Intermittent Failure with Propex HS2000 propane heater

Thread Starter


Joined Apr 24, 2023
I have a Propex HS2000 Propane heater for my van. It is broken. It has a rudimentary diagnostic system, that flashes a code for various errors. It is flashing 5 times, indicating a issue with combustion airflow. I did ensure that the airflow passages are clear.

I emailed the manufacturer an got no response, and I talked to the local distributor and they will happily swap parts, but they wont help diagnose and do not have a schematic.

I then dissasembled it and looked at the airflow sensor. It is a minature passive fan, with black/white optical encoder sensed by a photocell/ir emitter combination. This attaches to a small op-amp circuit to convert the sine wave to the square wave then goes into a PIC uProc.

I put a scope on the sensor output and it shows a 0.15V on the sensor for about 5 secs, then shuts of power to the sensor, shuts down the unit and flashes the diagnostic code.

So I took the board out of the unit, took a bunch of hi-res pics, put it back together, and it now all works. Just for fun I did another scope shot to measure both the input analog and output digital signal.

Nothing like a problem that cannot be reproduced at will. Assuming there is a cold joint or cracked resistor or poor connection, I broke out the trusty chopstick and poked around. I could not get it to fail.

Any tricks on how to find a non-reproducable, intermittent fault?



Joined Jan 23, 2018
Welcome to the group of people who fix things by magic. It happens that way some times, because the defect was not visible but the work that was done fixed the problem.