P0720

What P0720 Means

Code P0720 points to a fault in the output speed sensor circuit of the automatic transmission. The sensor measures output shaft speed, which the transmission control module uses to determine vehicle speed and select the correct gear ratio.

The transmission control system (for example, ZF 8HP or Steptronic) receives the signal from the sensor and compares it against data from other sensors (input shaft speed, throttle position). A broken wire, a failed sensor, or a problem with the control module (DME/EGS) makes the signal unreliable, and that sets P0720.

Symptoms

• Shift problems: harsh shifts, delays, or the wrong gear selected.
• The speedometer may not work or may read incorrectly.
• Limp mode (transmission locked in one gear).

Diagnosis and Repair

1. Read the fault codes with diagnostic equipment (ISTA, INPA) to confirm P0720 and catch any related codes.
2. Check the transmission output speed sensor: inspect the connector and wiring for breaks, corrosion, or damage.
3. Check sensor resistance with a multimeter against the manufacturer's spec (if available).
4. Check the sensor signal with an oscilloscope while turning the output shaft (on a lift, for example).
5. Replace the sensor with a new one if needed.
6. Check the control module (EGS/DME) for power or ground faults, and check the wiring to the module for continuity.

Common diagnostic mistakes: replacing the sensor without checking the wiring and connectors; ignoring related codes that point to a control module fault.

Secondary Problems

Ignoring P0720 can leave the transmission stuck in limp mode, which hurts handling and safety. Driving for a long time with a failed sensor can accelerate wear on the clutches and other transmission parts because shift timing is off. Fix the fault as soon as you can.