Solved/Help that you may need: 2003 GMC Yukon GEN 3 P59 Computer would NOT write!

[Kocapuff]

Answer to my ,2003 GMC Yukon Denali, 6.0 LQ4, GEN 3 P59 Computer would NOT write when it was installed in the Yukon.

My 2003 Yukon and HP Tuners worked perfect for about 4 years now.. All the sudden no more "Quick write" and then no more "write" while the computer was in the vehicle. The "write" would continue to "time out". So Frustrating.

I had to buy a bench harness and it easily wrote to my P59 PCM out of the vehicle.

I tried all of the Forum posts answers of pulling the HVAC Fuses, Radio and Amp Fuses, and the SEO fuse. I also removed my Light fuses! Nothing Helped!!

So I tried removing just the FUSE: VEH CHMSL - Vehicle and Trailer High Mounted Stoplamp

AFTER REMOVING THE "VEH CHMSL" fuse in the interior left side of dash fuse box, I could "write calibration" Again as long as "Use High Speed" is NOT checked!

Obvisouly there is some interference on this line. This worked for me and I was pulling my hair out with all the extra work, so I wanted to share with everyone..

I hope it saves others some frustration. Again "Use High Speed" still didn't work, but at least I can write calibration with my P59 Pcm in my vehicle again....

[blindsquirrel]

Implausible, unless something non-factory has been tied into that circuit somewhere. It doesn't go thru the BCM, it doesn't even do anything inside the relay block, just in on one pin and straight back out again on another.

screenshot.05-02-2025 14.46.42.png

[bk2life]

bench harness and write to P59 PCM out of the vehicle.....

this is the way.

[gtstorey]

Is the "flasher module" a true module or just an old fashion flasher relay?

[blindsquirrel]

screenshot.05-02-2025 15.25.42.png screenshot.05-02-2025 15.28.12.png

AFAIK, CHMSL supply is only routed through the flasher module so it can isolate the regular stop lamps from the CHMSL when it needs to (turn, hazards do not use the CHMSL).

[RDF1]

I had a 2003 yukon last year just about kick my butt.
Finally after bricked several PCMs i found out that removing the TBC BATT fuse that it would flash no problem With EFILive.
But HPTuners i couldnt flash it unless it was a bench flash.

[Alvin]

Usually pulling the Radio and Radio amp fuse under hood will allow for a slow read/write.

I did make a harness for problematic trucks (early CTSV can be a do this BS too) Serial data straight from DLC to the PCM. I just re-pin them... 1 wire only for tuning. Allows for normal reading and writing.

You can have these fail where the PCM bricks for 24 hours. If one looks to like it is going to be a pain I'll wire that stand alone DLC just to be safe.

[Alvin]

If anyone is interested I can have my in house wiring guy build some more of these DLC straight to PCM harnesses.

It's a male /Female DLC jumper (to get power/ground) with one wire you run temporary to the PCM. Depin the OEM wire and stick this one in it. Reverse when done tuning.

[Kocapuff]

Update to all:

I previously installed a "Dorman 920-025 Front Passenger Side Power Window Switch" on the Passenger door, the GM module went bad. I purchased the Dorman Module off Amazon. Well I couldn't program my keys correctly since install. I was playing around today with trying to get the "Use High Speed" to work and write and nothing was working... So I noticed when I was trying to "write" to the PCM without the High Speed Checked, that the Passenger Mirror Kept moving all around....... I found it very odd.

So today my new GM Genuine Parts 19115820 Front Passenger Side Door Lock and Side Window Switch with Side Window Lockout Switch and Module got delivered. I first took off the door panel and totally unhooked all the connectors to the Dorman Module. I tried "write calibration" with and without "high speed" checked... IT WORKS both ways and I love having the high speed for fast write again like it use to work!.... no more issue with trying to write to my computer... It was Dorman Passenger door module controller!

I don't know what fuse i could of pulled to try to disable that passenger door module. So if someone knows, let everyone know... 100% this was my issue... Hope it helps some people...