Rheem Tankless Water Heater Troubleshooting Codes

Your Rheem tankless water heater is throwing a code. The display is blinking something like "11" or "51," and now you are standing there wondering if you need a plumber or just a reset button. It is a frustrating spot to be in, especially when you are halfway through a shower.

Rheem Tankless Water Heater Troubleshooting Codes are actually a built-in diagnostic system. The manufacturer uses a standardized two-digit code set to tell you exactly what the unit is sensing: gas failure, overheating, flow issues, or a simple maintenance reminder. Understanding what each code means and what to check first can save you a service call and get your hot water back faster.

Quick Answer

Rheem error codes tell you what is wrong. Read the code. Look it up in the list below.

Reset the unit. If the code returns, check the specific cause for that code. Most codes are caused by a clogged filter, a dirty flame sensor, a blocked vent, or overdue maintenance.

Do not keep resetting a lockout code. Fix the root problem first.

The Fast Path: Reset, Wait, and Read the Code Again

Start here before you do anything else.

Read the two-digit code on the display. Write it down. Then power cycle the unit.

Turn the breaker off for 30 seconds, then flip it back on.

Wait at least five minutes. A temporary fault like a gas bubble, a voltage dip, or a sensor glitch will often clear itself. If the code disappears and the heater fires normally, you are done.

If the same code returns within a few minutes, the unit is telling you a real problem exists. Do not keep resetting it. Persistent codes mean something is wrong and needs to be addressed.

Move into the decision branches below.

How Rheem Error Codes Work

Rheem tankless water heaters use a self-diagnostic control board. The board monitors key components such as the gas valve, flame sensor, flow sensor, and outlet temperature continuously during operation. If a measurement falls outside the acceptable range, the board locks the unit and displays a code.

There are two categories of codes.

Active lockout codes mean the unit has shut itself down and will not operate until the code is cleared and the root cause is fixed. Codes 11, 12, 14, 51, and 52 are all lockout codes.

Notification codes are reminders. Code 90 tells you it is time to descale the heat exchanger. Code 99 is a catch-all that may indicate a minor issue or an expired maintenance interval.

Notification codes do not lock the unit, but you should address them soon.

The exact code layout varies slightly between older blue-dial models and newer touchscreen units. Per Rheem official technical documentation, the two-digit display is consistent across both lines, so your code reading applies to either.

Troubleshooting by Code: Step-by-Step Decision Branches

Ignition and Gas Supply Codes (11, 12, 14)

These codes mean the burner will not light or is losing flame.

If you see code 11, the igniter has tried and failed to light the burner. First check the gas shutoff valve. Is it fully open?

Many pros report that a partially closed valve is the single most common cause. Next, check other gas appliances in the house. If they also will not light, you may have a supply problem before the heater.

If code 12 appears after the unit has been running for a while, the flame sensor is not detecting the flame. Clean the flame sensor rod with fine sandpaper or a Scotch-Brite pad. Mineral deposits can coat the rod and block the signal.

If code 14 appears, the burner is not firing at all. This often traces to a blocked vent termination. Check the vent outside for debris, bird nests, or heavy snow buildup.

Each code in this group requires a specific branch. If the gas valve is open and the vent is clear but the code returns, the next step is measuring gas pressure at the manifold. That requires a manometer and a licensed gas fitter.

The National Fuel Gas Code (NFPA 54) requires that any work on gas appliances be performed by a qualified professional.

Overheating and High-Limit Codes (51, 52)

Code 51 means the outlet water temperature has exceeded the high-limit threshold. That threshold is typically around 170°F. Code 52 means the high-limit switch has tripped.

Both point to the same root causes.

First, check the water inlet filter. A clogged screen restricts flow. The unit overheats because water is moving too slowly across the heat exchanger.

Clean the filter by turning off water, unscrewing the inlet, and rinsing the screen.

Second, check for scale buildup. Hard water deposits coat the heat exchanger surfaces over time. They reduce heat transfer efficiency and cause the internal temperature to spike.

If you have not descaled the unit in the past 18 months, that is very likely the issue.

Third, check the bypass valve setting. If the bypass is set too far toward hot, it can restrict the cold water mixing needed to keep outlet temperatures stable.

Flow and Sensor Codes (61, 71, 76)

Code 61 points to the inlet thermistor. Code 71 points to the outlet thermistor. These sensors measure water temperature.

If they fail or send erratic readings, the board assumes a problem and locks the unit.

Code 76 is more common. It means the flow sensor, also called the water turbine, is not sending a signal. This is often a simple mechanical issue.

Turn off the water supply. Open a hot water tap to relieve pressure. Then remove and inspect the flow sensor assembly.

Look for a stuck rotor or debris blocking the blades. A quick rinse under tap water usually solves it.

If the rotor spins freely and code 76 returns, the sensor itself may be faulty. You can test it with a multimeter per the Rheem service manual.

Maintenance and Notification Codes (90, 99)

Code 90 is your friend. It interrupts operation every 18 to 24 months as a reminder to descale the heat exchanger. If you see code 90, you are not broken.

You are overdue for maintenance. Flush the unit with a descaling solution per the manual.

Code 99 is a catch-all. It can mean a previous code that has self-cleared or a minor error the board does not consider critical. Reset the unit and monitor it.

If it returns, go back through the other codes to see if the underlying issue is still present.

Mistakes That Keep Codes Coming Back

The number one mistake is resetting without diagnosis. If you power cycle the unit and code 11 returns within ten minutes, the problem is still there. Repeated resetting can damage the ignition system or cause the board to lock permanently.

Mistake two is skipping the inlet filter clean. Aggregate reviews report that over 60% of flow-related codes (61, 71, 76) are resolved by a simple filter cleaning. People replace expensive sensors when a 10-cent screen was clogged.

Mistake three is ignoring venting rules. Rheem indoor units require a specific direct-vent termination. If the vent run is too long, has sharp bends, or is blocked, the negative pressure prevents proper combustion.

That shows up as codes 11, 12, or 14.

Mistake four is using the wrong gas pressure. Natural gas units require 4 to 14 inches water column. LP units need 8 to 14 inches.

A propane tank that is running low can cause flame failure that triggers code 12 repeatedly.

Mistake five is not winterizing properly. Condensing models have a condensate drain that can freeze. A frozen drain backs up and triggers overheating or high-limit codes.

If your code appears only during freezing weather, check the drain line first.

Safety First: When to Stop and Call a Pro

Some codes mean "do not keep trying."

If code 11 or 12 returns immediately after a reset, shut off the gas supply and call a licensed plumber or gas fitter. A unit that repeatedly fails to ignite is a safety hazard. Unburned gas can accumulate and create an explosion risk.

If you smell gas at any point, do not operate any switches or electronics. Leave the building. Call the gas company from outside.

Let them clear the line before you touch the heater again.

As of 2026, the National Fuel Gas Code (NFPA 54) requires that any work on gas appliances be performed by a qualified professional. Opening the gas valve or burner assembly yourself voids the warranty per manufacturer terms. If your troubleshooting has reached the point of testing gas pressure or replacing the control board, hire someone who carries the proper license and insurance.

Thermal shock is another real risk. If a heat exchanger has scale buildup and you attempt to run cold water through it while it is hot, the sudden temperature change can crack the metal. That is a full replacement event.

Let the unit cool completely before working on it.

Maintenance That Prevents Future Error Codes

The best troubleshooting is the kind you never have to do. A consistent maintenance routine stops most Rheem error codes before they appear.

Descaling is the single most important task. Mineral scale builds up inside the heat exchanger over time. It insulates the metal from the water, causing the unit to overheat and trigger codes 51 or 52.

For average municipal water, descale every 18 months. For hard well water, do it every 12 months.

The process is straightforward. You pump a vinegar or citric acid solution through the service ports using a bucket and a small pump. Rheem sells a flush kit, but any standard tankless descaling kit works.

Clean the inlet filter at the same time. Shut off the water. Unscrew the inlet connection.

Pull out the screen. Rinse it under a faucet. A clogged filter is the leading cause of code 76 (flow sensor failure) and code 51 (overheating).

Doing this once a year takes five minutes.

Check the vent termination each spring and fall. Look for bird nests, leaves, spider webs, or ice buildup. A blocked vent causes ignition failures and flame loss that show up as codes 11, 12, or 14.

If your unit is a condensing model, flush the condensate drain line with warm water every six months. A clogged drain backs up and can trigger high-limit codes or cause the unit to shut down in cold weather.

Keep a simple log. Note the date of each descaling and filter cleaning. If a code reappears, you will know exactly when maintenance was last done and whether it is due again.

For official service intervals and model-specific diagrams, refer to the Rheem product manual available on the manufacturer's website.

Frequently Asked Questions

What does code 76 mean on a Rheem tankless water heater?

Code 76 means the flow sensor is not sending a signal. The water turbine inside the sensor is likely stuck or blocked. Turn off the water, remove the sensor assembly, and check the rotor for debris.

Cleaning it resolves the issue in most cases.

How do I reset my Rheem tankless water heater?

Turn the unit off at the breaker. Wait 30 seconds. Flip the breaker back on.

The unit will go through a self-check cycle. If the code was a temporary fault, the heater will fire normally within two to three minutes.

What does error code 12 mean on a Rheem tankless water heater?

Code 12 means the flame sensor has lost the flame signal after the burner was lit. The sensor rod may be dirty. Clean it with fine sandpaper.

If the code returns, check gas pressure and venting before calling a pro.

Why does my Rheem tankless water heater keep shutting off?

A unit that fires then shuts down usually has a flow problem or a flame sensor issue. Low water flow below 0.5 GPM will cause it to cycle on and off. A dirty inlet filter or a scaled heat exchanger are the most common culprits.

How often should I descale my Rheem tankless water heater?

Rheem recommends descaling every 18 months for municipal water and every 12 months for hard well water. If you see code 90 on the display, you are overdue. Flush the heat exchanger with a descaling solution as soon as possible.

Can I fix error code 11 myself?

Yes, if the cause is a closed gas valve or a blocked vent. Check the gas shutoff valve first. Open it fully.

Check the vent termination for debris. If both are fine and code 11 returns, call a licensed gas fitter. Do not keep resetting.