What, exactly, does error code 12 signify?

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I have a 1994 Toyota Corolla, 1.8L engine with an AT and 231K miles on the clock. Runs very well, doesn't use of leak oil, good on gas, and the transmission is solid. However, about a year ago the check engine light came on, accompanied by an occasional, random misfire. The error code given, code 12, refers to "engine revolution signal missing," and it's still there, along with the miss. Any help on where to being looking will be greatly appreciated.