Rheem tankless water heater gets code 90 when running multiple faucets.

[heaterhome]

Hi everyone, I have a Rheem tankless water heater model ECOH200DVLN-2 which is only a couple of years old. It worked fine the first year and now it will randomly turn off about once every few weeks with error code 90. I have noticed it usually cuts off if more than one hot faucet is running (such as a bathtub or shower). I have even seen it cut off when running a bathtub at full speed and then suddenly slowing down the water by turning on the hand shower. I have done lots of things to try and fix. I keep the temperature at 130, but it has cut off at 140, 130, 125, and 120.

-I took the internal fan out and cleaned it (it wasn't very dirty)
-I checked and cleared out the horizontal exhaust flue (there was a small amount of water in it, but I sucked it all out)
-I cleaned the water filter (it wasn't very dirty)
-I flushed the system for 2 hours (the vinegar didn't come out very dirty, so I guess it wasn't dirty inside?)

What should I do next? What could the problem be?

 

[Breplum]

Generally one calls the tech desk that the mfr offers. They may or may not be enthused to deal with a home diy person.
The brand that we are experts on, Navien, will most often not help a homeowner who calls because tools and some familiarity makes the process go so much faster.
Your parts will be covered by the warranty.

 

[Sponsor]

 

[GReynolds929]

There should not be standing water in the venting. It should be graded back to the water heater and exit through the condensate drain. If you have cleared the vent, have you performed a hard lockout reset?