P0870 Transmission Fluid Pressure Sensor/Switch C Circuit

P0870 code definition

A P0870 code indicates that the powertrain control module (PCM) has determined that the pressure of the transmission fluid is not within the desired range.

What the P0870 code means

The P0870 is an OBD-II generic code and it will occur if transmission fluid increases or decreases to a level that is not within the programmed limits.

What causes the P0870 code?

There are several reasons that the P0870 can occur:

• Low transmission fluid
• Faulty transmission high pressure pump
• Defective transmission (also called a slipping transmission)
• Defective temperature sensor
• Engine overheating
• Failing or defective PCM

What are the symptoms of the P0870 code?

The P0870 code triggers the code and also the service engine light. Most mechanics or technicians also say that a major sign that this code is going to appear is the smell of the transmission fluid as it overheats, and the smoke that may appear. Drivers will experience transmissions that slip out of gear or which are impossible to put into gear and use.

How does a mechanic diagnose the P0870 code?

Though the P0870 is diagnosed with an OBD-II scanner, one of the very first steps is to check the transmission before doing any other diagnostics as this may be the source of the code. The technician should also look at freeze frame determine when and where (in the order of events) the code was set. Once they check the transmission, they can reset the fault codes and test the vehicle to see if the code reappears.

The technician will use the scanner to diagnose any electrical issues with the system and examine the wiring, repairing and replacing components when needed. Unplugging the electrical connector and testing the sensor for voltage is also part of the process and if it is not in compliance, it may need to be replaced.

Common mistakes when diagnosing the P0870 code

The P0870 code can cause many mistakes because it may lead to a technician feeling that the only problem is a transmission leak. This is why the first step is always to do a thorough diagnostic on the transmission to be sure that high temperatures have not damaged it internally. Only when the transmission has been assessed fully and repaired should the diagnostics on the sensor be performed.

How serious is the P0870 code?

The P0870 code is very serious because it can easily lead to the destruction of the transmission. Mistaking it for low fluid levels, rather than a full-blown leak is a common blunder. This code should not be taken lightly because it may mean a much costlier repair down the road.

What repairs can fix the P0870 code?

To overcome and repair the P0870 code, a mechanic must:

• Perform a full transmission inspection and diagnostic to be sure that any leaks are repaired and that the right amount of fluid is in the tank. If the fluid smells very burnt before the inspection and the code is set, it could be that a new transmission is needed.
• Retesting after transmission repairs is necessary and if the code returns, a diagnosis of the electrical circuitry for the sensor is needed. Visually examining the wiring and replacing or repairing anything that is corroded, melted, shorted, or disconnected is necessary.
• If the code persists, the transmission temperature sensor must be unplugged and tested for its reference voltage. If it is within the manufacturer's guidelines, test the ground and resistance. If it fails, replace it.
• Checking the PCM is important as this could also be the issue, though it is rare.

Additional comments for consideration regarding the P0870 code

Don't rush to replace the sensor when it could be a transmission leak. Also, be aware that control modules can fail, such as the PCM, but it is not a common reason for this code to appear.

Need help with a P0870 code?

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