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Dear Orange850T,
Hope you're well and stay so! If the speed sensor (#3547699) is the factory-original item, it is about 30 years old and well past the end of its service life.
On 940s - with which I'm most familiar - the RPM sensor (crankshaft speed sensor) fails because a crack in the the sensor's wiring insulation sheath lets moisture get to the wires. That can disrupt the sensor's signal. When that happens on 940s, the engine stops: loss of the sensor's signal causes the engine control module to shut down the fuel pump. (This is a safety "feature": should an accident damage a fuel hose/pipe, loss of the sensor's signal stops the fuel pump and thus limits fire risk.)
So, the 3-1-1 code could have been "set" because a failed wiring harness caused signal loss, not because the sensor itself had failed.
Alternatively, corrosion on the sensor's wiring harness connector may have disrupted the signal. To remove corrosion - which may not be visible - an aerosol corrosion-remover, e.g., Deoxit, should be used on the sensor wiring harness connector. Sometimes, an invisibly-thin layer of corrosion can disrupt a circuit.
A sensor taken from a salvage yard car might work. But that sensor, too, could be about 30 years old and so at the end of its service life.
A replacement sensor is available for under US$50, see: https://www.fcpeuro.com/products/volvo-reference-sensor-rpm-850-s70-v70-c70-960-s90-v90-3547699 .
In short, I'd get a factory-fresh sensor - and having used a corrosion-remover on the wiring harness connector - I'd install it.
Hope this helps.
Yours faithfully,
Spook
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