97 Cadillac DeVille PO503 Code: Brake Switch Issue and Speedometer Behavior Explained

Model: 97 cadillac deville Fault Code: P0503 Posted: 2006-03-30 15:00

I'm experiencing a sudden speedometer malfunction on my 1997 Cadillac DeVille. During the last drive, the speedometer started reading backwards—from 45 mph down to zero—then the brake light illuminated. The dashboard displayed 'Service Brake Switch.' After clearing the code, PO503 reappears immediately. I've noticed that the speedometer functions normally from 0 to about 40 mph but becomes erratic above that threshold. I'm looking for a reliable flowchart or diagnostic steps to troubleshoot this issue. Specifically, what does servicing the brake light switch involve? Does the brake pedal switch require adjustment or replacement? Any insights into the location of the speedometer sensor, acceptable ohm resistance values for a healthy sensor, and wiring specifications would be greatly appreciated. Thanks, Mike