My truck is a 1997. For replacing oxygen sensors, is it necessary to disconnect the battery?

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Is it necessary to disconnect the battery first before replacing the oxygen sensors on my 1997 GMC 1/2 ton pickup. I disconnected the battery before replacing two of them with no problem. But then I had a mechanic replace the other two which were harder to get at.He didn't disconnect the battery first and now thye check engine light is on. Is it because of his not disconnecting the battery?