My security light stays on & won't let my 99 gmc 6.5 diesel sierra start any solutions

SOURCE: 2001 GMC SIERRA . ABS LIGHT STAYS ON

Wheel speed sensors misadjusted, damaged , or metal shavings and other debree, since they are magnetic can give false readings from one of the wheel circuits. Over sized tires can give of false speed readings as well. there are many other causes including bad relay, control module, e.t.c. It is best to have the proper scan tool to diagnose an ABS problem

SOURCE: 94 GMC truck diesel starts,runs and then dies and will not start

I have the same problem and have trouble codes 35 and 36. I found out that there is a computer module that needs replaced as it sits low in a valley and gets hot and seperates. I am reseaching about the a/m fix that installs the new module away from heat.

SOURCE: 1995 GMC Sierra Z71 ABS light stays on

yo man, idk if this is exactly what your problem is but my 1991 gmc sierra was doing the same fuckin thing and ****, and i took it to the gm dealer and had them look at it and my connectors and my ignition switchs were fried, also my truck was having a problem shitfing once those went but i also had that problem, perhaps yours is somehwta liek that as well... hope i could help

SOURCE: my 2000 gmc sierra won't start. changed the

Just maybe it's mechanical.Internal to the steering column is an activator rod that manually makes the contact in the ignition switch.It's the cast piece that is molded to the activator rod that breaks.Two things you can do;tilt the steering wheel all the way up and try again to start(the cast piece spreads before it breaks,by tilting you gain a little extra throw on the switch).This will only work a couple times before complete failure of the casting.The other is to move the cover that's just below the tilt lever enough to get to the rod and move it with a pliers to start.If it's mechanical these things do work,I ran my truck with a vice grip for a couple weeks until I could get the parts for the repair,but replacing the broken part is smarter and won't scratch up your column like mine.

SOURCE: Excessive black exhaust on 2003 GMC Sierra Duramax Diesel.

This is what i would try (cheapest first) redexe/wyyns
injector cleaner,double dose,then i have put 1gal gas in 10gal diesel to increase burn temp (you may encounter a hot start problem temporarilly)then if still smoking, get a compresion check on the engine(this will tell you volumes)
by the way the NATO tanks put in 1/3 gas in diesel to stop it freezing in arctic conditions.any more than 1/3 can result in problems.