I am new here, but I have owned a 99 Sable since October of 06. Since then I have liked the car, but have had numerous issues with it. I bought the car with 45.5k. It now has 73k on it. Since I have owned the car I have replaced the coolant tank (which exploded), a clogged heater core, LF door ajar switch, brakes(twice!), tires, spark plugs(52k), cabin air filter, recall for coil spring shields, fuel filter, K&N air filter, idle air ctrl valve, battery, and numerous cosmetic repairs due to accidents(thanks mom!) and some/factory upgradeshttps://viewmorepics.myspace.com/ind...ageID=28280914
I would say the car is sharp however the issues bothering me as of now are:

"Service engine soon" light comes on regularly when I take a long drive. I always get a code of P1744-"Transmission slippage detected." Is my transmission really toast? It has happened several times.

My fuel economy is horrible. I am getting less than 20 mpg...highway. In town is more like 16mpg. Having replaced the plugs and the air filter the next logical steps are wires and o2 sensor. Any recommendations?

it does sound like the clutches (or torque converter, or both) are going bad in your transmission. I would get it serviced before it does die and costs even more to repair. it will not be cheap however. those front wheel drive transmissions can be expensive. I would make sure to have at least $2000 on hand just in case it is bad. if it''s just clutches, it shouldn''t be more than 5-800 to fix. (unless you live in a high rent district like LA or Manhattan. the wires and O2 sensor should be fine. it''s probably that transmission that''s sucking down all the fuel mileage.