On boot up the G2 runs a series of self diagnosis tests. While it runs through these tests if it finds something that is not right it will stop at the point of diagnose and display an error has occurred in the form of an "E" code on the display. Examples would be E1, E2, E3, etc.
For the purpose of this article we will use E1 as an example, but the same applies to all E codes.
E1 according to service manual is the Z80 chip failed. This may not be true, in fact in most cases I have seen, the E code that points to a failure of a component, such as in this example E1, the Z80 chip, the chip is not at all the problem.
Most of the times I have found the real problem to be some other component, or a bad solder joint some where within the circuit that the Z80 is in.
So are the E codes reliable? Not really but they can and do lead you to the circuit of the problem. In the case of the Z80 circuit there are many components and solder joints where any one of could be the cause of the failure. Components in that circuit could be anything from a capacitor, resistor, transistor, clock (crystal), diode, or even one of the other processors the Z80 communicates with.
There are basically four processors in a G2. The Z80, EPROM, Lexi chip and the WCS chip. Where the Z80 is the first chip check on power up diagnose, if the Z80 sends out a signal to any one of these other processors and the processor does not send a signal back to the Z80, the diagnose will show the Z80 failed.
Now you need to find which processor failed and why. A component, a bad solder joint, or another processor? To find the problem means probing around with an oscilloscope, with power and or a signal sent through the circuit.
With this in mind E1 does not always mean the Z80 chip is faulty. As mentioned above E codes do not reliably pin point a malfunction. In short if you see an E4 do not panic it may not be as big of a problem as that.