My 99 cougar will not go in any forward gears but will in reverse

Possible your linkage is broken. Look at the linkage where it attaches to the transmission and verify the plastic connections aren't cracked.

Check the cable under the gear lever. Look for broken pin or missing pin joining the lever to the cable

Have you ran an Oil pressure test? low oil pressure (20lb)or no oil pressure(0lb)and the lifters will not open the valves enough to let fuel in to the cylinder to start your engine!

re adjust the cable per the shop manual.

theirs an adjustment that mounts to the tranny it could have slipped out of adjustment

check the oil lever in the transmisson assuming it's automatic, if the fluid level is fine then with it in gear running and the brake on look to see if the drive shaft is turning, if it is then you have a prboblem with the drive shavft depending on engine size and wheather it's front or rear wheel drive. if it's not turning then you have a transmisson problem. If this is helpful please rate.

the problem is the forward clutch pack has failed in the trans or the hydraulic pressure that applies that clutch pack is leaking. u are going to need an overhaul

Your transmission shifts are controlled by the PCM, Powertrain Control Module. It can be scanned for transmission fault codes which can guide your diagnostics. Autozone can do this for free. You would have to get the car there or find a technician who will bring their hand held scanner to our car.

But, try this first. Disconnect the battery for 1/2 hour. This will clear all the shift tables and the Keep Alive Memory (KAM). Reconnect battery and see if you now have forward gears.

If no luck with fault codes, this could be a possibility that requires a trans teardown.

MERCURY: 1994-2000 MYSTIQUE
1999-2001 COUGAR
ISSUE: Some vehicles may exhibit no forward gear ranges due to the misalignment of the forward/coast clutch cylinder snap ring with respect to the legs of the forward clutch piston.
ACTION: During assembly of the forward/coast clutch cylinder, the gap in the select fit retaining ring should be located midway (i.e., 45 degrees) between adjacent legs of the forward clutch piston. Refer to the following Service Procedure for details.
SERVICE PROCEDURE

1. Prior to installing the forward and coast clutch plates into the cylinder, note the location of the four (4) forward piston legs ( Figure 1 ).
2. Mark the top of the cylinder with a felt tip marker midway between any two adjacent legs (i.e., 45 degrees or 4 external spline teeth from the center of any leg) ( Figure 2 ). NOTE: THE FORWARD PISTON LEGS WILL NO LONGER BE VISIBLE WHEN CLUTCH PACK AND RETAINER IS INSTALLED.
   
3. Following the Service or Workshop Manual procedure, install the clutch plates.
4. Position the ends of the snap ring such that the gap is aligned with the mark on the top of the cylinder (the 45 degree mark) ( Figure 3 ).
5. Perform end clearance check as required.

Does the fluid smell burt or is it discolored

sounds like your brake/shift interlock solenoid is not releasing.
Your owner manual will tell you how to bypass in emergency.
Check your fuse first. fuse panel on instrument panel.

Please rate as fixed if this helps.

Fuse 23 - 1 5 amp
Fuse 34 - 7 1/2 amp

check enigne light not related.
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The selector lever lock can be moved mechanically out of position ”P" when for example the battery is disconnected or completely discharged.
Using a ball point pen or similar item as an unlocking lever, push forward and hold in position.

Move the selector lever out of position "P".

NOTE: If the selector lever is moved back to ”P" it is re-locked.

Selector Lever Lock