Takagi T-KJr intermittently shuts down w/ error code 21 - "Pressure Switch"

[Mark - T-KJr]

Hello, I have a Takagi T-KJr. which intermittently shuts down when the shower is being used by the tenant. He is turning the hot water on full so the flow rate is exceeding the 0.75 gpm threshold flow rate. He can get the hot water to come back on by turning on the kitchen sink faucet and then returning to the shower. The code reader is showing a "21" error code ("pressure switch"), both currently and past. The pressure switch is a diaphragm device with a tube that goes to the fan motor. It has a vacuum rating of 980 Pa and is supposed to measure the air draw of the fan. I have replaced the pressure switch, both with a new one of the same vacuum rating (although not the Takagi part number for the T-KJr) and I have swapped pressure switches with another T-KJr that was operating correctly. I have also cleaned out the tube per the Takagi maintenance manual. The intermittent shut down continues. I'm hoping someone can pull back the curtain on the error code "21" and tell me if something other than the pressure switch could cause this.

Does the fan assemble every need cleaning? Could this cause an error code 21?
For example, I have found the squirrel cage blades inside bathroom ceiling fans to be covered in gunk after years of use, degrading the effectiveness of the fan.

Would something caught inside the stainless gas exhaust pipe cause a code 21?

I installed 4 T-KJr's in the building in 2005. The error code 21's started in late 2024 and to my knowledge have only occurred on one of the machines, but the most heavily used one.

Thanks for any insights.

 

[GReynolds929]

Buildup on the fan blades could cause out of spec performance. Have you pulled the fan to check it. Have the flame rods been cleaned or checked?

 

[Mark - T-KJr]

I clean the flame detector and ignitor rods every time the water heater stops working. This has heretofore fixed the problem. Now it doesn't. And the 21 code is coming up. The fan is a big problem to access. It's behind a lot of stuff. That's why I wanted to know if anyone has identified that as a problem before I go back there.