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Try this: wire your voltmeter probes directly to the sensor output wires (try scraping some insulation and soldering a small wire to the output wire, and once you finish the test you can re-insulate by covering it with liquid electrical tape). Monitor this real time with the voltmeter in the cabin so you can gain a sense of what in fact is going on when the "check" lamp goes on. This will also tell you how responsive the sensor is by measuring the breadth and time of the sweep across the scale.

If idling in traffic, or high heat, causes the trouble, then look for symptoms related to high underhood temperatures, including a corroded electrical connector, a faulty injector, etc. Let us know what happens.

By the way, in the newer OBD-II cars, one can connect a scan tool directly into the data port and measure these outputs in real time, very easily. The earlier OBD-I is not as simple to measure: Volvo sells a breakout box and special connector ($$$$$$) to do this. Nobody can afford it.