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Q: If my speedometer is off by 30 mph, could it be the sensor? on 1999 Pontiac Grand Am

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Speedometer works but is off by 30 mph.
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Pofessional scanner will tell if it is or not!! Scanner live data will show what the computer sees and if it differs from odometer it's the dash cluster, if it's the same ,,possible sensor!
You are welcom for the advice: however since that was given;
"The things that pass for knowledge i can't understand"
The ECM uses the sensor output for the speedometer, but also for the transmission and mixture controls. If it was a sensor, the trans would shift into O/D at 20 mph. It sounds more like the decoder chip or the circuit board at the back of the cluster that changes the pulse signals to a voltage signal that runs the speedo. Plug in a code reader that has live data capability and take it for a drive. Verify that the live data reads the proper mph's compared to the dash. If they are different, it's the C/B or decoder.
Took the car to oreilly's about the engine light and the o2 sensor is bad on it. Would that cause the speedometer to act up?
In rare cases, yes. The PCM has a common ground for the heated O2 sensor, Trans input sensor and a few other components. If there is an internal short in the heater system, it is possible it is grounding battery voltage through the O2's high/low signal circuit, scrambling the speed circuit signal.

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