P0870 – Transmission fluid pressure (TFP) sensor / switch C -circuit malfunction

By Reinier (Contact Me)
Last Updated 2017-10-10
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P0870 Mean?
2. Where is the P0870 sensor located?
3. What are the common causes of code P0870 ?
4. How expensive is it to fix code P0870 ?
5. What are the symptoms of code P0870 ?
6. What are common solutions to code P0870 ?
7. How serious is code P0870 ?
8. How safe is it to still drive the car with code P0870 ?
9. How difficult is it to repair code P0870 ?
10. What are the common mistakes when repairing code P0870 ?
11. How do you troubleshoot code P0870 ?
12. Codes Related to P0870
13. Get Help with P0870

What Does Code P0870 Mean?

OBD II fault code P0870 is a generic code that is defined as “Transmission fluid pressure (TFP) sensor / switch C -circuit malfunction”, and is set when the PCM (Powertrain Control Module) or TCM (Transmission Control Module) or both, detect a failure in the Transmission Fluid Pressure Sensor / Switch C Circuit. Note that while this code refers primarily to malfunctions/failures in the Transmission Fluid Pressure Sensor’s control or signal circuit, this code can also set as the result of mechanical failures or issues in the transmission itself, or in the valve body assembly that controls gearshifts.

All automatic transmissions depend on pressurized transmission fluid both to operate efficiently, and to make gearshifts smoother. In terms of operation, the transmission uses an engine-driven pump to pressurize the fluid. The pressurized fluid is then directed into and out of complex hydraulic circuits via shift solenoids that are controlled by the TCM, and in a fully functional system, the PCM and TCM (and various other control modules) work in conjunction to control the shift solenoids to ensure smooth, predictable gearshifts.

To monitor the transmission fluid line pressure continuously, the transmission is fitted with a pressure sensitive sensor that converts the actual fluid pressure into an electrical signal. This signal is transmitted to the PCM/TCM and other affected control modules via the CAN (Controller Area Network) bus system to enable the PCM/TCM to control the transmissions’ shift pattern effectively based on operating conditions and driving style.

Thus, the input data from the Transmission Fluid Pressure Sensor is a critical factor in effective transmission control. Therefore, to protect the transmission and other drive train components, the PCM/TCM will set code P0870 when a fault, failure, or malfunction occurs in the Transmission Fluid Pressure Sensor’s control or signal circuit(s). In addition, the PCM/TCM may also illuminate a warning light, and initiate a fail-safe or limp mode to protect the transmission.

Where is the P0870 sensor located?

On most applications, the Transmission Fluid Pressure Sensor is screwed in to the transmission casing, and is readily accessible. However, on some applications, the Transmission Fluid Pressure Sensor is screwed into the valve body, which is located inside the transmission.

The image below shows a typical Transmission Pressure Sensor that is located outside of the transmission. Note that the appearance of these sensors differ between applications, and that Transmission Pressure Sensor are NOT interchangeable between applications, or even between models in a given model range.

What are the common causes of code P0870 ?

Some common causes of code P0870 could include the following-

• Defective Transmission Fluid Pressure Sensor
• Damaged, shorted, burnt, disconnected, or corroded wiring and/or connectors
• Dirty, degraded, or contaminated transmission fluid
• Mechanical failures of the transmission or valve body
• Failed or failing PCM/TCM. Note however that control module failure is rare, and the fault must be sought elsewhere before any control module is replaced

How expensive is it to fix code P0870 ?

If the Transmission Pressure Sensor is located outside of the transmission, repairing this code should cost well under $100 if the repair is done on a DIY basis on most applications. However, replacing an internally located sensor could cost between $100 and about $250, which includes a new transmission filter, oil pan gasket, and new transmission fluid that can sometimes only be obtained from the dealer.

The cost of removing, disassembling, or replacing the transmission can run to several thousand dollars, with the final cost depending on the application, the nature of the fault, and local labor charges.

What are the symptoms of code P0870 ?

Some common symptoms of code P0870 could include the following-

• Stored trouble code and possibly an illuminated warning light
• Several additional transmission related codes may be present along with P0870
• Gearshifts may be harsh, erratic, or unpredictable
• Some gears may not engage
• Transmission may not shift at all
• Varying degrees of transmission slippage may occur
• Transmission may be locked in a failsafe or limp mode until the fault is corrected

What are common solutions to code P0870 ?

Common solutions to code P0879 could include the following-

• Inspection, testing, and replacement of the Transmission Fluid Pressure Sensor
• Inspection, and replacement/repair of damaged wiring
• Replacement of transmission fluid
• In rare cases, replacement/rebuilding of the transmission or transmission components such as the valve body
• In even more rare cases, replacement and programming of the PCM/TCM

How serious is code P0870 ?

How serious this code is depends on the nature of the failure. However, since the nature of the failure is not always immediately apparent, code P0870 should be considered as serious by default, since catastrophic transmission failure can occur if the fault is not rectified in a timely manner.

How safe is it to still drive the car with code P0870 ?

Ideally, vehicles on which code P0870 is present should not be driven, since the transmission could be forced into a limp mode unexpectedly. This poses an obvious safety risk when the vehicle suddenly loses power or the ability to shift in heavy or fast-flowing traffic.

How difficult is it to repair code P0870 ?

Provided that the Transmission Fluid Pressure Sensor is NOT located inside the transmission, this code should not present the average non-professional mechanic with undue difficulties, since repairs typically involve simple testing and wiring repairs outside the transmission.

WARNING: However, non-professional mechanics should take note that this code is typically NOT repairable if the Transmission Fluid Pressure Sensor is located inside the transmission, and especially if the transmission has a so-called, “lifetime” fill. In these cases, it is virtually impossible to refill the transmission to the correct level without specialized equipment and knowledge, so if the transmission does have a “lifetime” fill, do NOT drain the transmission by removing the oil pan, but refer the vehicle to the dealer or other competent repair facility for professional diagnosis and repair, instead. Attempting DIY repairs on these transmissions will almost certainly result in serious or even fatal damage to the transmission.

What are the common mistakes when repairing code P0870 ?

Common mistakes when diagnosing/repairing this code typically include the following-

• Condemning and replacing control modules when the fault is almost always caused by damaged or corroded wiring that is external to the transmission
• Replacing or rebuilding transmissions when the fault is more likely to involve clogged fluid passages or sticking valve shuttles in the valve body. Note that this is a common occurrence on older vehicles on which the transmission fluid has not been replaced according to the maintenance schedule
• Replacing or rebuilding transmissions on which the actual fluid pressure had not been checked as a first step in the diagnostic procedure. This is an important step, since it can confirm or eliminate mechanical failures inside the transmission as a cause of the code

How do you troubleshoot code P0870 ?

NOTE: A dedicated transmission fluid pressure gauge is required to diagnose this code accurately.

Step 1

Assuming that the transmission does NOT have a “lifetime” fill, 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 the transmission is user-serviceable, check the fluid level, as well as the condition of the fluid. If the fluid is up to the mark, but it has a dark, tarry consistency or has a burnt odor, replace the fluid as a first step in the diagnostic procedure. Failure to do this could result in a misdiagnosis, wasted time, and damage to the transmission later on.

Step 2

If the transmission fluid is serviceable, refer to the manual to determine the correct procedure to follow to test the actual fluid pressure in the transmission. Take note though that this procedure MUST be performed EXACTLY as described in the manual in order to obtain accurate and reliable test results.

Compare all obtained test readings with the values stated in the manual. If any test result deviates from the stated values, refer the vehicle to the dealer r other competent repair facility for professional diagnosis and repair, since removal and disassembly of the transmission may be required to diagnose the cause of the problem.

Step 3

If however, the actual test pressures agree with the values stated in the manual, it would be safe to rule out mechanical failure of the transmission as the cause of the code.

Refer to the manual to locate the Transmission Fluid Pressure Sensor, as well as the color-coding, function, and routing of all the associated wiring. If the sensor is located outside of the transmission, inspect it for visible signs of damage or corrosion, and replace it with an OEM sensor to ensure proper operation if such damage is found.

Step 4

If the sensor is free of visible damage or corrosion, perform a thorough visual inspection of all associated wiring. Look for damaged, shorted, disconnected, or corroded wiring and/or connectors, ad make repairs or replace wiring as required. Clear all codes after repairs are complete, and operate the vehicle for one complete drive cycle before scanning the system again to see if the code returns.

Step 5

If the code persists, perform resistance, continuity, ground integrity, and reference voltage (where applicable) tests on all associated wiring as per the instructions in the manual. Be sure however, to disconnect all wiring from all relevant control modules during this step to prevent causing damage to one or more control modules.

Compare all obtained readings 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: Pay particular attention to the resistance and continuity values of the signal circuit, since an abnormal resistance could result in the PCM receiving an incorrect input signal.

NOTE 1: Be sure to include testing the internal resistance of the sensor in this step, since its resistance is a reasonably good indicator of its overall condition. Replace the sensor if its resistance or continuity values do not agree with the values stated in the manual.

Clear all codes after repairs are complete, and operate the vehicle for one complete drive cycle before scanning the system again to see if the code returns.

Step 6

If the Transmission Fluid Pressure Sensor is located inside the transmission, proceed as follows-

Make sure the transmission has cooled down to prevent scalds by hot transmission fluid. Remove the oil pan, but be sure to place a suitable container under the transmission to prevent fluid spills.

Locate the Transmission Fluid Pressure Sensor in the valve body, and perform all tests as described in Steps 4 & 5.

NOTE #1: Pay particular attention to the resistance and continuity values of the signal circuit, since an abnormal resistance could result in the PCM receiving an incorrect input signal.

NOTE #2: : Bear in mind that if any deviations from stated values are found, it is necessary to replace the transmission’s entire internal wiring harness, since the metal wear particles in the transmission fluid can cause short circuits in repaired wiring.

Once it is certain that the fault had been found and repaired, and that ALL tested electrical values fall within specified ranges, be sure to fit a new transmission fluid filter, and to use a new gasket when refitting the oil pan.

Refill the transmission with fluid from sealed containers of the grade and formulation recommended by the manufacturer as per the instructions provided in the manual.

WARNING: DO NOT reuse the old fluid. Doing so will almost certainly introduce dirt and contaminants into the transmission that could result in transmission damage or failure.

Clear all codes after repairs are complete, and operate the vehicle for one complete drive cycle before scanning the system again to see if the code returns.

Step 7

While Steps 1 through 6/7 will resolve code P0879 in nine out of every ten instances, it is likely that intermittent wiring issues, intermittently sticking parts in the valve body, or other intermittent mechanical issues in the transmission can cause the code to return.

Note that while it might be possible to resolve an intermittent electrical fault on a DIY basis by repeating all electrical tests until the fault is found and repaired, diagnosing mechanical issues generally requires removal and disassembly of the transmission. Thus, if a mechanical issue is suspected, the better option is always to refer the vehicle for professional diagnosis and repair.

Codes Related to P0870

• P0871 – “Transmission Fluid Pressure Sensor/Switch “C” Circuit Range/Performance”
• P0872 – “Transmission Fluid Pressure Sensor/Switch “C” Circuit Low”
• P0873 – “Transmission Fluid Pressure Sensor/Switch “C” Circuit High”
• P0874 – “Transmission Fluid Pressure Sensor/Switch “C” Circuit Intermittent”

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