heater circuit of the LH O2 to make sure that is operating correctly otherwise replace it as well. The heater circuit warms up the O2 to get it to operating temp ASAP so it can do its job. O2's do not operate effectively when the engine is cold or not at the proper operating temp.
The tip on the thermostst is a good one to monitor. These Z32 thermostats fail in many modes like opening to far. not closing all the way , etc. I prefer the OEM thermostat for a DD.