Connections for these probes are very susceptible to voltage generation with dissimilar metals. Be sure that any other wire connections that are made to the probe wires are made on both wires identically. Also, if a switch has been installed to monitor each cylinder on a single indicator, you must switch both wires and not switch just one wire and gang the others together. This introduces big errors. Before you start chasing baffling and such, be sure your CHT indication is calibrated!

The EPI 800 does not switch the wires, but the output from the individual  A to D converters.  If the wires were installed as delivered, the system is reasonably accurate.