[Edited: RSR -> main system relay]
Great. The CPS (RPM sensor) fails much more commonly than the system relay. OBD DTM 3 should help indicate which one is the issue, which is why I recommended it to you. Sorry, it was another recent 240 thread where someone had recently replaced their CPS, which is why I suggested the relay was a good possibility in your case. The relay is still a possibility for you as you do have both fuel and spark.

The CPS lives on top of the trans bell housing straight down right behind the block with the connector up above on the firewall. It can be a little awkward to get to. They can get brittle with age and you don't want to break it off (extraction then becomes a major pain), so only use as much force as needed to rock it loose and don't ever stick a wrench on it trying to turn it. I just noted that ART has fully documented it for you below in his recent reply to you https://www.brickboard.com/RWD/volvo/1684253/220/240/260/280/crank_position_sensor_failed.html

If the ECU can't get a CPS signal to see the engine turning over (even though the starter motor may be turning it) then it won't supply gas through the injectors in order to prevent flooding. When the signal from a failing CPS becomes weak then starting and stalling increasingly becomes an issue, or it can just suddenly fail. In addition to the sensor failing, the insulation of the CPS cable often becomes damaged, splitting down where it joins the sensor or chafing on metal tubing or the firewall behind the block. A failing CPS is often temperature sensitive and may start to fail intermittenly, especially when hot, which can make early diagnosis difficult. A can likewise become tempermental as it fails, also making diagnosis difficult. You need to do your OBD testing when there is a no start condition in order to catch the problem in the act. The good news is that both the CPS and tsystem relay are available in the aftermarket and not overly expensive. Before shopping elsewhere, do check out FCPEuro for Volvo pricing and what they consider to be quality aftermarket brands.

After you've done the DTM 3 test, if the injectors do pulse, which would suggest the relay is okay, then there is a DTM 2 test you can do to try to confirm it's the CPS. However, in my experience it sometimes passes the test when the CPS is intermittent or weak and hasn't outright failed. Please note the description of the CPS test in the FAQ is slightly incorrect. I documented the correct procedure in another post here recently. I'll repeat it here for you.

The DTM 2 CPS test procedure for LH 2.4 (should be same as LH 3.1) should read:
"With the LED flashing in DTM 2, try starting the engine. If the LED turns off then flashes the 3-3-1 code then the RPM signal is correct. If the engine won't start, run the starter motor until the the diode turns off. If you now get the 3-3-1 code, the RPM signal is correct, otherwise continue diagnosis. If the diode continues to flash rapidly then the ignition system must be checked, the fault could well be the RPM sensor, but possibly could be elsewhere, continue diagnosis."
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now