These steps apply to both the "B (3340/3442)" and "MS (331/441)" variants of this printer. The ONLY DIFFERENCE with the MS version is that the firmware is tweaked to use a different toner and will take a larger black toner compared to the "B" variant. It is a binning variation where they use the same parts between the two units, but differentiate them with firnware. You can buy one or the other and get the same unit but cannot interchange toner.
They are so close, they even use the same 40k page drum unit (which is honestly a far cry better, back before these the "entry level drum" was ~20-30k pages, not much better than the common 12k page Brother drum back then)!
With that out of the way, here's the issue: The sensor inside the toner compartment is probably broken and needs to be replaced, or there is paper under the cover, which can be cleared by taking the two screws marked in red (use a stubby screwdriver unless you want to remove the side panels and top cover -- there is no working room without removing the top panel on this model). Use a small object like a toothpick to see if the sensor moves freely. If it does not, the sensor is broken and needs to be replaced, as it is creating a false jam by being stuck in the closed position too long.
NOTE: Using compressed air inside the sensor may potentially fix it as well, but the way they bury the optical sensors on the newer laser printers makes this less effective and needed. Still may be worth a try as a last-ditch effort.
The sensor is towards the bottom of the printer and is a bit tricky to replace AND requires significant disassembly :-(. I have attached photos to show how much of the printer is inaccessible from the bottom with this particular unit:
Here is the SM for the printer:
Lexmark B3340 B3442 Ms331 Ms431 Ms439 M1342 Service Manual
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@switchhear let's see if our printer guru @nick has any idea about this.
crwdns2934271:0crwdnd2934271:0 oldturkey03 crwdne2934271:0
@oldturkey03 I'll have to look at mine and see where the sensors are before I scored a MS621 to replace a temperamental HP. There's a few similar MS variants of it with the same firmware tweak I mentioned on the CS431.
crwdns2934271:0crwdnd2934271:0 Nick crwdne2934271:0