I'm sure others will chime but, but I have to reitterate on studying up sensor values, DMM testing procedures, and "outside of the box" logic when using these.
Can literally cost thousands of dollars if placed in the wrong hands.
I think back to the old redbrick mt2500 snap on tool. Codes out the wazoo for every sensor on the vehicle practically. After a friend had replaced every sensor, the codes persisted. He then replaced pcm. You can see how quickly he racked up $600 in parts. After the new pcm failed to solve the problem, he began to unhook it and notice a broken/corroded main power wire for the pcm....
On the other hand they are amazingly handy sometimes. Had I reached for one right away I would have noticed the ford PATS antitheft system was being triggered by the "spare" (unprogrammed) key in the ignition switch. Having a neon with no spark or fuel, and knowing how the ASD relay system works, allowed me to watch for cranking rpms. There wasn't any, which lead me directly to checking crank sensor instead of trying to fix 2 "separate" problems. Just a few examples.
Really have to know what kind of things could trigger out of range readings. Is that MAP REALLY bad, or does the hose have a hole/cut/kink/clog? Are MAF counts wrong due to a torn intake air hose? Is the right bank actually lean or is it sucking air from a exhaust manifold leak? Is the -40* ect reading due to a bad sensor or is it just unplugged?