GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2008 🢒 Vivant 🢒 Transmission/Transaxle 🢒 Automatic Transaxle - ZF 4HP 16 🢒 DTC P0721

Circuit Description

The vehicle automatic transmission (A/T) output speed sensor (OSS) is a magnetic inductive pickup that relays information relative to vehicle speed to the transmission control module (TCM).

Vehicle speed information is used by the TCM to control shift timing, line pressure, and torque converter clutch (TCC) apply and release.

The OSS mounts in the case at the speed sensor rotor, which is pressed onto the spur gear. An air gap of 0.1-1.3 mm (0.004-0.05 in) is maintained between the sensor and the teeth on the spur gear teeth. The sensor consists of a permanent magnet surrounded by a coil of wire. As the differential rotates, an AC signal induces a higher frequency and voltage measurement at the sensor. The sensor resistance should measure infinity at 20°C (68°F). The sensor can measure from 20-8,000 hertz.

Conditions for Setting the DTC
    • The transmission speed is less than 8,160 RPM.
    • The increasing rate of output speed is more than 45,000 RPM/seconds.
    • The system voltage is greater than 8.5 volts.
    • The engine is running.
    • DTCs P0705, P0725, and P0727 are not set.
    • The TCM or engine control module (ECM) is defective.

Action Taken When the DTC Sets
    • The malfunction indicator lamp (MIL) will illuminate.
    • The TCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Failure Records buffer.
    • The running vehicle remains in actual gear.
    • Open lock up clutch

Conditions for Clearing the MIL/DTC
    • The MIL will turn OFF when the malfunction has not occurred after 3 ignition cycles.
    • A history DTC will clear after 40 consecutive warm-up cycles without a fault.
    • Clear the DTCs with a scan tool.

Diagnostic Aids

When DTC P0721 sets, the possible cause of the fault could be the OSS.

Step

Action

Values

Yes

No

1

Did you perform an the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Install the scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Record and clear the DTCs.
  4. Raise and support the drive wheels.
  5. Start the engine.
  6. Ensure the gear position is D.
  7. Observe the output speed with a scan tool.

Is the speed within the specified range?

190-9,762 RPM

Go to Diagnostic Aids

Go to Step 3

3
  1. Turn the ignition to LOCK.
  2. Disconnect the transmission wiring connector.
  3. Measure the resistance between terminals 1 and 2 of the transmission wiring connector.

Is the resistance within the specified range?

∞ ohms

Go to Step 9

Go to Step 4

4
  1. Remove the oil pan.
  2. Disconnect the output speed sensor wiring connector.
  3. Measure the resistance between terminal 1 of the output speed sensor wiring connector and terminal 2 of the transmission wiring connector.
  4. Measure the resistance between terminal 2 of the input speed sensor wiring connector and terminal 16 of the transmission wiring connector.

Is the resistance within the specified values?

0 ohms

Go to Step 6

Go to Step 5

5

Repair the malfunctioning terminals, as necessary.

Is the action complete?

--

System OK

--

6
  1. Turn ON the ignition, with the engine OFF.
  2. Measure the voltage of terminal 1.
  3. Measure the voltage of terminal 2.

Is the voltage within the specified range?

11-14 V

Go to Step 7

Go to Step 8

7

Repair the malfunctioning terminals, as necessary.

Is the action complete?

--

System OK

--

8

Replace the output speed sensor.

Is the action complete?

--

System OK

--

9
  1. Disconnect the transmission wiring connector and transmission control module (TCM) connector.
  2. Measure the resistance between terminal 1 of the transmission wiring connector and terminal B10 of the TCM wiring connector.
  3. Measure the resistance between terminal 2 of the transmission wiring connector and terminal B5 of the TCM wiring connector.

Is the resistance within the values shown?

0 ohms

Go to Step 11

Go to Step 10

10

Repair the malfunctioning terminals, as necessary.

Is the action complete?

--

System OK

--

11
  1. Turn ON the ignition, with the engine OFF.
  2. Measure the voltage of terminal 1.
  3. Measure the voltage of terminal 2.

Is the voltage within the specified range?

11-14 V

Go to Step 12

Go to Step 13

12

Repair the malfunctioning terminals, as necessary.

Is the action complete?

--

System OK

--

13

Replace the TCM. Refer to Transmission Control Module Replacement .

Is the action complete?

--

Go to Step 14

--

14
  1. After the repair, use a scan tool clear info function and road test the vehicle.
  2. Review the DTC info.

Did the last test fail or is the current DTC displayed?

--

Go to Step 2

System OK