59.f0 error on HP Color Laserjet CP3525

We had a Color Laserjet CP 3525 with a 59.f0 error.  Generally, with these errors, you pull the transfer unit out (ITB) and push it back in.  That usually soves it, because the transfer unit wasn't properly seated.  Not this time...

The way to troubleshoot this is posted in several spots online.  I followed the conventional list.  The error means that the transfer unit isn't engaging against the toner cartridges.  There's a hub inside the machine that turns against a gear on the transfer unit.  As it turns, a flag on the transfer unit rotates into a sensor on the machine.  So what you do is:

  1. Make sure the gear on the transfer unit isn't stuck--that the engagement mechanism moves properly.
  2. Make sure the hub which turns the gear (the one in the machine) works properly.
  3. Make sure the sensor flag is turning on the transfer unit when the gear turns.
  4. Make sure the sensor on the machine is working properly electronically.

That's it.  That's as far as any of the guides will take you.

  1. The gear on the transfer unit was stiff, so we had to replace the transfer unit.  But we still had the same error after replacing it.
  2. The hub on the machine seemed to turn properly.  When runnning the component test from the machine's control panel, it would rotate once, click, rotate again, and click.  This is what it's supposed to do.
  3. The sensor flag was dipping down into the sensor.
  4. The sensor, when checked from the control panel, was reading "on" and "off" properly.

Everything appeared to work as it should... yet we still had the 59.f0 error.

It turned out to be the fuser drive assembly (which also happens to drive the transfer engage).  As shown in the pics below, the gears that drive the transfer engage have slots in them and are timed together.  They were out of alignment.  This means they were still turning, but weren't turning correctly--the transfer unit was engaged improperly against the toner cartridges.

Replacing the fuser drive assembly is not a small job.  Follow the service manual exaclty on this one, and don't skip a single step... especially when putting back the cartridge drive gears.