Noritz Code 20 indicates a combustion fan motor failure or speed error. The primary fix is checking for debris in the fan or replacing the motor and control board if the speed sensor fails.
Code 20 is a hard safety lockout, so your Noritz won't fire at all until the blower problem gets sorted. If you ignore it and somehow force the unit on, you're risking CO buildup from incomplete combustion. I usually see this on older units or anything installed outside where moisture and critters get into the intake. Don't sit on this one.
Think of the combustion fan as the lungs of your Noritz. When I show up for a Code 20 call, I'm looking to see if the fan is physically stuck or if the board just lost communication with it. It's one of the more involved repairs because you've got to open up that sealed combustion chamber to really get a good look at the assembly. Parts aren't cheap either, usually $250-500 all in.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Fan motor bearing failure from age or moisture40%
Fan speed sensor failing to report correct RPM24%
Control board output circuit to fan motor failed22%
Foreign object or debris blocking the fan wheel14%
Symptoms You May Notice
Code 20 flashing on the controller and the unit won't start no matter what you do.
Complete silence when you call for hot water. No fan hum, no ignition attempt, nothing at all.
A grinding or squealing noise from inside the unit right before it throws the code and shuts down, which means those motor bearings are shot.
The unit actually starts its sequence, the fan spins up, then Code 20 appears about 5-10 seconds in. That usually points to a speed sensor or RPM mismatch rather than a dead motor.
Intermittent hot water that works fine some days and codes out on others, especially in cold weather when bearing grease thickens up and the motor struggles to get up to speed.
Can you reset a Noritz waterheater to clear the 20 code?
Cut power at the breaker or unplug the unit for 30 seconds, then power it back up. You can also use the remote controller's reset function if your model has one. But honestly, don't expect it to stick. If the underlying problem is a dying motor or a flaky speed sensor, that code's coming right back the second the unit tries to fire up again. Use the reset to confirm the fault's still active, then get the repair scheduled before it fails at the worst possible time.
Tools Required for Diagnosis
Phillips #2 screwdriverFlathead screwdriverMultimeter (AC voltage setting)Flashlight or headlampShop vac (for clearing intake blockages)Combustible gas detectorElectrical contact cleaner sprayNeedle-nose pliers
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
ComponentComponent Under Test
Expected Range110–125 VAC
ConditionIf Open (OL) or infinite, replace component.
Replacement Parts
If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.
Part Name
OEM Number
Estimated Price
Combustion Fan / Blower Assembly100073060 · $80–$160
100073060
$80 – $160
Fan Speed Sensor (Tachometer)100073039 · $25–$55
100073039
$25 – $55
Frequently Asked Questions
What does the combustion fan do in a Noritz tankless water heater?
It's basically the breathing system for the whole unit. The blower pulls in fresh air for combustion and then pushes all the exhaust gases out through the vent pipe. Noritz is really strict about fan speed because if the air-to-fuel ratio is even a little off, you get incomplete combustion and that means carbon monoxide. The system monitors RPM constantly, and if the fan drops below the required speed even briefly, it shuts everything down. It's not being dramatic, that safety logic is literally what keeps CO out of your house.
How much does a Noritz fan motor replacement cost?
Budget $250 to $500 all in for a professional repair. The OEM Noritz fan motor is a precision part with a built-in speed sensor, usually $150-250 just for the part depending on your model. Add an hour or two of labor to tear down the cabinet, swap the motor, reseal the combustion chamber properly, and do a gas pressure check afterward. And don't cheap out on an off-brand motor. The tachometer specs have to match exactly or you'll just be looking at Code 20 again in three weeks.
Can I replace the Noritz fan motor myself?
Honestly, I'd steer most homeowners away from this one. You're working inside the sealed combustion path, and if those gaskets don't go back on perfectly, you've got an exhaust leak inside your living space. That's not a theoretical risk, I've seen it happen. On top of that, after you mess with the blower, someone really should check the gas pressure to make sure the air-to-fuel ratio is still dialed in for your altitude. If you're a mechanically confident person who's done appliance work before and has a CO detector, it's doable. But it's not a beginner job.
Why does Code 20 appear on a unit that's only 4 years old?
Environment kills these motors faster than age does. If your unit's outside or in a damp crawlspace, moisture works its way into the motor housing and corrodes the bearings. I also see early failures on units that short-cycle constantly because they're oversized for the load, and every hard start wears down that motor faster than normal. Check your intake screen too. A partially clogged screen makes the motor work harder on every single cycle, and that extra strain adds up fast. Four years is young but it's not unheard of in rough conditions.
Will a Noritz with Code 20 produce carbon monoxide?
Code 20 is a lockout, so the unit won't fire at all. That's the safety system doing exactly what it's supposed to do. The real danger is if you bypass the lockout or if there was a period before the lockout where the fan was weak but still running, because a lazy fan means a lazy flame and that's where you get CO production. Don't try to trick the unit into running. Get the fan fixed, make sure you've got a CO detector in the space anyway, and leave the gas alone until the blower problem's resolved.
What's the part number for the Noritz combustion fan motor?
It varies by model, so you really need your model number off the rating plate before ordering anything. For the NR98-DV series it's been part number 100013724 but don't quote me on that for your specific unit. The NRCP199-DV uses a different assembly entirely. Call a Noritz distributor with your model and serial number rather than guessing. Wrong motor means wrong RPM specs and you'll just get Code 20 right back. Noritz's parts line is usually pretty straightforward about confirming the right part if you have the serial number ready.