P0503 Fault and OD Light Flashes at 70MPH on 1998 Ford F150
My 1998 Ford F150 is showing a flashing OD light specifically when I reach around 70MPH, which then disappears after turning off the truck and restarting it. The issue only occurs at or above 70MPH. I have already replaced both the vehicle speed sensor (VSS) in the transmission and the associated plastic gear, yet the OD light continues to flash under the same conditions. The persistent diagnostic trouble code is P0503 — indicating an intermittent or erratic high reading from the vehicle speed sensor. Given that all known VSS components have been replaced, what other potential causes could be responsible for this issue?
Some 1998 Ford F150 models were reported to have faulty speed sensor clusters or wiring harnesses that could cause intermittent signals, even after replacing the VSS. This might be a possible root cause worth investigating.