Cause
Faulty output speed sensor
The sensor reading the transmission's output shaft, failed or reading erratically. It is the reference against which several other measurements are checked, which is why its faults surface as codes that appear to be about something else — gear ratio monitoring, speedometer drive on many vehicles, torque converter lock-up, and the correlation checks that compare it against wheel speed and road speed.
Most are magnetic or Hall sensors reading a toothed ring, so the ring matters as much as the sensor. A chipped or corroded tooth produces a dropped count at one point in every revolution — erratic in a way a failed sensor is not, and worth distinguishing because the repair is different and deeper.
Swarf is the cause worth naming. These sensors are magnetic and sit inside the gearbox, so they collect the fine metal debris that any transmission produces. A build-up on the tip degrades the signal gradually. On a gearbox that has had a hard life, a bearded sensor is telling you about the gearbox rather than about itself.
Before condemning it, consider that the reading may be honest: a correlation fault can equally mean the tyres or the final drive are not what the module assumes — which costs nothing to establish and is far commoner than a failed sensor.
Typically a moderate job.
Codes this can cause (10)
Codes where this is a likely explanation first, rather than merely a possible one.
- P0720 Output Shaft Speed Sensor Circuit Likely for this code High
- P0721 Output Shaft Speed Sensor Circuit Range/Performance Likely for this code High
- P0722 Output Shaft Speed Sensor Circuit No Signal Likely for this code High
- P0723 Output Shaft Speed Sensor Circuit Intermittent Likely for this code High
- P0730 Incorrect Gear Ratio Likely for this code High
- P215B Vehicle Speed - Output Shaft Speed Correlation Likely for this code Moderate
- P076F Gear 7 Incorrect Ratio Moderate
- P077C Output Shaft Speed Sensor Circuit Low Moderate
- P077D Output Shaft Speed Sensor Circuit High Moderate
- P215C Output Shaft Speed - Wheel Speed Correlation Moderate
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