Question: My 2004 Ford F-150 started slipping in reverse. All forward gears are functioning properly. If I rev up the motor in reverse, it seems like it wants to go, but the light comes on and I get a P0720 code. I looked up the code and replaced the output speed sensor, and changed the fluid and filter.
Answer: DTC P0720 means the PCM detected a loss of output speed sensor input while in operation, which could be caused by a bad sensor. In your case, the loss of reverse is producing the DTC. The PCM sees you're in gear, foot on the throttle, but you're going nowhere. So it assumes the output speed sensor isn't working, but we know the actual cause is the transmission's output shaft isn't turning. The fact that you feel the truck wanting to move (slipping) strongly suggests an internal mechanical/hydraulic problem, not an external control issue. The fault may be in the reverse clutch assembly, low/reverse band, low reverse/servo, or an obstruction in a number of hydraulic circuits. The fact that it shifts fine in all forward gears tells us line pressure is good and the transmission pump is OK. A complete transmission overhaul is recommended most of the time. A specific fix will likely require removal and teardown of the transmission anyway.
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