Need some help with a 240T Lambda Sond problem...

I can't get the frequency to fluctuate when trying to set the mixture.
Should be somewhere around 45-55% duty cycle, but it's stuck at a steady
70%. Tried a different computer with no luck, and am at wits end. No
clue where to go from here.

Per Bentleys, readings were taken at the Red wire near the battery. Per Volvo green manual, getting a buzz from the frequency valve, O2 sensor wire is not grounding out with shielding. Volvo manual says the computer is the problem, but a known good box gave no difference. Frequency still stays at 70+%. O2 sensor is new.

Any help/suggestions would be appreciated.

Thanks-

Kerry