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For 1990-2009 cars only
Makes 🢒 Saturn 🢒 2002 🢒 SL1/SC1 🢒 DTC 🢒 DTC P0721

DTC P0721 - OSS Sensor Performance
Object Number: 870991 Size: MF

Circuit Description

The output speed sensor (OSS) produces an AC voltage of different amplitude and frequency depending on the velocity of the output shaft. The OSS produces the signal from 16 evenly spaced machined notches on the transaxle differential carrier. The output sensor produces a readable AC voltage signal when vehicle speed is above 5 km/h (3 mph). This OSS signal is sent to the powertrain control module (PCM), which is used to determine output shaft RPM. Missing or extra pulses in a revolution of the output shaft will be interpreted as noise by the PCM. Noise may be caused by an intermittent open, short to ground, or short to voltage. The level of noise is based on engine and output shaft speed. DTC P0721 sets when the OSS signal voltage at the PCM changes too rapidly indicating a noisy signal.

DTC Parameters

DTC P0721 will set if the PCM has detected OSS signal noise when:

    • The speed noise parameter = YES
    • The condition exists for longer than 0.8 seconds
    • The condition exists twice in one ignition cycle (2000)
    • The condition exists once in one ignition cycle (2001)*
    • The vehicle speed is greater than 24 km/h (15 mph)

DTC P0721 diagnostic runs continuously when vehicle speed is greater than 24 km/h (15 mph).

DTC P0721 is a type A DTC for 2000, type B DTC for 2001.*

*Applicable for vehicles built after June 2000.

Diagnostic Aids

Use scan tool to monitor SPEED NOISE while driving vehicle. SPEED NOISE will = YES if noisy or missing OSS pulses are being received.

DTC P0721 can set due to an:

    • An intermittent open anywhere in the OSS circuit
    • An intermittent short to ground or short to voltage on circuit 401
    • A loose sensor
    • Electromagnetic interference (EMI) caused by high current carrying wiring routed near the OSS wiring. EMI can alter the amplitude and/or frequency of the OSS signal and create invalid OSS pulses.

An intermittent open anywhere in the OSS circuit or intermittent short to ground/short to voltage on circuit 401 will cause TRANSMISSION OSS parameter on scan tool to momentarily drop out.