Disconnect the battery to unplug sensors?

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I'm pretty sure it's usually advised to disconnect a car's battery before unplugging anything electrical. Obviously it makes sense to disconnect the battery when doing real electrical work, but is it really necessary in all cases? For instance, do you really need to disconnect the battery just to unplug the MAF sensor so you can take off the airbox cover? What could go wrong if you don't?
 
I am trained in automotive fuel injections systems for both ODB generations; and while it is generally advisable to disconnect the battery before any car repair; in reality - none of the sensors are powered while the key is in the off position.
 
Ignition switch off. I'll tell you a story one day years ago at job far far away I was given a carb to change out ,well heck I am good so why disconnect the battery. Well what happened is some gas spilled out ,,no big deal right ? though I did not notice the choke cable was broke and upon removing the carb the piece of cable fell down and shorted the starter battery+} to the solenoid [a Chevy starter] which caused the eletric feul pump to run which when added to the already spilled feul started burning pretty good {remember the shorted starter sparked which started the feul to catch on fire} well all I could think is how am I going to explain this to the owner of the company I worked for? But due to luck and quick thinking I pulled the neg cable from the battery which stopped the feul pump and used compressed air to blow out the fire . All this was in an instant and no damage was done . I really was lucky .So as a rule disconnect the battery and if not be carefulllllllll!!
 
Some stuff is ground sensed or grounded through the computer. Could still become active or blow the power supplying fuse with key off.
 
That doesn't sound good. I will admit that one time unplugged the MAF sensor to check on the air filter in my car, 5 mins or so after turning the car off. After I was finished and went to start the car again, the CEL was on. Plugged in my ScanGauge and found a MAF sensor error code. Cleared the code and never had any problems. I think the ECU stays on for a period of time after you shut the engine off; I've never had another CEL from the car but every other times I've unplugged the MAF it's been after the car was sitting for a while.

I might have to change my practices.
 
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