I have a Dell E525W Printer. What is a CDT Sensor and where is it at?

We need an actual photo. The diagram does not illustrate where it is accurately enough.

@abstracted Look at the age of the question and answer. Most people do not have CLJ's existing for exact reference photos. Unless someone has it in production and needs to do it anyway, it won't happen. I only somwewhat recently got to a point I could have a color laser and afford to own it and it took 3 rounds of failure to get one I like.

Check a similar looking Lexmark; Dell rebranded a lot of those. You can tell by the toner ;). Square-ish toner on the color series (usually on the right panel)=Lexmark, round toner=Xerox. For mono, it was 95% Lexmark/5% Brother; again, Brother and Lexmark drums are very different. The toner styles are also different (Lexmark=rounded, Brother=flat), as well as Brother having a corona wire that isn't considered "Maintenace free", which Lexmark does. You're supposed to run the wiper every toner change with Brother. The drums match this shape variation.

This is a Lexmark. "Dell Color" is Lexmark Unison too, by the way (unless it was a Xerox), but they both use Pantone colors so Unison vs Xerox has little matter. Dell just gave up badge engineering and just sells them as straight Lexmark and Xerox machines.

As much as I loved HP ColorSphere back in the day, the dynamic security printers abandoned that level of CLJ color performance. Whatever HP has now has lessons from ColorSphere, but Canon Color V2 and Unison are the superior systems today. I love the HPs for their mostly "don't cry about clone toner damage" policy*, but losing ColorSphere's consistency was a dealbreaker. I didn't want to deal with a well used machine with 150k pages+a potentially tired fuser. I'll HAPPILY take a mono 4000 series with 200k and just brush it off and run it at 500k+ without a care, but the color machines just aren't machines you disregard the counter on unless it has serious issues. Half a million pages gets expen$ive with color.

*OEM chips only since dynamic security; clones can be killed at ANY TIME, so never update the firmware with 3rd party. Old printers just nagged.

@abstracted check the original diagram in the original answer. It's the square holes.

It's actually rectangular. I see a dim red light inside the left access hole but not inside the right one. After cleaning both access ports with q tips, it did not fix the error... I'm wondering if there's supposed to be a dim red light on the right side one as well? Any thoughts out there?

@petermezes you are right. Pardon the geometrical differences :-)