Luckily I have a 740 with the same FPR

Don, do you have more than one 700? I'm going to assume you don't have Regina, only because I remember you swapping parts between two LH2.4 cars last week.

At first glance, your readings confirm the sensor is working. It is reporting a mixture something leaner than ideal.

The reading starts at .54V when cold because that is the midpoint bias voltage on the ECU's comparator (coming from the ECU) and when a lambda sond is cold it is high impedance.

The output of the sensor, warm, only varies if the mixture varies, so a non-varying sensor output does not indict the sensor itself. The ecu is supposed to use that lean information to richen the mixture by increasing the injector pulse duration. The sensor output then varies as the ECU makes correction in response to the new lambda output, cycling about once a second.

Could be it is trying to, but is stymied by low rail pressure, a vacuum leak, an injector or two not working, or an exhaust leak.
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Art Benstein near Baltimore