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P1391 – What Does It Mean and How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


P1391 is an OBD-II Code that refers to Transmission Component Slipping (Acura, Buick, Cadillac, Chevrolet, General Motors, Isuzu)


A P1391 code is a manufacturer specific diagnostic trouble code related to Acura, Buick, Cadillac, Chevrolet, General Motors and Isuzu vehicles that occurs when the PCM/ECM detects the vehicle exceeding acceptable RPM parameters before a shift occurs. 

Maximum line pressure will be commanded and TCC apply/torque converter clutch apply will be disabled and this transmission DTC P1870 will be set.


Common causes for this code include:

  • Transmission fluid that is dirty, beat-up, or contaminated
  • Defective or jammed shuttles in the valve body
  • Defective pressure sensors that can influence TCC modulation timing and/or pressure
  • Defective/worn pressure pump
  • Transmission fluid level is low
  • Overexertion of towing capacity
  • Incorrect grade/formulation of transmission fluid


The symptoms of a P1391 code are:

  • Illuminated Check Engine Light
  • Shifting delays
  • Lack of fourth gear/overdrive
  • Hard shifts
  • Increased engine RPMs
  • Bad performance


To diagnose a P1391 DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data for failure.
  2. Visually inspect the transmission and the related components for damage.
  3. Check all the wiring and connectors for damage.
  4. Check the fluid level and quality of the transmission fluid.
  5. Measure the engine RPMs when shifting.
  6. Perform a pressure test on the transmission.
  7. Check for any towing-related problems.
  8. Change the transmission fluid, if needed.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a P1391 code:

  • Not thoroughly inspecting all the wiring and connectors for damage.
  • Not checking the fluid level or quality of the transmission fluid.
  • Not measuring the engine RPMs when shifting.
  • Failing to change the transmission fluid, if needed.

How serious is this?

A P1391 code is a serious code and should be addressed as soon as possible. If the problem is not corrected, it could lead to further damage to the transmission.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Repair or replace any wiring that is damaged
  • Replace control valve body with the revised TCC regulator and isolator valves
  • Replace the defective pressure sensor
  • Replace the defective shuttle valve
  • Replace the defective pressure pump
  • Flush and replace the transmission fluid
  • Replace TCC clutch
  • Check and adjust the towing capacity, if needed

Related codes

A P1391 is related to and may be accompanied by the following codes:

P0700 – Transmission Control System Malfunction

P1740 – Torque Converter Clutch Malfunction


In conclusion, the P1391 code is a diagnostic trouble code that refers to a problem with the transmission slipping. There are many potential causes for this code, and the most common solutions are to repair or replace any damaged wiring, replace the control valve body, replace the defective pressure sensor, flush and replace the transmission fluid, and/or replace the TCC clutch. If you are experiencing any of the symptoms associated with this code, it is best to have a technician diagnose and correct the problem as soon as possible.