My CP4025 printer does not print yellow

HP SM:

Hp Clj Cp4020 Cp4520 Service Manual

I've run into this issue on 2 Lexmark C3326 units (the yellow channel goes out of whack enough that it's forever skewed and can NEVER BE FULLY FIXED, just compensated for); I've tried :-(. Since it's not an issue I can completely correct, I have learned to mask it enough that you need to look until it drifts again and needs to be sorted. When it occurs, I compensate for it by adjusting it so it's masked enough to at least run the toner out on the failing C3326 so I can replace it with a CS machine (such as the CS/CX431 for the 6/6.7k page HY toner vs the 2/2.5k toners on the C/MC3326), and not waste the toner I have.

The only reason I have bothered to maintain them for this long (and went through 2 failed units, and a 3rd I only got to use the C33 only toner up) is I have $500 in toner I can't use on a C3426; otherwise, I'd have hit up eBay for a gently used C3426 to finish the toner, potentially run the C3426 or if I had repeat issues like the C3326 get a CS series since those are not problematic in my experience (or if it happens the adjustment isn't a chronic need since Lexmark has a more reliable registration system on the CS/CX models).

When I have adjusted these Lexmark C3326s from he-double hockey sticks, I use the trans flag (or really, anything with those colors laid out I can immediately tell) in color after making my adjustments paired with the Lexmark internal test pages. It works amazingly well to spot how off it is without involving the (sometmes fails where it's unfixable) problematic Cyan and Magenta channels (while ignoring the working yellow channel). This might give you an idea of what you can do to find a "benchmark" test page should HP have limited to no internal reference pages you can use as a checkpoint. It's a weird workaround (I know), but for me given the cyan and magenta channels line up so well (and the yellow usually never fails chronically) I can spot registration clashing and skew issues immediately.
You lean how to find less toner hungry test pages when your toner set is $400 and 4 adjustments costs you $800, and the 3rd is the last reliable one.

This is why I use it for the final post adjustment test:

As you can tell, this C3326 remains a bit off but it's not too far gone that it wasn't correctable (yet). I have one more chance to avoid the swap.

As of yesterday (Sep 2), the C3326 has lost it's ability to control image skew so badly I no longer can run it when accuracy MATTERS. I had to pull out a MC3326 standby unit to finish off my toner stockpile so I can get another C3426 or a CS431. These C/MC3326 units are GARBAGE. Never again, I want another C3426 or a CS431 once I exhaust this supply of toner.

You've swapped the toner, and I assume you pulled any tabs that split the toner hopper from the drum. That said, I'd double-check since it's a crapshoot. I say this because the newer 2016+ HPs never shipped with a toner hopper tab. I say this because many older ones did for years (with some potentially being redesigned to remove the plastic toner hopper tab, mainly long-running models like your CP4025). HP no longer sells this one, but they still sell the updated model and have done so since 2009; it's the last non-DS HP they make. These can't be retrofitted with the HP DS firmware because it has legacy firmware and Canon RIP (which can't be easily patched to add it). That said, HP shipped their toner cartridges with a single-use shipping tab with such an old engine). However, third-party toners never got past using it (even in 2016+ HP DS models where OEM supplies no longer use it). I can tell because the CP4025 has the pre-~2010 HP logo.

If the toner has no tab or has been completely removed, you'll want to look at the back of the controller. When the controller passes POST and diagnostics, these enterprise HPs have a smiling face icon ( :) ) and a heart on the back; both need to be on to rule out a controller issue. If it isn't on, you generally have a bad controller, but if it's on or the printer missed the fault for whatever reason, this MIGHT lead you to a false positive; treat it accordingly for an issue that can't be immediately detected like this (at least easily). This has been retained even in the current enterprise models.

READ: If you see this and do not own an LJ Enterprise machine like this, your printer will not have this; it's an enterprise-only feature reserved for "Class A" (enterprise) printers.

While possible, I don't think it's your ITB (even though it's a wear item on these; the CP5525 moved to the lifetime belt). If the ITB had an issue, all four channels would have issues (or problems on multiple channels, like fading or streaks). It's also a long-life part before they got them built to a point where it's a lifetime part; unless you're nearing 150k pages without a replacement, skip this until you exhaust all but the controller.

NOTE: If you need a new ITB, the HP part number is CE249A.

Once you confirm the controller has at least passed the POST testing, I would look at the laser first and make sure the connections are good. I would try reconnecting them on the laser end and the connected controller; as a last resort, it's worth trying to replace the laser assembly cables if it means avoiding a laser replacement...

READ: If the cable replacement doesn't work, it likeluy points to a faulty laser assembly. In which case, when you find a new laser once it's installed you will have to adjust the CMYK registration so it's centered becsause the new laser will likely not fit the paramaters set for the old one without skewing the registration.
This is why I don't repair the C3326s and replace them when they lose their way and are hard to recover without a lot of paper and toner waste. I have had to do my THIRD adjustment on the current C3326 and had to buy a MC3326 as an interim unit so when it's beyond correction, I can use my toner surplus up :-(. THIS IS A $400 FAILURE PER TONER SET ON THE C3326!

Start with the automatic registration adjust if it's available (it should be on that model; most good color lasers from the last ~10 years have sensors which can self correct on the fly and do most of the work adjusting the registration.) That said, generally a bit of human touch to finalize it is needed with a complete laser swap if you want it to be completely dead on (not as crucial with the established lasser as it "knows" it but with replacements, no you have to step in and finalize the registration margins).

If you have replaced the toner, laser assembly cables, laser and it STILL has issues, the controller is bad. You'll need to grab the info from the controller with the configuration page and tunnel into the printer using HP PJL commands without the HP service tools.