I had Autozone read my codes for service engine light always being on. They found one they didn't ever see: P3999. Anyone know this code???

 

The list I have doesn't list it. Is the truck demonstrating any symptoms that may point to the problem? Any rough idle or running, random stalling, anything like that?

 

There is no such code as far as I can tell, are you sure they read it correctly?

http://mechanicdoctor.com/

 

Sometimes it revs way up, then if I turn it off and start again it usually is normal. The idle does jump up and down slightly. The person I bought it from said it was always on, not to worry unless check engine light comes on. I think she said that the ecu was replaced with a 96 model and that was why the rev up and idle jump. Any truth to that? I have owned it for 16 onths with no problems, just trying to get it all nice. It has 118000 and I only drive it 2000/ year.

 

What have you done to the truck so far? Are you using premium gas? How often do you drive it?
Every day? Once a week? Once a month?

 

No mods to truck. I usually drive it on weekends. I have just started running high octane. I was using regular octane, and i am sure the previous owner did as well. Why the need for premium? It does run dramatically better now that i run premium. Again, this truck runs great, I just want to keep it that way. Also, for diff fluids, what is the life of fluids. Is it important to change yearly even if i drive 2000 miles a year. By the way tis site rocks! You guys are great!!!

 

I have to disagree with the person you bought the truck from. Service engine (RED light) is bad. That means it's something that needs to be fixed. If it weren't serious the light would be yellow, like the check engine light.

I would think that if the ECU was replaced the problem lies in there somewhere. I think it is not recognizing something from the 97 engine that wasn't on the 96 and throwing a code that doesn't exist. I could be way off but that's what I am thinking. Could also be something just isn't plugged in or a loose connection that's freaking out the computer.

 

On a D1, the red engine light is an emissions reminder that has to be reset with a Rover scanner, or a paperclip. Comes on about every 82K Miles.

 

There is a "how to" in the tech section on www.discoweb.org on how to reset the "Red Service Soon" light.

As for what to do once a year, change the oil every 6 months no matter if you drive it 2000 miles a year or 500 miles a year.
Check the diffs fluid and transfer case fluid levels everytime you change the oil, and then change then every 2 years no matter the miles.
Do the same with your trans fluid and coolant as well as the brake fluid.

You need to change the not because they are going to wear out on you but because of water contamination.
Condensation will collect in these fluids and because you drive it so little they will not get hot enough to evaporate the water.