usually needs replacement.
3. Check airflow: Ensure the blower motor is running at full speed.

Error Code 15

The Problem

• Boiling safety warning: The heat exchanger temperature exceeded 207°F.

The Solution

1. Descale the unit: Scale acts as an insulator, causing the heat exchanger to overheat. Flush the system immediately.
2. Check the inlet valve: Ensure the water inlet valve is fully open to allow maximum water flow.
3. Test the thermistor: The temperature sensor might be giving a false high reading.

Error Code 16

The Problem

• Outlet temperature too high: The water leaving the unit is hotter than the thermostat setting, roughly 199°F on many models.

The Solution

1. Restrict flow: If the incoming water is very warm (like in summer), the heater might struggle to get the flame low enough. Increase water flow.
2. Clean the heat exchanger: Remove scale build-up.
3. Check sensors: Inspect the outlet thermistor for resistance accuracy.

Error Code 21

The Problem

• Air intake filter switch: The unit senses a dirty or blocked air intake filter screen and will not fire until the airway clears.

The Solution

1. Clean the intake screen: Turn off the power, remove the air intake filter screen, and wash out dust and lint.
2. Inspect the switch wiring: Check the connector on the filter switch for a loose or corroded pin.
3. Clear the intake path: Make sure nothing outside is restricting the air intake terminal.

Error Code 24

The Problem

• Switch malfunction: The operating switch or remote control is not communicating correctly.

The Solution

1. Hard reset: Disconnect the remote control wires and power cycle the unit.
2. Check remote wiring: Ensure the wires connecting the remote to the board are not pinched or shorted.
3. Verify grounding: Confirm the unit has a solid earth ground.

Error Code 29

The Problem

• Condensate blockage (Condensing models): The neutralizer or drain line is clogged.
• Heat exchanger temperature too low: The system isn’t transferring heat effectively.

The Solution

1. Check the drain: Ensure the condensate drain line is not frozen or clogged with debris.
2. Clean the neutralizer: If your unit uses a neutralizer box, ensure the rocks inside are not solidified and blocking flow.
3. Clean the air inlet: Wash the air inlet screen to ensure proper combustion.

Error Code 31

The Problem

• Inlet thermistor failure: The sensor measuring cold water temperature is faulty.

The Solution

1. Inspect wiring: Check the wires connecting the thermistor to the board for breaks.
2. Clean the sensor: Remove scale deposits from the sensor probe.
3. Replace thermistor: If the wiring is good, the sensor likely needs replacement.

Error Code 33

The Problem

• Outlet thermistor failure: The sensor measuring hot water temperature is faulty.

The Solution

1. Check connections: Ensure the thermistor is plugged securely into the control board.
2. Clean the sensor: Remove any mineral build-up.
3. Measure resistance: Use a multimeter to ohm out the thermistor. Replace if out of range.

Error Code 34

The Problem

• Air intake thermistor failure: The sensor measuring the air temperature is reading incorrectly.

The Solution

1. Check environment: Ensure the unit is not operating outside its temperature limits.
2. Inspect wiring: Look for damaged wires leading to the air intake sensor.
3. Check fan blade: Ensure the cooling fan is spinning freely.

Error Code 45

The Problem

• Water leak detected: The internal leak sensor found moisture inside the casing, so the unit shuts down to protect the electronics.

The Solution

1. Find the leak: Open the front cover and look for drips at the fittings, heat exchanger, or drain line.
2. Dry the sensor: Wipe the leak sensor and the area around it once you stop the source.
3. Call a pro for a cracked exchanger: A leaking heat exchanger is not a DIY repair and usually means replacement.

Error Code 51

The Problem

• Gas valve fault: The main gas solenoid valve is malfunctioning.

The Solution

1. Inspect terminals: Check for loose wires on the gas valve solenoid.
2. Test voltage: Verify the control board is sending voltage to the valve. If not, the board may be bad. If it is, the valve is bad.

Error Code 61

The Problem

• Fan motor fault: The combustion fan is not spinning at the correct speed.

The Solution

1. Spin the fan: With power off, spin the fan by hand. It should move freely. If it feels gritty or stuck, replace the motor.
2. Check voltage: Measure the voltage supplied to the fan motor.
3. Check venting: Excessive vent length or bends can overwork the fan.

Error Code 72

The Problem

• False flame detection: The flame sensor detects fire when the unit should be off.

The Solution

1. Check for moisture: Look for water leaks dripping onto the flame sensor or wiring.
2. Clean the sensor: Remove carbon build-up from the flame rod.
3. Verify ground: A bad electrical ground often causes sensor errors.

Error Code 76

The Problem

• Poor communication: The remote control and the main board have lost a clean signal, often from wiring or a loose connector.

The Solution

1. Reseat the remote wires: Power down, then unplug and firmly reconnect the remote control wiring at both ends.
2. Inspect the cable: Look for a pinched, corroded, or shorted communication wire and replace it if damaged.
3. Power cycle: Cut power for 30 seconds and restore it to re-establish the link.

Error Code 99

The Problem

• Improper venting/combustion: The unit cannot vent exhaust gases properly.

The Solution

1. Clear the flue: Check for bird nests or debris in the exhaust pipe.
2. Flush the unit: A clogged heat exchanger can restrict airflow and cause this error.

Why Your Tankless Heater Requires a Reset

If you find yourself resetting your heater constantly, it is likely overloaded. Even powerful tankless units have limits. If you try to run two showers, a dishwasher, and a washing machine simultaneously, the unit may trip to protect itself.

Constant resetting is not a fix; it is a bandage. If the demand is too high, you may need a second unit or a larger model. If you are sure you aren’t overloading it, you likely have a faulty part that requires professional replacement.

How to Reset a Rheem Tankless Water Heater

Unlike older tank-style heaters, modern Rheem tankless units do not have a pilot light you need to light with a match. They use electronic ignition.

If your unit locks out with an error code, follow these steps to reset it:

1. Turn off hot water taps: Ensure no water is flowing through the system.
2. Power cycle the unit: Locate the power cord or the circuit breaker. Turn the power off/unplug it, wait 30 seconds, and turn it back on.
3. Check the remote: If you have a remote control panel, press the “Power” button to turn the system off and then on again.
4. Hard reset: Some errors require you to press and hold the “Min” and “Max” buttons on the main control board simultaneously to clear the code. Refer to your manual for your specific model’s hard reset procedure.

FAQs