C912 won't print both sides [duplex] anymore

[pixels]

Hi crew,
I have a Lexmark C912 with Ethernet print server that has served me well for many years.
A few weeks ago it suddenly would not print both sides.
I've had the duplex unit out, cleaned it with compressed air, checked all the belts and gears, checked the plug and wiring, can't see anything wrong??

Duplex is enabled in the printer [finishing menu], is added as an option in the print driver properties > config tab and is available to select in the print driver properties.
The duplexer does seem to be getting drive during self test after power on, though it's hard to be sure.
I've tried re-installing the driver - nope.

I'm an electronics tech who is pretty computer savvy and I know this printer very well, but this issue has got me stumped!!
V. frustrating! Everything I throw at it, just ends up with me in the bin!! grrrr!!!!

The one thing I'm yet to try is to print from a different machine.
This machine is win 7 x64.

I have a feeling that an update from MS is what's killed it, as I have dual boot with XP Pro x86 to keep my Matrox rtx10 capture card operational [no drivers from Matrox for win7]
After installing a batch of security updates from MS for xp, it/they killed my capture card driver.
Re-installing Matrox x tools [with drivers] got my capture card working again, so.....but, re-installing the driver for the c912 should have corrected any issue....you would hope..... everything else on the c912 works fine, just the duplexer won't do both sides.
I know it's not going to work even before the first sheet has been delivered, as I can hear the feed tray click another sheet out before the first sheet has made it to the delivery [bin 1], at which point it normally reverses that sheet back into the machine via the duplexer which in turn feeds it back around to the input side.
Must try another computer

Any help, thoughts, advise much appreciated
Cheers

 

[pixels]

Some more info, in fact I thought I had it! I checked the fingers that the sheet lifts up when it's being delivered in to the standard [top] bin by holding them up by hand and I got no response/error from the printer...ahha.
I then cleaned the sensor with a brush and a can of compressed air and then it did work, giving me the 'bin full' message on the printers panel, but still the duplex doesn't work!!
It knows the duplex unit is there, because if I physically remove it, all the duplex options disappear from the printers finishing menu and from the driver if I click start>devices and printers>right mouse/properties on Lexmark>configuration tab>ask printer, the tick disappears from the duplex option and reappears if I refit the duplexer and go through the same procedure again - the tick for duplex reappears.

So, I found one dirty sensor [bin full] and now that works, so I have a feeling that there's another sensor somewhere that is giving me grief.

 

[pixels]

Finally got a chance to try this from another computer - nope, still only prints one side, so it's definitely something to do with the printer.
Maybe it's something to do with the clutch on the drive gear.
Any clues from anyone much appreciated, this is now officially driving me nuts!!!!
Cheers

 

[pixels]

Ok, getting somewhere now, though still perplexed! You can imagine my shock when I went to print some more blank invoices, paper for which is in trays two and three [standard 80gsm A4 in tray 1], with duplex in the driver set to 'use printer settings' which I thought was 'Duplex - off', but the first oddity I noticed was that the 1st sheet was being delivered to the top [standard bin] instead of out the end [bin 1], but then instead of dropping into the standard bin, it was reversed back into the machine to go through the duplexer!! 'What the!!!'

I've since discovered that duplexing works fine from trays 2 and 3, but not from tray 1??????
So at least I have a workaround now, I can simply put the bond A4 into tray 2 and duplex from there. But what the heck's the matter with tray 1????
The printer was set to duplex on [my fault] which explains why it started to duplex the invoices. Setting the printer back to duplex off restored 'on' control to the driver, which is the best way as you then have a simple mouse click right underneath to choose long edge or short edge duplexing.
Fixing that setting didn't fix tray 1, that still won't duplex???? odd??????

 

[pixels]

Some more info here, if I load a sheet of a4 into the manual tray and tell the printer to print from that tray [choosing manual paper in the driver] and to print it both sides [duplex], it only prints one side then asks me to load another sheet into the tray so it can print the 2nd page of the document, but if I cancel that job, load another sheet and instead choose 'Multipurpose feeder' [same tray], it prints duplex no problem.