On Sat, 27 Feb 2021 07:49:51 -0800 (PST), trader_4
wrote:

I remember taking a Pontiac that had an AC problem to a place that
specializes in auto AC. They had the car for two days, told me they figured
out it was a bad switch in the panel. When I drove away, the AC worked,
but the check engine light was on. I didn't know what to do, figured they
probably caused it, but would deny it, etc. I already had the service manual
for the car, so I decided to check it out myself. Back then you could short
two pins on the diagnostic connector and it would flash a code sequence on
the instrument panel. The code indicated a problem with the coolant temp
sensor. Seems unrelated, right? So I trace it down, there is no ground coming
from the main engine computer to the sensor. That was very bad news.
But fortunately the module was easy to open up, which I did. Didn't take long
to fine a blown out circuit trace on the PC board. I bridged it with some solder,
put it back together, problem solved.

So, how did this happen? Obviously some incompetent hack trying to diagnose
the AC was jumpering 12V here and there and blew it out. I was very fortunate
to be able to diagnose and fix it. Imagine if I took it back, was the typical customer.
You think they were going to admit what they did and pay for a $700 computer?
The dope that did it may not even realize he did it, but you'd think he must have
seen some spark when he tried to connect this to that.

Many years ago I bought a used Toyota that was almost $5000 less expensive
than all of the similar vehicles in the area at the time. I didn't realize
until later that the automatic climate control didn't work. Everything
worked manually, but the auto part just blew full cold. The Toyota dealer
said it was a bad computer and would cost $3800 plus labor. Googling, I
found someone with the same problem and the responder asked what the
diagnostic code said. Hmm, a diagnostic code? Checking the factory service
manual, I saw that holding 3 buttons on the nav unit while turning on the
ignition would initiate a self-test. My codes were 11 and 21, with 11 being
'no daylight detected from the daylight sensor", which was normal because
it was dark, and 21, which said "no response detected from the cabin air
temp sensor", with a picture showing where that sensor is and how to remove
and replace it. Reaching up under the dash, it was immediately obvious that
the sensor was unplugged. I plugged it in, which cleared code 21, and it
worked for the next 15 years when I finally sold it.

Sometimes you get lucky, like I did, but I agree with y'all who are saying
that proper training is necessary these days. Without training, it's easy
to get into a mode where various parts get replaced, hoping each time that
the issue will be resolved.