Last week the idiot light came on in my truck.
"What is it? What's wrong?"
"Everything seems fine. What is malfunctioning?"
Thus, our definition... idiot light. It can tell you something is wrong. But not what it is.
I checked everything I could. The only possible physical problem (though not likely) was the air filter that could have used a changing. (Done now.)
I studied up on the internet. It didn't take long to figure out that the odds-on favorite was an oxygen sensor. So I learned myself on 'em.
My 2002 Toyota Tundra has a 4.7L V-8 engine. Apparently there are oxygen sensors "upstream" and "downstream" of the catalytic converters on each side. They measure the content of oxygen in the exhaust against the outside air, and have the engine adjust it's fuel/air mixture accordingly. (They also heat themselves to begin working correctly while the engine is heating up.)
I learned from other's problems and discussions on the internet that, besides dealers and mechanics, auto parts stores also have "code readers" that can tell you why your idiot light came on. And if it is indeed an oxygen sensor, you can borrow the tool you'll need to complete the job. (The O2 sensor is shaped like a spark plug with wires attached to the tip - the special tool is a slotted socket.)
I chose to go to the Advance Auto Parts store because they hosted a car wash put on by the youth group of my friend Darren's Masonic Lodge. The man there plugged the handheld computer into my truck, and it read a report that my truck had saved in it's memory: a past problem with the O2 sensor, a current problem with the O2 sensor, and a confirmation of that problem. The computer said it was bank 2, sensor 1: the passenger side, upstream sensor. For about $120 I could get the OEM part (not in stock.) Or for about $65, a universal sensor - I would have to splice some wires together. I got the universal sensor, air filter and a packet of anti-seize lubricant (turns out there was some included), and paid my deposit on the loaner toolkit.
I studied the process, made sure it was within my capability, and set to work. I had a bit of a challenge reaching the wire plug connector and detaching it carefully. Luckily, I found a picture on the internet that gave me a view of how the locking clip worked. Through various searches, I read about how other people had run into slight issues. I encountered a few - nothing significant - worked around them.
So after an hour or so of studying and shopping, and a couple/few hours of careful work, the idiot in my truck has stopped telling me to check the engine. (Though sometimes he still sings along to the radio ;)