Q: my nissan xterra 2006 4.0l v6 automatic transmission isn't shifting from first. No codes are showing on the computer

Fault codes may not be present if there is a hydraulic fault in the transmission. First thing to check is the transmission fluid level and condition. Low/poor fluid condition can keep the transmission from shifting properly or shift at all. Transmission fluid should be a reddish pink color. Of it is burnt - black.brown and foul smelling, changing the transmission fluid and filter MAY help. But, from years of experience, if the transmission fluid has deteriorated to a burnt black, there are more than likely internal issues with the transmission. The front pump may have failed and not providing the proper fluid pressure to change gears. Also, if the clutch packs are burned up - yes automatic transmissions have clutches - they will not engage or change gears.

It may be best to have the vehicle towed to a local transmission specialist for further diagnosis.