How to Fix Rheem Water Heater Error Code 90: Abnormal Combustion (Full Guide)

A Rheem Error Code 90 indicates Abnormal Combustion. This means your water heater’s onboard computer has detected that the air-to-fuel ratio is off, or the exhaust isn’t clearing the system properly. Essentially, the unit is “suffocating” and has shut down as a critical safety precaution to prevent carbon monoxide buildup or hardware damage.

If you’re staring at a cold shower or hearing the unit rev up and then click off abruptly, don’t panic. I’ve seen this a thousand times in the field. While it sounds serious, most of the time it’s just a matter of cleaning out the “lungs” of the machine. It’s fixable, and I’m going to walk you through exactly how to get that flame burning blue and steady again.

Comprehensive Repair Guide

Before you touch anything, turn off the power to the unit at the circuit breaker or the plug, and shut off the gas supply valve. Safety isn’t a suggestion in this trade; it’s a requirement. Once the unit is cold and safe, follow these steps.

1. Inspect the External Vents: Grab a ladder and go outside. Check both the intake and exhaust pipes. I’ve pulled everything from dead squirrels to plastic grocery bags out of these. Ensure there is at least 12 inches of clearance around the terminals. Even a thick spider web with debris can be enough to trip the sensitive air pressure switch.
2. Clean the Air Intake Filter: Inside the unit (usually behind the front panel), there is a small screen or filter on the air intake. Unscrew the front panel using your Phillips head screwdriver. Locate the filter—it’s often a plastic mesh. Pull it out and wash it with soap and water. If this is clogged, the unit is literally gasping for air.
3. Clear the Blower Fan: Locate the blower motor at the bottom of the unit. Using compressed air or a vacuum, clean the fan blades. Dust buildup here changes the aerodynamics of the blades, preventing them from moving the volume of air the system expects. Warning: Do not spin the fan too fast with compressed air as it can damage the motor bearings.
4. Inspect the Heat Exchanger Fins: Look up into the burner assembly. If you see white powdery buildup (scale) or black soot on the fins, you need to clear it. You can use a soft brush or a vacuum. If the blockage is deep inside the “fins,” you may need a professional to perform a chemical flush or a deep cleaning, but a good vacuuming often does the trick for DIYers.
5. Check Electrical Connections: Use your needle-nose pliers to ensure the wires leading to the fan and the flame rod are snug. A loose wire can cause “ghost” errors where the computer thinks the fan is failing when it’s really just a bad connection. If you have a multimeter, check the fan for continuity to ensure the motor hasn’t burnt out.
6. The Master Reset: Once everything is clean and reassembled, turn the gas back on, then the power. Open a hot water tap to its maximum flow. If the 90 code persists, you may need to go into the “Tech Mode” on the display to clear the history, but usually, a simple power cycle (unplug for 60 seconds) will reset the logic board.

Detailed Diagnosis: Root Causes

In my thirty years of turning wrenches, I’ve learned that combustion issues boil down to three things: Air, Fuel, and Path. When Error 90 hits, one of these is out of whack. Here is why your unit is throwing a fit:

• Venting Obstructions: This is the #1 culprit. Whether it’s a bird’s nest in the intake pipe, a beehive in the exhaust, or even heavy snow blocking the external terminals, any restriction prevents the unit from breathing. If the “out” path is blocked, the exhaust gases (CO2) back up into the combustion chamber, snuffing out the flame.
• Clogged Heat Exchanger Fins: Over years of service, dust, pet hair, or soot can build up on the delicate fins of the heat exchanger. Think of this like a radiator in a car; if air can’t pass through those fins, the heat builds up too fast and the combustion becomes unstable. This is usually due to poor air quality in the room or a lack of annual maintenance.
• Blower Motor Degradation: The fan is responsible for pulling in fresh oxygen and pushing out waste. If the motor bearings are wearing down or if the fan blades are coated in grime, the fan won’t reach the “RPM” (revolutions per minute) required by the computer. The sensor detects this low airflow and triggers the 90 code to prevent a fire hazard.
• Gas Pressure Fluctuations: If your gas regulator is failing or if you have multiple gas appliances running at once on an undersized line, the “fuel” part of the equation drops. Without enough gas to sustain the flame at the commanded blower speed, the mixture becomes too “lean,” causing abnormal combustion.

Symptoms of Rheem Error Code 90

When your Rheem unit starts throwing a Code 90, it doesn’t always just quit silently. Usually, the machine tries to tell you it’s struggling before it finally locks out. You might notice the digital display flashing “90” or “10” (which is a precursor to a 90). Physically, the water will likely be lukewarm or completely cold because the burners refuse to stay ignited for more than a few seconds.

Pay close attention to the sound: if you hear the blower fan revving unusually high—like a jet engine trying to take off—that’s the unit trying to force air through a blockage. You might also hear a repetitive clicking (the igniter) followed by a “whoof” sound or a total shutdown. In some cases, you’ll smell a bit of “heavy” exhaust outside near the vent, which is a surefire sign that the combustion isn’t as clean as it should be.

How to Prevent Error 90

You don’t want to be doing this again in six months. The secret to a long-lasting Rheem unit is environmental control. First, if your unit is in a garage or basement, keep the area around it clean. Sawdust, lint, and chemicals (like bleach or paint thinner) can corrode the heat exchanger or clog the intake. I always recommend installing a high-quality vent screen on the exterior pipes to keep pests out without restricting airflow.

Second, schedule an annual flush and vacuum. Most folks wait until the hot water stops to call me, but if you spend 30 minutes once a year vacuuming the fan and flushing the scale out of the heat exchanger with vinegar, you’ll never see a Code 90. Lastly, consider a surge protector specifically for your water heater. Modern Rheem boards are sensitive; a small power spike can wonk out the sensors that monitor combustion, leading to false error codes.

Frequently Asked Questions

Q: Can I keep using my water heater if it only shows Error 90 occasionally?
A: Absolutely not. An Error 90 means the gas isn’t burning correctly. This can lead to the production of Carbon Monoxide (CO), which is odorless and deadly. If the unit throws this code, it’s telling you it’s unsafe. Get it fixed before you use it again.

Q: How much does it cost to have a pro fix a Code 90?
A: If it’s just a cleaning, expect to pay a standard service call fee—usually between $150 and $250. However, if the blower motor is shot or the heat exchanger is irreversibly clogged with soot, you could be looking at $400 to $800 depending on your local labor rates and part availability.

Q: My vents look clear from the outside; could it still be a venting issue?
A: Yes. Sometimes the PVC pipes inside the walls can sag or pull apart, or a bird might have gotten down deep into the pipe where you can’t see it from the terminal. If you have a leaf blower, you can sometimes “blow out” the lines from the unit side to see if any hidden debris comes flying out the other end.