Was that with the O2 sensor plugged into the green harness lead, or disconnected from it? That does make a difference, as the ECU is using the O2 sensor output voltage to adjust mixture, which in turn is reflected by a change in the output voltage of the sensor, which changes mixture again, and so the loop continues. When the sensor is unplugged the ECU operates at a fixed cycle so you won't see much voltage variation unless you enrich the mixture by pulling off & plugging the vacuum line to the FPR (or BRIEFLY pinching the return line) or leaning it by causing a vacuum leak - pull off a vacuum hose somewhere. You should be able to induce a full lean (0.1v approx) to full rich (0.9v approx) condition and the sensor should respond accordingly. Some good notes about this and other O2 sensor tests in the FAQ. I had a similar situation with my 90 780 turbo - the LH2.4 ECU was cooked, in my case, and it was not responding to the O2 sensor. Voltage started off around .7 then dropped off and stuck around .18, but disconnecting the O2 sensor & forcing rich/lean conditions confirmed the sensor worked. However I think your LH2.2 ECU is generally considered pretty bombproof. Any driveability issues or other symptoms of something being amiss?
-Chris