Under the dash, near the steering column will be a small, green box. There should be 2 or 3 (can't remember) wires coming off of it heading up in the direction of the speedo. On the end of the wiring is a black plastic optic eye. That is your VSS. it bolts to the back of the speedo & is for the purpose of sending a speed signal to the ECM & cruise, if your car has it. It does NOT control the speedo inself, our cars use a cable driven unit. If the speedo itself is not working correctly, the VSS won't get a reading & therefore show a code 24. Find out what the issue with the speedo not reading is, & the code shoudl go away.