For 1990-2009 cars

### Circuit Description

A normal function of the transmission control module (TCM) programming is to perform an internal check that verifies the EEPROM data. When the TCM cannot read the EEPROM data, then DTC P1621 will set. DTC P1621 is a type B DTC.

### Condition for Running the DTC

The ignition switch is in the RUN position.

### Condition for Setting the DTC

TCM is unable to read to EEPROM data.

### Action Taken When the DTC Sets

• | The TCM flashes the sport mode lamp and sends a MIL request to the ECM on the second consecutive drive trip that the diagnostic runs and fails. The ECM then illuminates the MIL.

• | The ECM records the operating conditions in the Freeze Frame at the time of the MIL request from the TCM.

• | The transmission operates in the default mode (maximum line pressure, command 4th gear, inhibit TCC, freeze shift adapts).

### Conditions For Clearing the MIL/DTC

• | The ECM turns off the MIL after three consecutive drive trips during which the TCM sends no MIL request.

• | A History DTC clears after forty consecutive warm-up cycles, if no failures are present by this diagnostic or any other emission related diagnostic.

• | The scan tool clears the MIL/DTC.

• | The TCM cancels the DTC default mode actions when the ignition switch is Off long enough in order to power down the TCM.

### Diagnostic Aids

If TCM reprogramming has recently been completed, this could be the reason for P1621 to set.

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. Performing this step verifies that an operational abnormality did not take place during TCM power-up, such as Electromagnetic Interference (EMI).

#### DTC P1621 TCM Long Term Memory Performance

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check ? | -- | Go to Step 2 | Go to A Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Install the Scan Tool .
2. With the engine OFF, turn the ignition switch to the RUN position.

Important: Before clearing the DTC, use the Scan Tool in order to record the Failure Records. Using the Clear Info function erases the Failure Records from the ECM/TCM.

3. Record the DTC Failure Records.
4. Clear the DTC.
5. Turn the ignition switch to the OFF position for at least 30 seconds.
6. With the engine OFF, turn the ignition switch to the RUN position.

Did DTC P1621 reset?

--

Go to Step 3

Go to Diagnostic Aids

3 | Replace the TCM. Refer to Transmission Control Module Replacement . Did you complete the replacement? | -- | Go to Step 4 | --
4 | Perform the following procedures in order to verify the repair:

1. Select DTC.
2. Select Clear Info.
3. Turn the ignition switch to the RUN position for more than 2 seconds.

Did DTC P1621 reset?

Go to Step 1

System OK