1999 Mercury Sable o/d off light

Transmission Control Switch (TCS) & Transmission Control Indicator Light (TCIL) Switch is mounted on shift lever handle. Switch sends a signal to PCM. The PCM turns on TCIL and toggles Shift Solenoid No. 3 (SS3), disabling 4th gear. Malfunction of switch will cause lack of 4th gear (overdrive) disable function.
Either the switch, or the SS3 solenoid in the tranny or the wiring to it has a problem. Is the check engine light on?

First thing you need to do is have the codes read, Autozone does it free, Post the results with the code number(s). Even if the service engine light is not on, there should be freeze frame data stored.
If it won't make it to Autozone, buy a cheap scanner, they can be had for 30-40 dollars, a good investment, since they cover all cars sold in the USA from 96 on.
If the light is blinking, it means possible damage to the Catalytic converter. Misfires. Service it right away!
Puchase readers here. Post all codes here, and we can look into it.

OK, that code has nothing to do with the OD flashing, so to fix the OD, pop the switch out of the gear shift, and test it with an ohm meter, with button pressed it should read under 5 ohms, probably zero ohms, and released it should read OD, if not replace the switch, if good, you will need to test the wires at the switch, to the transaxle. same readings you want as close to zero on the same color wires. if they are OK, the shift solenoid 3 is NG, and some tranny dismantling is in order. P0443=

Special equipment is used to test this system and there is a tecnical service bulliten on that code. Here is the section that covers the code.
SBDS STATIC LEAK TEST 1. Perform SBDS startup. 2. Select "Powertrain" from the Main Menu. 3. Enter the Vehicle Identification Number (VIN) and then press "Continue." 4. Select "Electronic Engine Control" under Categories and "DCL Display" under Tools, then press "Continue." 5. If applicable, select the appropriate vehicle qualifier and then press "Continue." 6. Follow the instructions on the connection screen, make sure vehicle is in Park or Neutral, chock the tires, start vehicle, and press "Continue." 7. Select "Table." 8. Select EVM (units of mode and %), ECV (units of mode), FTP (units of volts), IAC (units of %), RPM, and then press "Continue." 9. Use the pendant on the Portable Vehicle Analyzer (PVA) to select the ECV (on/off) using the "#" button and then press "Yes." The ECV should now have an "=" sign next to it. Pressing the "Yes" button turns the ECV on and off. Turn the ECV on (shuts off venting of system). 10. Repeat the procedure in Step 9 to turn the EVM on (opens system to engine vacuum). The FTP voltage should drop to 0.8 volts or less (voltage should be attained in approximately 1 minute). If the FTP voltage does not reach 0.8 volts or less, then a large leak has been detected (follow the latest PC/ED Service Manual diagnostics contained on SBDS or SBTS). 11. Use the pendant on the PVA to turn the EVM off (holds system pressure constant), FTP voltage should stabilize within 20-30 seconds and then hold at a constant voltage. If the FTP voltage changes more than 0.4 volts in 1 minute, then a small leak has been detected (follow the latest PC/ED Service Manual diagnostics contained on SBDS or SBTS). NOTE: Refer to the latest Powertrain Control/Emissions Diagnosis (PC/ED) Service Manual on SBDS or Service Bay Technical System (SBTS) for information on DTCs P0443, P0446, P0452, P0453, P1450, and P1451. NOTE: If all of the parameters for step 8 are not available using the latest SBDS CD-ROM release, return to the Main Menu and use the following VIN for EVAP testing only: XXXXM91V6VXXXXXXX. NOTE: If the vehicle stalls, it will also be necessary to raise the engine speed to 1500 rpm. This can be accomplished by using the pendant on the PVA to increase the IAC (%) to raise the rpm. NOTE: Commanding the ECV on for more than 5 minutes may cause damage to the canister vent solenoid.
2/17/2010 ...