|Code||Fault Location||Probable Cause|
|P0868|| Transmission fluid pressure (TFP) sensor -low |
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|Wiring, short to earth, TFP sensor, ECM/PCM/TCM|
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What Does Code P0868 Mean?
OBD II fault code P0868 is a generic code that is defined as “Transmission fluid pressure (TFP) sensor –low”, and is set when the PCM (Powertrain Control Module) detects that the transmission fluid pressure is lower than the minimum allowable value.
All automatic transmissions use pressurized transmission fluid to engage/disengage clutch packs and other components such as the lock-up torque converter clutch in order to accomplish gearshifts. However, this pressure is not constant, and in some cases, pressure is temporarily relieved when certain transmission are moved. In a fully functional system, an internal pressure pump (that is most often driven by the engine) provides the pressure. The pressurized fluid is then directed by the PCM into various hydraulic passages via electronically controlled solenoids and other valves to cause the transmission to shift.
Thus, when the pressure falls below a limit set by the manufacturer, the PCM or TCM (Transmission Control Module) is no longer able to initiate gearshifts, since a set minimum pressure is required to initiate a gearshift on the one hand, and to maintain the gear selection on the other hand. To maintain a selected gear, the transmission fluid pressure must be sufficient to keep the currently selected clutch pack from slipping.
To monitor the transmission fluid pressure, all transmissions are fitted with pressure-sensitive sensors that transmit the actual fluid pressure to the PCM on a continuous basis. In most cases, this sensor is supplied with a 5-volt reference voltage by the PCM. As the fluid pressure changes, the resistance in the sensor also changes, with more or less current (known as the signal voltage) passed back to the PCM. The PCM interprets the received signal voltage as pressure, and will set code P0868 and illuminate a warning light when it senses that the transmission fluid pressure has fallen below a specified value. Note that in some cases, the PCM may also initiate a fail-safe or limp mode in order to protect the transmission.
Where is the P0868 sensor located?
While the transmission fluid pressure sensor is always located on the transmission casing, most transmissions are fitted with a variety of other sensors as well, which can sometimes make it difficult to identify the fluid pressure sensor correctly.
Therefore, non-professional mechanics are strongly urged to refer to the manual to identify the fluid pressure sensor correctly to avoid a misdiagnosis. The image below shows a typical transmission fluid pressure sensor, but note that the appearance and location of this sensor varies greatly between applications.
What are the common causes of code P0868 ?
Some common causes of code P0868 could include the following-
- Defective transmission fluid pressure sensor
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
- Low fluid levels due to fluid leaks on the transmission
- Dirty, contaminated, or degraded transmission fluid
- In some cases, a defective transmission fluid temperature sensor might contribute to the setting of code P0868, although this is rare
- Mechanical failure of transmission components, and in particular, the pressure pump
- Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any control module is replaced
How expensive is it to fix code P0868 ?
In most cases, a simple replacement of the transmission fluid pressure sensor should cost well under $100 if it is done on a DIY basis. Note that most repair shops will charge a minimum of one hours’ labor to replace this sensor, which could add another $80 – $120 to the bill, depending on location.
Rebuilding or replacing a transmission can run from around $1000 to several thousand dollars, depending on the application. Note that in these cases, labor charges vary greatly, so shop around for the best rate per hour. Be aware though that replacing a transmission can take from about four hours, to more than ten hours, depending on the application and how much reprogramming of control modules needs to be performed.
What are the symptoms of code P0868 ?
Some common symptoms of code P0868 could include the following-
- Stored trouble code and an illuminated warning light
- Additional transmission related codes may be present as well
- Varying degrees of transmission slippage may occur
- Transmission may not engage some, or any gears
- Transmission may overheat
- Transmission may be locked into a fail-safe or limp mode until the code is resolved
- In some cases, serious and even fatal damage to the transmission may occur
What are common solutions to code P0868 ?
Common solutions to code P0868 include the following-
- Inspection and replacement of a defective transmission fluid sensor
- Inspection and repair/replacement of damaged wiring and/or connectors
- Topping off the transmission fluid level, but note that this must be accompanied by the repair of fluid leaks
- Replacement of unserviceable transmission fluid
- In rare cases, replacement or rebuilding of the transmission might be required
How serious is code P0868 ?
Code P0868 should be considered as serious by default, since fatal transmission damage could occur if the code is not resolved in a timely manner.
How safe is it to still drive the car with code P0868 ?
Ideally, vehicles on which code P0868 is present should not be driven, since there is a possibility that the transmission could fail, or enter a limp mode unexpectedly. This could cause crashes if the vehicle should suddenly lose power while driving in traffic.
How difficult is it to repair code P0868 ?
How difficult this code is to repair depends on the nature of the problem. For instance, merely testing and replacing the fluid pressure sensor should not present the average non-professional mechanic with undue difficulties.
However, diagnosing other failures such as mechanical issues within the transmission itself requires removal and disassembly of the transmission, which are procedures that are best left to the dealer or another competent repair facility.
NOTE: A dedicated transmission fluid pressure gauge could be helpful to diagnose this code.
What are the common mistakes when repairing code P0868 ?
Common mistakes when dealing with code P0868 could include the following-
- Failing to repair fluid leaks before attempting to diagnose the code
- Failing to test the actual transmission fluid level and pressure before replacing the fluid pressure sensor
- Failing to check that the PCM actually delivers a 5-volt reference voltage, which should be checked at the PCM connector
- Failing to recognize that dirty, contaminated, or degraded transmission fluid can cause internal blockages in the transmission, which blockages can sometimes cause false or misleading pressure readings even if the pressure sensor and its control circuit(s) are in perfect working order.
NOTE: Be aware that if fluid leaks are present, it might not be possible to check the fluid level on transmissions that have “lifetime” fills, since these transmissions are not fitted with dipsticks or other measuring devices. In these cases, the better option is to refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair.
How do you troubleshoot code P0868 ?
Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
NOTE: If other codes are present along with P0868 and these codes precede P0868, resolve these codes in the order in which they were stored before attempting a diagnosis of this code. Failure to do this will result in a misdiagnosis.
Assuming that there are no additional codes present and that the transmission is user-serviceable, check the fluid level and condition as a first step. If the fluid level is low, fill the transmission to the proper mark as per the instructions in manual. Clear all codes after topping off the fluid level and operate the vehicle for one complete drive cycle before rescanning the system to see if the code returns.
NOTE #1: Take note that unlike engine oil, the transmission fluid is not consumed. Thus, if the fluid level is low, there must be a fluid leak present, which must be repaired before the vehicle is placed back into service. Failing to do this will not only cause a recurrence of the code, but could cause transmission failure later on.
NOTE #2: If the transmission oil has a dark color, or has a thick, tarry consistency and/or a “burnt” odor, the fluid is no longer serviceable, and must be replaced. Note that failing to do this could result in transmission failure.
Assuming that the fluid is serviceable and up to the proper mark, refer to the manual to identify and locate the fluid pressure sensor. Also determine the function, color-coding, and routing of all associated wiring to avoid a misdiagnosis later on.
Perform a thorough visual inspection of the sensor, its connector, and wiring. Look for damaged, burnt shorted, disconnected, or corroded wiring and/or connectors, and make repairs or replace wiring as required. Clear all codes after repairs are complete and operate the vehicle for one complete drive cycle before rescanning the system to see if the code returns.
If the code persists but the wiring is free of visible damage, perform resistance, continuity, reference voltage, and ground integrity checks on all associated wiring. Be sure to disconnect all wiring from the PCM and other relevant control modules during this step to prevent damaging one or more control modules.
Compare all obtained reading with the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.
NOTE #1: Pay particular attention to the internal resistance of the fluid pressure sensor, since this value is a reasonably good indicator of the sensors’ overall condition. Replace the sensor with an OEM replacement if its resistance deviates from stated values.
NOTE #2: If there is no voltage in the reference voltage circuit at the sensor connector, refer to the manual to identify the CORRECT pin on the PCM connector in order to check that the PCM is actually delivering a reference voltage. Test this pin strictly in accordance with the instruction in the manual; if no voltage is present on this pin, replace the PCM. If the reference voltage is correct at the PCM connector, find and repair the open circuit in the reference voltage wire.
Since sensor failures and wiring issues are the most common causes of code P0868, the steps outlined above will resolve the problem in nine out of every instances of this code.
However, if the code persists but all electrical values check out relative to the manufacturers’ specifications, it is possible that a mechanical failure had occurred in one or more transmission components. While testing the actual transmission fluid pressure as per the instructions in the manual might eliminate or confirm this, actually diagnosing and repairing the issue requires skills and equipment that most non-professional mechanics do not possess.
Thus, if a mechanical failure of the transmission is suspected, the wiser option is to refer the vehicle to the dealer or other competent repair facility for professional assistance.