Oki C9400 Colour Printer: Prints very light

I would shake the toner and see if that helps; sometimes when laser printers sit this occurs when they settle or the toner gets too low. While it is rare, sometimes moisture in the air wrecks the toner.

The other common failure point besides what you have already checked is the imaging drum; as they age, this can occur over time, especially in older models where the drums did not have the endurance they do now where more modern laser printers design it so that the imaging drum and other parts is independent of the toner to get the per toner cost down and rate these for anything from 12k pages, up to 40k or even 60k+ pages (ex: the Lexmark CS drum/developer is split, but good for 150k pages; their common mono lasers hover at 40k or 60k with ones that need a freight delivery going far beyond that at times).

Okidata and Brother has always done this split drum design for years, even when the others built it into the toner carts.
Once the durability was good enough for it, the holdouts (even Lexmark) began to switch over on NEW models, except HP's Canon engine lineup. The legacy machines like the office sized Optras still combined it for a long time (as well as the ones which shipped as a unit because you can't easily change it to split parts; even with how they built those Optra toners being easily split for refilling). Some have ALWAYS been like this like the large Lexmark color lasers (such as the legacy Optra Color series, CS, CX, models which require a freight delivery) and Oki lasers.

I've run into this issue a few times on Reman HP 80X toners as well as with my M401 due to the fact they reuse old drums in those things to cut costs. Based on what you are describing I suspect the Cyan imaging drum did not age well but the rest somehow did.

Given the age of the C9400 (and Okidata no longer selling in the US) Okidata no longer sells consumables, drums, users, or transfer belts for yours, you may need to look on the secondary market. That said, you can probably find a compatible/remanufactured/OEM NOS imaging drum for these for ~$50 or so (P/N for the Cyan drum is 41514707), and try your luck. Sometimes if the unit is otherwise good but a potential future nightmare it MIGHT BE worth throwing a little bit of money at it to see if you can push a bit of extra runtime out of it even with things like consumable sales being a thing of the past.

This idea of buying NOS drums or doing home rebuilds isn't new; it's been done forever and when someone has an old model like the Samsung CLP-300 the OEM drum and toners were discontinued with the HP buyout of the printer division. They continue to produce the newer toner design since they use it in the newer Samsung engine lasers; but not the CLP-300 style toner and older.

These workarounds still have limitations; once the supply of easily available drums Oki stopped making are gone, that's it; you're done once it's also worn out and can't find another. Even the fuser and transfer belt is considered "consumable" in Oki lasers, so you can only do this stuff for so long.