Me, monitoring live engine stats on my phone while trying to diagnose a problem, and still trying to claim that I'm not "a car guy".
I've been reminded today that I went looking for a car that wasn't "smart" and I have to remember not to treat it as such.
The fans on the radiator are supposed to kick in at a certain coolant temperature - 90 degrees. I can see from the gauge on the dashboard and it's confirmed by the ODB that it was passing that, without that happening. So what was going on? The relays are good, jumping the contacts manually fires things up - all seems good. It's just not happening.
Well, turns out there are two temperature sensors - because this is a "dumb" car. There's an analogue one that returns the actual temperature for various purposes - and there's another switch in the coolant that's dumb as hell and closes at 90 degrees (triggering the fan relay) and opens again at 85.
Swapped out the second one and it seems to be behaving now. the old one was coated with a layer of crust, so it almost certainly wasn't triggering at the right temperatures.
@mike There are always some design decisions that don't quite make sense. We had a guy replace our cabin fan on the Xterra because he needed the work and the fan had a bunch of crap in it. Apparently there is a camshaft position sensor right beside the fan and he loosened it a bit by accident. Next day the car turns completely off while my wife is driving in traffic...there had to be a more graceful failure mode.
@daniel @mike ugh. Was a similar problem on a circa 1990 Holden Astra that a company I worked for had. Car service two-three days ago, ran fine. Stopped for petrol and when we put the keys back in ... nothing. 2 hr wait for mechanic who without looking, asked "had it svcd recently?", reached in and pushed wiring harness plug back in -- apparently the wiring was too short & had a habit of being pulled out during an oil filter change