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. If the EEPROM does not acknowledge the data request from the TCM, then DTC P1621 will set. DTC P1621 is a type B DTC.

### Condition for Running the DTC

The ignition is ON.

### Condition for Setting the DTC

The EEPROM does not acknowledge the data request from the TCM.

### 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 external anomaly, such as Electromagnetic Interference (EMI), did not take place during TCM power-up.

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

1. Install the Scan Tool .
2. Turn ON the ignition with the engine OFF.

Important: Record the Failure Records before clearing the DTCs. Using the Clear Info function erases the Failure Records from the ECM and the TCM.

3. Record the DTC Failure Records.
4. Clear the DTC.
5. Turn OFF the ignition for at least 30 seconds.
6. Turn ON the ignition with the engine OFF.

Did DTC P1621 reset?

--

Go to Step 3

Go to Diagnostic Aids

3 | Caution: When you are performing service on or near the SIR components or the SIR wiring, you must disable the SIR system. Refer to Disabling the SIR System. Failure to follow the correct procedure could cause air bag deployment, personal injury, or unnecessary SIR system repairs.

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 ON the ignition with the engine OFF for more than 2 seconds.

Did DTC P1621 reset?

Go to Step 1

System OK