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P0781 - Gear selection, 1-2 shift malfunction

Error codes 2021

Content:

Error codeFault locationProbable cause
P0781 Gear selection, 1-2 shift malfunction Wiring, TR sensor, shift solenoids, gear mechanics errors

What does code P0781 mean?

The OBD II trouble code is defined as "1-2 Shift Malfunction" and is set when the PCM (Powertrain Control Module) detects a malfunction or abnormality in the hydraulic circuit (or sometimes electrical) that controls shifting between first and second gear fully automatically controls transfers.

Shifting operations in fully automatic transmissions are carried out by a microprocessor that controls shift solenoids that manage the effective transfer of pressurized transmission fluid between different hydraulic circuits. By switching the pressure fluid from one circuit to another, different gear ratios are established by reconfiguring the planetary gears in the transmission. This reconfiguration has the same effect as changing gear on a manual transmission to achieve different gear ratios.

Automatic transmissions use an internal pump driven by the engine through the torque converter to maintain the pressure needed by the transmission clutch packs to transfer power through a planetary gear set. Therefore, if a solenoid fails or prevents pressurized fluid from moving freely from one circuit to another, there will not be enough pressure in the affected circuit to shift into gear or maintain the selected gear ratio.

Although design specifications vary widely from manufacturer to manufacturer, all applications use input data such as engine load and speed, gearbox turbine speed, throttle position, and vehicle speed to calculate the optimal gear ratio for their operating conditions. By always ensuring that the selected gear ratio corresponds to the current operating conditions, the engine performance is maximized and the lowest fuel consumption is achieved.

The code setting parameters also differ between manufacturers, but generally speaking, code P0781 is set when the PCM detects a mismatch between the desired gear ratio and the gear ratio the transmission is currently in. In this case, a fault code will be stored, but the CHECK ENGINE warning light may or may not come on as some applications require multiple failure cycles before warning lights come on.

The picture below shows the typical position of shift solenoids in an automatic transmission. Notice the healthy, bright red color of the transfer fluid.

What are the common causes of code P0781?

The most common causes of code P0781 are low gear oil levels and old, deteriorated gear oil. Other possible causes are

NOTE: The causes for codes P0782, P0783, and P0784 are largely the same unless they are the same as those for code P0781. In addition, the symptoms are similar, except that different gears are affected. The diagnostic and repair procedures that apply to P0871 can also be applied to codes P0782, P0783, and P0784.

  • Wrong or unsuitable gear oil

  • Clogged or clogged internal fluid passages.

  • Open, short-circuited, or damaged cables and corroded connectors.

  • Shift solenoids defective.

  • Mechanical malfunctions in the transmission.

  • TCM / PCM errors, however this is very rare.

  • What are the symptoms of code P0781?

    In some cases, you may experience no symptoms other than a stored DTC and a lit CHECK ENGINE indicator. Other possible symptoms can include:

  • The presence of other shift-related error codes, such as B. P0780 - "Shift disruption"

  • Gear slip.

  • No gear engaged.

  • Hard, unpredictable or unpredictable gear changes.

  • Overheating of the transmission.

  • Increased fuel consumption.

  • The transmission can go into a "limp home" mode.

  • How can you fix error code P0781?

    NOTE: It is important to note that faults in the electrical control circuit can result in code P0781 also being set. Do not automatically assume that the 1-2 solenoid is defective. Do a thorough check of all related wiring and connections to determine if there is no electrical problem causing code P0781. It is also recommended that you have a good repair manual for the vehicle on hand to determine the location, color coding, and routing of any associated cables.

    Step 1

    Extract any existing error codes and record all freeze frame data for future reference in case an intermittent error is found later.

    step 2

    Check all associated cables for continuity (disconnect all control modules before starting the continuity test), ground and resistance and compare the values ​​obtained with the values ​​given in the manual. Repair the wiring as necessary.

    step 3

    If no electrical problems are found, check that the gear oil is at the correct level, is not dirty and / or contaminated, and has no burnt odor.

    NOTE 1: This check may not work on all vehicles as many newer vehicles do not have transmission dipsticks. If necessary, it may be necessary to take a sample of gear oil from the liquid cooler lines. Refer to the manual to locate the cooler lines, but be careful not to get debris into the liquid cooling lines.

    NOTE 2: Gear oil that appears dark or has an unusual smell must always be considered insufficient and replaced. Be aware that recent engine overheating could affect the transmission fluid. So make sure that the engine has not overheated in any way in the recent past.

    Step 4

    If the gear oil is suspicious in any way, it must be replaced. However, this can be a problem in vehicles with a lifetime full gearbox as it is sometimes impossible to know when the fluid in these gearboxes is at the correct level. Too much fluid is just as bad as too little. Therefore, follow the fluid change instructions in the service manual or take the vehicle to a dealer for further diagnosis and repair.

    Step 5

    For vehicles where fluid replacement is not a major problem, remove the transmission oil pan and inspect for large pieces of metal, clutch friction material, or other signs of mechanical failure of the transmission.

    When such deposits are found, it is often cheaper to replace the entire transmission with an overhauled unit than to carry out repairs. However, if no foreign objects are found, do a thorough check of all internal cables and connections. Refer to the manual to identify the affected solenoid. Note, however, that the magnet itself is part of the electrical control circuit and must therefore also be checked for continuity and resistance.

    NOTE: DO NOT attempt to repair any burned, shorted, or otherwise damaged wires and / or solenoid valves found during the inspection. Most, if not all, applications require the entire internal wiring harness to be replaced for a reliable repair.

    Step 6

    If the internal wiring appears to be normal or after the internal wiring harness has been replaced, connect a code reader and try manually activating the affected solenoid. The scanner will inform you whether the magnet is working or not. If so, enable it repeatedly to check for intermittent errors. If the magnet does not work and all cables and connections are in good working order, replace the magnet. After replacing the solenoid, test the system again to make sure it is working properly.

    Step 7

    Replace the transmission oil pan (insert a new filter and gasket), fill the transmission with the correct fluid and type, and test the vehicle with an attached scanner to monitor the system during normal operation.

    There should be no adverse readings and the code should not be returned. If the fault or one or more symptoms reappear, the operating pressure in the transmission is very likely too low. Refer to the manual for the correct procedure for checking the pressure provided by the pump.

    NOTE: It is important to follow the instructions in the manual carefully for an accurate result. For example, some applications require the gear oil to be at a certain temperature during a pressure test. Please read the manual for this.

    If the operating pressure of the transmission does not fall within the range specified in the manual, make the necessary repairs or contact a dealer for further diagnosis and repair.

    Step 8

    If the code and / or symptoms persist despite all attempts at repair, you suspect a defective TCM. Keep in mind, however, that ECU failures are rare and that the fault must be found elsewhere before ECUs are replaced. To do this, the exchange must be programmed.

    In some cases, there may be an intermittent failure that you may need to have aggravated before an accurate diagnosis and definitive repair can be performed.

    Codes related to P0781

  • P0782 - 2-3 shift malfunction

  • P0783 - 3-4 shift malfunction

  • P0784 - 4-5 switching malfunction

  • NOTE: In some cases the code P0780 “Shift Malfunction” may be present with the related codes above.