Rheem Tankless Water Heater Troubleshooting

That blinking light on your Rheem tankless water heater is trying to tell you something. Rheem Tankless Water Heater Troubleshooting starts with understanding what those error codes mean so you fix the right problem the first time. Most issues fall into a handful of categories that homeowners can safely check on their own.

Manufacturer specifications indicate that over 70 percent of service calls for tankless units stem from three root causes: a clogged inlet filter screen, a partially closed gas valve, or a simple power interruption. As of 2026, the diagnostic system on these units has not changed significantly from prior generations. Twenty minutes of smart checking can save you a service fee.

Quick Answer

Check the error code on the display first. Each code points to a specific problem. Start with the free checks like the inlet filter screen and the gas valve.

If the code keeps returning after a reset, the fix changes. Match your code to the right branch below.

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Start With the Error Code: What That Blinking Light Actually Means

How to Read the Display and Memory Codes

Your Rheem tankless unit stores the last five error codes in its memory. The display shows a two-digit or three-digit number like 682 or 110. Press and hold the temperature down button for three seconds to enter memory mode.

The unit scrolls through stored codes so you can see what happened even if the display went dark.

Do not clear the code until you have written it down. The code tells you which system triggered the lockout. Without that number, you are guessing.

Why Most Rheem Units Lock Out After Five Failed Ignition Attempts

The control board counts every failed ignition try. After five attempts in a single cycle, the board locks the unit and displays an error code. This safety feature prevents unburned gas from accumulating inside the combustion chamber.

If your unit hits this lockout repeatedly, the underlying issue is almost always one of three things: a gas supply problem, a vent blockage, or a failing igniter assembly. Do not keep resetting the unit without investigating the root cause. The lockout is there to protect you and the equipment.

The Branching Path: Matching Rheem Error Codes to Fixes

Each error code leads to a different branch of the troubleshooting tree. Here is what the most common codes mean and the first action to take for each.

Error Code 682 and 752: Ignition and Flame Failure

Error 682 means the burner did not light at all. Error 752 means it lit briefly then went out. Both point to the gas supply or the flame sensing system.

Start by verifying the gas shut-off valve is fully open. Then check the inlet filter screen for debris. If both are fine, the igniter or flame rod may need professional replacement.

Error Code 110 and 111: Air Intake or Exhaust Blockage

These codes indicate a restriction in the combustion air path. Error 110 points to the intake side. Error 111 points to the exhaust side.

Go outside and inspect both vent terminations. Birds, rodents, leaves, and snow can block the openings. Clear any obstruction and reset the unit.

If the code returns, the vent run itself may be restricted.

Error Code 121, 122, and 129: Thermistor Issues and Overheating

Thermistors are temperature sensors mounted on the inlet and outlet water lines. Code 121 or 122 means one of those sensors is reading outside its expected range. Code 129 means the unit detected an overheat condition.

Scale buildup inside the heat exchanger is the most common cause. A descale flush often resolves all three codes.

Error Code 290: Combustion Fan Failure

The combustion fan pulls air through the burner assembly. When it fails, the unit cannot sustain a flame. Listen at startup.

If you do not hear the fan spin up, the motor may be seized or the circuit board may not be sending power. This repair requires a licensed technician.

Error Code 172: Communication or Control Board Error

Code 172 usually points to a wiring issue between the main control board and the remote display. Check the communication wire connections at both ends. If the wiring is secure and the code persists, the control board may need replacement.

That is a job for a qualified service professional.

The No-Error-Code Branch: When the Unit Runs But Misbehaves

Cold Water Sandwich: What It Is and When It's Normal

A cold water sandwich is a burst of cold water between hot water runs. It happens because the unit idles and the water in the heat exchanger cools down. When you reopen the tap, that cold slug comes out first.

This is normal and not a malfunction.

If the cold burst lasts more than a few seconds, check the minimum flow rate. Many Rheem units require at least 0.4 gallons per minute to activate. Low-flow fixtures can cause the unit to cycle on and off, creating multiple cold sandwiches.

Temperature Fluctuations: Gas Pressure, Flow Rate, and Scale

Inconsistent water temperature usually comes from one of three sources. First, gas pressure that is too low or too high will cause the burner to underfire or overfire. Second, the unit may be undersized for the simultaneous demand.

Third, scale buildup inside the heat exchanger reduces heat transfer efficiency and causes the outlet temperature to swing.

Measure the inlet water temperature with a thermometer. If it changes significantly during the season, you may need to adjust the temperature rise setting on the unit.

Low Flow or No Flow: The 0.4 GPM Activation Threshold

Showerheads and faucets with flow restrictors can fall below the activation threshold. If the unit does not fire when you open a single tap, try running a second tap at the same time. If the unit fires, low flow is the issue.

Remove the flow restrictor from the fixture or install a different showerhead.

The Free 10-Minute Checks Before You Call Anyone

Inspect the Inlet Filter Screen

The inlet filter screen traps sediment before it enters the heat exchanger. A clogged screen is the single most common cause of error codes on Rheem tankless units. Turn off the water and gas.

Remove the inlet isolation valve assembly. Pull out the screen and rinse it under running water. Reinstall and restore service.

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Verify the Gas Supply Valve Is Fully Open

Partial gas flow causes ignition failures and code 682. The shut-off valve should be turned fully counterclockwise. Do not leave it halfway open.

If you have a propane system, check the tank level. An empty or low tank will not deliver enough gas pressure to sustain the flame.

Check the Vent and Condensate Drain

The vent termination can become blocked by debris, insect nests, or snow. Go outside and visually inspect both the intake and exhaust openings. The condensate drain line needs to be clear and sloped downward.

A blocked condensate drain can trigger code 110 or 111 because the unit cannot properly route combustion gases.

Power-Cycle the Unit the Right Way

A simple reset clears transient error codes. Turn off the gas shut-off valve. Flip the circuit breaker to the off position.

Wait 60 seconds. Flip the breaker back on. Turn the gas valve back on.

Check the display. If the same error code reappears immediately, the issue is a hard fault, not a glitch.

When to Stop and Call a Licensed Pro

Gas Pressure Testing and Manometer Readings

Proper gas pressure is critical for a Rheem tankless heater. Incoming gas pressure must be within the range listed on the unit's data plate. For natural gas, that is typically 5 to 7 inches water column.

For propane, 10 to 13 inches water column. Measuring gas pressure requires a manometer and a service tee. This is not a DIY task.

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Replacing the Igniter, Flame Rod, and Thermistors

The igniter, flame rod, and thermistors are replaceable parts, but accessing them requires removing the combustion chamber cover. That means breaking the gas seal and potentially disturbing the burner assembly. A licensed technician has the tools and training to do this safely without damaging the heat exchanger.

Why Vent Material and Length Are Non-Negotiable

Rheem units are certified for specific vent materials and maximum lengths. For condensing models, that means schedule 40 PVC. For non-condensing models, category III stainless steel.

Using the wrong material or exceeding the maximum vent length can cause flue gas recirculation and carbon monoxide exposure. The vent spec is printed on the unit's rating plate.

Mistakes That Turn a Small Fix Into a Big Invoice

Resetting the Error Code Without Fixing the Cause

When code 682 reappears, resetting a second time seems harmless. It is not. The control board logs each failed ignition attempt.

Unburned gas can accumulate inside the combustion chamber each time the unit tries and fails.

The lockout is not a bug. It is a safety feature that protects your family from gas accumulation. It also protects the igniter from repeated electrical stress.

If a code survives one reset, stop resetting and start diagnosing.

Descaling the Wrong Side of the Unit

Flow direction matters during a descale. The pump should push solution into the outlet service valve and out through the inlet service valve. Reversing the flow pushes loose scale deeper into the heat exchanger, creating a bigger plug.

The solution needs to circulate for at least 45 to 60 minutes to break down mineral deposits. Use a submersible pump rated for gentle flow and follow the manufacturer's recommended flow rate.

Ignoring the Condensate Trap

Condensing Rheem models produce acidic water as a byproduct of combustion. That water drips through a condensate trap before reaching the drain line. If the trap is dry, clogged, or sitting empty, exhaust gases can back up and trigger codes 110 or 111.

Check the trap during every maintenance flush. Clean it out, then refill it with clean water before reinstalling. This one minute of maintenance prevents a service call later.

Overtightening Service Valves and New Parts

Rheem's service valves are brass, but the fittings and sensors around them are softer. Overtightening can crack a thermistor housing or strip a plastic cap. Tighten with hand pressure, then add a quarter turn.

If it leaks, give it a small nudge rather than forcing it.

The Decision Guide: Fix It Yourself, Call a Tech, or Replace It

Here is the general rule. If the unit is under 7 years old and the error code points to maintenance items, fix it. If the unit is over 12 years old and the heat exchanger is leaking, replacement usually wins.

If you are not comfortable working with gas appliances, call a licensed technician. That is not a failure. It is the safest route, and it protects your warranty.

When a Repair Is Worth It

Replacing the igniter on a 5-year-old unit costs roughly $100 to $200 in parts, plus about an hour of labor. Replacing the whole unit costs $2,000 or more. A descale flush, inlet filter cleaning, or condensate trap fix costs under $50 if you do it yourself.

Error codes like 121 or 129 are often fixed with a proper descale. If the unit fires and delivers hot water after that, you just saved the service fee.

When Replacement Beats Repair on an Older Unit

Older Rheem units, especially non-condensing models from a decade ago, lose value quickly. A failed control board on a 10-year-old unit can cost $600 installed. If the heat exchanger is leaking, the repair can approach the price of a new unit.

Warranty coverage can change that calculus. Check coverage details on Rheem's official website. The [U.S.

Department of Energy](https://www.energy.gov) notes that properly maintained tankless water heaters can last 20 years or more. A unit that old is already at the end of its typical service life.

How to Explain the Problem So the Tech Fixes It Fast

Skip the vague description when you call. Say something like: "My Rheem shows error code 682. I have verified the gas valve is open, cleaned the inlet filter, and checked the vent." Include what you already did.

That saves a diagnostic hour. It also means you are less likely to pay premium labor while a technician rechecks basics you already cleared.

Rheem Tankless Water Heater Troubleshooting: Common Questions

How often should I descale my Rheem tankless water heater?

At least once a year for most homes. If your water hardness is above 10 grains per gallon, do it every 6 months. A consistent schedule prevents scale-related error codes like 121 and 129 from appearing in the first place.

What does error code 682 mean on a Rheem tankless?

It means the burner failed to ignite within the allowed number of attempts. Start with the gas shut-off valve and the inlet filter screen. If both are clear and the code returns, the igniter, flame rod, or gas control board likely needs a licensed technician.

Can I reset a Rheem tankless water heater myself?

Yes. Turn off the gas, flip the breaker off for 60 seconds, flip it back on, then open the gas valve. If the same code reappears instantly, stop resetting and call a professional.

Repeated resets mask a real fault and allow gas vapor to build up.

Why is my Rheem tankless not getting hot water?

Most of the time, it is a flow issue or a blocked filter. Confirm the unit meets the minimum 0.4 GPM flow rate. Check the inlet filter screen before assuming the heater is broken.

If you get hot water briefly then cold, scale buildup is the more likely culprit.