I have a York Heat Pump/AC which provides an "X/L" signal for error reporting to a thermostat. The York documentation says...
X/L Output
The X/L terminal of the heat pump control is typically connected to the
X/L input of the room thermostat. The thermostat uses this signal to
notify the homeowner of a problem with the heat pump using an LED or
LCD display. When the control energizes the X/L terminal, the thermo-
stat displays the flash code so the homeowner can see it.

When the control locks out the compressor because of a pressure
switch lockout, it will energize the X/L output as shown in Table 3. The
control has a three second delay between fault code flashes.

TABLE 3: X/L Output Categories
Condition -- X/L
Pressure Switch lockout - last mode of operation was heating -- 2 flashes
Pressure Switch lockout - last mode of operation was defrost -- 3 flashes

My cheap-o thermostat does not not have an X/L or L or L-A terminal so it won't display any information.

However, I'm pretty good at Arduino hardware and software. I have an oscilloscope currently watching the X/L line from my compressor unit, and it shows an identical signal to the R (24 volt supply) line.

There is less than a ma of current available on the X/L line, which is not enough to drive an LED.


What wave form will I see on the X/L signal line when it "flashes" an error code?

With that information, I hope to rig my Arduino to monitor and report.

Note that the X/L signal coding seems universal enough that several brands of thermostats support it, which implies that manufactures besides York may also support something similar.