'01sunfire will not shift into reverse or drive. What would cause this. Haven't had this problem before.

coward=FORWARD? SO R FAILS INTO DRIVE. OR LOW. PRNDL
well step one on any car or dAkota. learn what is behind that engine the full real name of the a/T MUST BE KNOWN FIRST
IT IS A 42RE OR 46RE BOX?? I GUESS 46
SO REV IS DRIVE. WELL FIRST MAKES SURE THE SHIFT LINKAGE IS NOT FAILING,
R MUST GO TO R ON THE BOX SIDE.
ATF FLUID LEVELS GOOD IN PARK AND HOT.
????
DO YOU KNOW HOW TO GET 1994, TCM FAULT CODES VIA SERVICE MANUAL STEPS>
THE BOX HAS AND ELECTRONIC GOVERNOR
IF BAD WILL NOT UPSHIFT IN DRIVE OR LIMITED.
makes sure to first set the shift cable adjustments so that all PRNDL detents in the box match up to lever in side car shifter. the service manual cover all that.
then this box has 4 pump pressure port that we use a 300psi gauge on to make sure pressures are good in REVERSE.
inside the box the REAR servo and and front clutch are both engaged if not reverse fails for sure pressures pressures wrong seen at black lines in my drawing below.,.

if the shifter manual valve below is mal-adjusted
reverse fails hard and is the most simple thing to work in this box SEE? if cable not adjusted, the valve is now offset and leaks and the black path here is dead.

reverse is NOT electronically shifted .

There are a few potential causes for the issue you are experiencing with your 2006 Chevy Silverado 1500 4x4 and the 4l60e transmission. Here are a few possibilities:

1. Clogged or dirty transmission filter: If the filter is clogged or dirty, it can cause problems with the transmission, including a loss of reverse gear.
2. Faulty solenoid: The solenoid is responsible for controlling the flow of transmission fluid, which in turn controls the shifting of the gears. If the solenoid is faulty, it can cause problems with shifting, including a loss of reverse gear.
3. Wiring issue: There may be a problem with the wiring that connects the transmission control module (TCM) to the solenoid.
4. Faulty TCM: The TCM is responsible for controlling the transmission, so if it is faulty, it can cause shifting problems.

To diagnose the issue, it is recommended to have a mechanic or dealership technician take a look at the car. They will have the necessary tools and expertise to properly diagnose and repair the problem

run fault codes
shifting to neutral would cause no problem but shifting to reverse will destroy the transmission and drive train slamming your face against the wind screen
If that hasn't happened then may be it is a selector light that is incorrect could be a problem harness connection to the transmission

Have someone check your motor mounts and your shift linkage for wear, binding, and or damage from car accident or from foreign object under the car.
Problem should be in one of those areas.

Transmission filter may be plugged. If it is, that indicates wear parts are worn out. Transmission needs to be rebuilt. The transmission control module can cause the problem as well as shift solenoids inside the transmission. No simple fix. You'll have to go inside.

it sounds like transmission trouble take it to a shop cause i dont think you wil have the tools to fix the trans at home

Check the transmission fluid level, reverse needs twice the pressure to activate that drive does. If low refill and try again. If fluid good, I`d say the tranny 3rd reverse clutch is worn out. Recheck the condition of the fluid on the dipstick, if it appears burnt or very dirty I`d still suspect internal tranny clutch issues. It also has a low reverse clutch, but goes into 1st gear as you said.

you need a used trans, or a rebuild, but dont let a rebuilder fill you full of ****, all you need is the reverse drum, and possibly some clutches. the teeth on the reverse drum strip off, and thats when you lose reverse.

Have you checked the transmission/gearbox fluid is at the correct level ? This will also cause this. Hope this is of help.

Some possible causes could be:

Vacuum leaks cause an engine to run lean witch can cause the vehicle to stall when put into gear.

Improper base idle adjustment can cause the vehicle to stall when shifted into drive or reverse

A malfunctioning or damaged carburetor can cause the vehicle to stall when shifted into drive or reverse

Improper ignition timing can cause the vehicle to stall when shifted into drive or reverse.

A malfunctioning or damaged transmission or torque converter can cause the vehicle to stall when shifted into drive or reverse