HP CLJ M651 Yellow Toner Slot Stopped Working

School Campus Lab Printer HP M651 network printer stopped printing Yellow toner. Replaced toner cartridge. Same problem. Eliminated the cartridge being the issue. Also disabled toner check and swapped cartridge colors around and it always came back to whatever color was in the Yellow toner slot not printing. I checked HP forums and found a similar issue which was resolved by opening the "front upper cover" and straightening the copper contact, I do not know how to open the front upper cover to gain access to the toner cartridge contact to see if it is bent. The M651 repair manual wasn't clear on this either. If anyone has any info on this, I would greatly appreciate it! We've just replaced the toner and the transfer and fuser kit. Would hate to have to ditch the printer after sinking all that time and money into it. Thanks