Diagnostic article Generic OBD-II
P0502: Two vehicle-speed paths disagree in the Duramax monitor
P0502 compares replicated output-speed information with a transmission speed message in this GM example.
Read before applying this article
Documented application
2009 General Motors diagnostic parameter file 09OBDG01 DURAMAX Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
The P0502 entry in GM's Duramax summary is labeled absence of vehicle-speed signal, but its actual strategy compares two speed paths. One is replicated transmission output-speed information; the other is vehicle speed delivered over CAN by the transmission controller.
A comparison needs both values
The published criterion is a difference between those two sources that persists for 45 seconds. Therefore, a single speed reading cannot show which side of the comparison is responsible. Retain both available values and the transmission-controller communication status when reviewing the original event.
The monitor is qualified by coolant temperature, engine speed and engine torque, with power take-off inactive. Those requirements explain why a parked or lightly loaded check may not reproduce its decision. They are operating prerequisites, not instructions to create a high-load road test.
Do not confuse the name with a voltage specification
The source does not give a universal low-voltage connector limit for this code. The generic circuit-low label and the source's absence-of-signal wording must be interpreted through the documented speed comparison and the exact vehicle service procedure.
A reasonable diagnostic distinction is whether one speed path is missing or whether two present values disagree. The code alone does not resolve that distinction. After correction, verify credible agreement under qualified conditions and completed monitoring. Clearing the fault or seeing a speedometer operate cannot prove both monitored paths are valid.
Source and applicability
2009 General Motors diagnostic parameter file 09OBDG01 DURAMAX Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established. This explanation describes the documented monitor, not a universal repair procedure.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Vehicle Speed Sensor A Circuit Low
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT