P1590 – TC-SST-ECU to ECM Communication Error in Torque Reduction Request (Mitsubishi)

By Reinier (Contact Me)
Last Updated 2022-02-07
Automobile Repair Shop Owner

Manufacturer Specific Definitions for P1590

Table of Contents

1. What Does Code P1590 Mean?
2. Where is the P1590 sensor located?
3. What are the common causes of code P1590?
4. What are the symptoms of code P1590?
5. Get Help with P1590

What Does Code P1590 Mean?

OBD II fault code P1590 is a manufacturer-specific trouble code that is defined by carmaker Mitsubishi as, “TC-SST-ECU to ECM Communication Error in Torque Reduction Request”, and is set when the PCM detects a communication failure between the TC-SST transmission control module, and the PCM (Powertrain Control Module, aka Engine Control Module, aka Electronic Control Module).

NOTE #1: In Mitsubishi-speak, the abbreviation “TC-SST” stands for Twin Clutch Sport- or Sportronic Shift Transmission. This is the brand name of a dual-clutch, six-speed transmission that was developed by a company named Getrag, specifically for use in Mitsubishi vehicles.

NOTE #2: DTC P1590 also applies to Mitsubishi vehicles that are fitted with CVT (Continuously Variable Transmissions). Note, though, that in the case of CVT-equipped vehicles, the definition of code P1590 changes to “CVT-ECU to ECM Communication Error in Torque Reduction Request”. In both definitions, the fault involves a communication failure between the transmission and the PCM, but in the case of CVT transmissions, the monitoring and fault-reporting systems use different logic and operational parameters to those that apply to the SST transmission

Before we get to the specifics of code P1590 as it applies to Mitsubishi vehicles, we need to explain the basic operating principles of dual-clutch transmissions. In practice, all such transmissions are essentially automated manual transmissions in the sense that actual gear shifts are executed by extremely fast hydraulic or electronically controlled actuators. The practical advantages of this are that gear shifts are not only smoother than on fully automatic transmissions, but gearshifts also happen faster- typically within a few milliseconds.

All dual-clutch transmissions also use two mechanical clutches; one clutch controls shifts from evenly numbered gear ratios to unevenly numbered ratios, while another controls shifts from uneven ratios to evenly numbered ratios. Put differently, one clutch controls shifts to first, third, and fifth gears, while the other controls shifts to second, fourth, and sixth gears. Depending on the transmission design, reverse gear is usually assigned to the clutch that controls uneven gears.

However, to make this system work, a dual-clutch transmission selects two gears at the same time. For instance, when first gear is selected, second gear is also engaged at the same time, but the clutch that controls second gear remains disengaged until a control module commands a shift to second gear. When this happens, the clutch that controls first gear disengages, while the clutch that controls second gear engages, but with an almost undetectable delay to prevent both first and second gears from being engaged at the same time.

This pattern repeats for the other gear ratios, but to prevent harsh and uncomfortable gear shifts during aggressive driving, the PCM “smoothes out” gear shifts by reducing the engine’s power output momentarily just before, and during the actual gearshift. As a practical matter, the PCM monitors input data from sensors that include the throttle pedal position sensor, the engine speed sensor, the engine temperature sensor, as well as operational data from the MAF (Mass Airflow) sensor and other engine sensors.

Based on this data, the PCM calculates the engine load and other operating parameters, all of which are used to reduce the engine’s torque output during a gearshift to make gearshifts more comfortable, and to prevent damage to transmission and engine components during gearshifts under high-load conditions.

Depending on operating conditions, the transmission control module will transmit a request to the PCM to limit, or reduce the engine’s torque output by a certain amount just before a gearshift happens to prevent damage to mechanical components. This request is transmitted to the PCM via a high-speed CAN (Controller Area Network) serial bus communication system, and if this system is fully functional, the PCM will reduce the engine’s torque output as requested.

However, if a defect or malfunction in this CAN system prevents the PCM from receiving the torque reduction request, the PCM will recognize that it cannot control the engine’s torque output during gearshifts, and it will set code P1590 as a result.

Note, though, that in most instances of this code, the PCM will usually also initiate a limp mode by severely limiting the engine’s power output both as a safety precaution and as a means of protecting the engine and transmission against damage.  Note also, that in all cases where a limp mode is enforced by the PCM, the limp mode will persist until the fault is corrected, which may include replacing and reprogramming one or more control modules.

Where is the P1590 sensor located?

This image shows some detail of the TC-SST transmission in many Mitsubishi vehicles. It should be noted, though, that while the presence of code P1590 on Mitsubishi vehicles effectively disables the transmission, the causes of code P1590 rarely involve failures in, or malfunctions of, the transmission itself. In most instances of this code on Mitsubishi vehicles, the principal cause(s) of this code involves issues with wiring and/or control modules outside of the transmission, but as mentioned elsewhere, this code can only be diagnosed on Mitsubishi vehicles with manufacturer-specific tools, equipment, and cabling.

What are the common causes of code P1590?

Common causes of code P1590 on Mitsubishi vehicles could include one or more of the following-

• Failed or defective PCM
• Failed or defective transmission control module
• Corrupted software in either (or both) the PCM and TCM (Transmission Control Module)
• Damaged, burnt, shorted, disconnected, or corroded wiring and/or electrical connectors in the relevant CAN bus system
• In some cases, one or more defective or malfunctioning clutch actuators could potentially contribute to the setting of this code on Mitsubishi vehicles
• In rare cases, a defective or malfunctioning MAF (Mass Airflow) sensor can cause code P1590 to set or contribute to its setting on Mitsubishi vehicles

WARNING:  In some cases, it might be possible to resolve this code by reprogramming the affected control modules with software that includes the latest updates, fixes, and patches. However, it should be noted that depending on the nature of the problem, the simple reprogramming of implicated control modules without first properly diagnosing the issue might have unpredictable results. These results could include a limp-mode, severely restricted engine power, harsh or uncomfortable gearshifts, or a failure by the transmission to select some or all gears.

NOTE: Be aware that diagnosing code P1590 on Mitsubishi vehicles requires the use of manufacturer-specific diagnostic equipment and cabling. We therefore strongly recommend that you seek professional assistance with diagnosing and repairing this code on any Mitsubishi vehicle.

What are the symptoms of code P1590?

Common symptoms of code P1590 on Mitsubishi vehicles include the following-

• Stored trouble code and an illuminated warning light that may or may not be flashing, depending on the exact nature of the problem
• One or more additional UXXXX (Communication) codes may be present along with P1590. Note that additional codes may include both generic and Mitsubishi-specific codes
• The vehicle will almost certainly be locked into a limp mode that will persist until the fault is found and corrected

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