P0502

P0502 means the VSS circuit is reading below the lowest allowable voltage or frequency, effectively a short-to-ground or open-circuit condition on the signal wire, with the engine running and the vehicle expected to be moving. The PCM sees flatline 0 Hz when it should be seeing 100+ Hz. Cheapest-first diagnosis: key on engine off, back-probe the VSS signal wire with a DMM, you should see roughly 5V or 8V reference depending on sensor type when disconnected. With the sensor connected and the wheel spun by hand (vehicle safely raised), you should see the voltage pulse between roughly 0.3V and 4.7V on a Hall-effect sensor, or an AC waveform of 0.5V to 5V peak-to-peak on a variable-reluctance sensor. A flat 0V reading means a short-to-ground in the harness, while a flat 5V reading means an open in the signal return. Check the ground wire continuity to chassis next, this is a $0 fix if a ring terminal has corroded under a body bolt. The expensive misdiagnosis is replacing the PCM when the ground strap behind the cylinder head is loose.

Vehicle-specific patterns: 1996 to 2002 GM trucks with NV3500 manual transmission set P0502 from a failed VSS at the tailshaft, the sensor tip cracks from heat. 2003 to 2007 Nissan Frontier and Xterra set P0502 from a broken signal wire where the harness flexes near the transmission crossmember. 1998 to 2004 Jeep Grand Cherokee WJ sets P0502 from the transfer case VSS connector filling with water from the cowl drain. 2000 to 2005 Toyota Tacoma sets P0502 from rodent damage to the VSS harness routed along the frame rail. Estimated repair: $60 to $320.