The Dell manual doesn't really help. Lights 1,2,3 is "some other failure has occurred".
If you are using a separate video card reseat the card. Reseat - with the PC powered off, open the case and remove the card. Then reinstall making sure its correctly and fully seated.
You can also try resetting the BIOS. With the PC powered off, remove the CR2032 backup battery on the motherboard for at least 10 minutes. Reinstall, observing battery polarity.
Dell forum member since 2002
Dell Inspiron 15 Gaming 5577 Laptop
Home Built Desktop PC with Gigabyte Designare Z390, i9 9900K CPU
Windows 11 64 bit Pro, 22H2, SSD drives. Cakewalk by BandLab and Studio One 4.6 Pro Recording Studio Software, MOTU Ultralite MK5 recording interface unit
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Dell S2719DGF Monitor
Same situation on T5610 (dual CPU, 2x E5-2650 v2)... turned out to be a bad CPU. Tested both processors using socket for CPU1. One of the processors (originally in CPU2 slot) would not boot, but other processor worked fine.
I SUSPECT Intel's Management Engine module update -fried- my CPU2 slot.
Dell, you owe me a new motherboard and a CPU. Bill it to INTEL.