500e RPM Measurements

[rjfjr1]

My mechanic has found a discrepency in RPM readings between several sources.

At idle, all three control units in the box on the passenger side of the engine, the shop's scanner and "fluke" meter read 500 +-20rpm, but the LH Module indicates 700-750.

At 1,500 RPM per the three control units and two external meters, the LH module indicates 2,000rpm.

Any ideas would definitely be appreciated. Could the faulty LH reading lead to hesitation/stalling off the line?

Can LH modules with different part #s all from 500e be swapped?

How does the LH module read RPM (source) and what does it do with it?

Thanks very much in advance!