I agree with Frank; O2 sensor would be the first place to look. Easiest test might be to actually monitor the sensor wires under the passenger floorboard with a good voltmeter. Sensor should generate approx. .2 to .8V, varying when warm. A DC of 98% corresponds to very lean, which would correspond to an O2 sensor voltage very low, say 0.2V. I would think that a mixture reading this far off might set the check engine light - apparantly a bad O2 failure is the only thing that does set it on this model. It might also show up as a fault code. Do you have access to a code reader? Really easy to make one.

An actual DC reading of 100% indicates a faulty O2 sensor, or no current to the EHA. Might try unplugging the EHA an see what happens.

Now, just to verify; many of us who have used the Sears-type duty-cycle voltmeter on pins 2 & 3 of the round connector, have wired it backwards: the red wire actually goes on the GND pin #2 to obtain the correct correlation of the DC (less than 50% for rich, higher for leaner). Of course either way, it should read around 50% when warmed and in closed loop.

DG