P1791 – OBD-II DTC

OBD-II DTC P1791 – 3-2 Downshift Error

DTC P1791 meaning

DTC P1791 refers to a “3-2 Downshift Error” in the vehicle’s transmission system. This code indicates that the transmission control module (TCM) has detected an issue when attempting to downshift from third gear to second gear. The error suggests that the expected downshift did not occur as intended, which can affect the vehicle’s performance and drivability.

DTC P1791 symptoms

Common symptoms associated with DTC P1791 may include:

• Unusual or harsh shifting behavior
• Delayed or failed downshifts
• Check Engine Light (CEL) illuminated
• Transmission slipping or erratic performance
• Increased engine RPMs without corresponding acceleration

DTC P1791 causes

Several factors can contribute to the occurrence of DTC P1791, including:

• Faulty transmission control module (TCM)
• Worn or damaged transmission components
• Low transmission fluid levels or contamination
• Electrical issues, such as short circuits or poor connections
• Faulty sensors, such as the throttle position sensor (TPS) or vehicle speed sensor (VSS)

DTC P1791 seriousness

The seriousness of DTC P1791 can vary depending on the underlying cause. While it may not pose an immediate safety risk, it can lead to:

• Reduced vehicle performance
• Increased wear on transmission components
• Potential for further transmission damage if not addressed
• Inability to drive the vehicle in certain conditions

How to diagnose DTC P1791

To diagnose DTC P1791, follow these steps:

1. Use an OBD-II scanner to confirm the presence of the P1791 code.
2. Check for any additional DTCs that may provide more context.
3. Inspect the transmission fluid level and condition.
4. Examine electrical connections and wiring related to the TCM and transmission sensors.
5. Test the functionality of the TCM and relevant sensors.
6. Perform a road test to observe shifting behavior and confirm symptoms.

How to fix DTC P1791 step-by-step instructions

To fix DTC P1791, consider the following steps:

1. Address any low or contaminated transmission fluid by performing a fluid change.
2. Repair or replace any damaged wiring or connectors related to the TCM.
3. Test and replace faulty sensors, such as the TPS or VSS, if necessary.
4. If the TCM is found to be faulty, consider reprogramming or replacing it.
5. Clear the DTC using an OBD-II scanner and perform a road test to ensure the issue is resolved.

How to erase DTC P1791

To erase DTC P1791, follow these steps:

1. Connect an OBD-II scanner to the vehicle’s diagnostic port.
2. Turn on the ignition without starting the engine.
3. Navigate to the “Erase Codes” or “Clear DTCs” option on the scanner.
4. Confirm the action to clear the codes.
5. Disconnect the scanner and start the vehicle to verify that the code does not reappear.

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