Lexus IS 300 P0846: Meaning, Causes, + Diagnosis

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P0846 Lexus IS 300

P0846 is a relatively common trouble code.  While your Lexus IS 300’s check engine light will come on, P0846 is a transmission-related trouble code, specifically the transmission fluid pressure sensor circuit.

It’s a generic code, so P0846 has the same definition for any vehicle made for the 1996 model year and up.  Before doing anything, check your IS 300’s transmission fluid.

Abbreviations used in this article:

  • PCM– Powertrain Control Module is the main computer module with the engine and transmission control in one unit. 
  • TCM– The Transmission Control Module controls the transmission shifts and is separate from the engine control module. Your Lexus IS 300 will either have a PCM or a TCM.  And, for this article, they are interchangeable.
  • TFPS– Transmission Fluid Pressure Sensor

P0846 Definition:  TFPS “B” Range/Performance

P0846 Meaning Lexus IS 300

Here’s the definition of P0846 for the Lexus IS 300 broken down by its individual parts. 

Transmission Fluid Pressure Sensor/Switch

The TFPS has two primary jobs:

  1. Convert the transmission fluid pressure into a signal that the PCM/TCM can use to control shifts.
  2. Verifies that the PCM/TCM’s request for a transmission shift has occurred.

Circuit “B” Range/Performance

The TFPS converts the hydraulic pressure data into a voltage signal that your IS 300’s TCM/PCM can use to control the transmission.  P0846 occurs when this signal is out of the normal operating range for this signal.

P0846 Symptoms:  Lexus IS 300

Here are the most common symptoms associated with P0846:

  • Limp Mode– Your IS 300 will shift differently and won’t have access to all of its gears.
  • Check Engine Light– There will often be other codes that occur with P0846.  They will usually be related to voltage and power.
  • Erratic Shifting– Shifts may feel soft (RPM slowly drops during the shift), or they may feel too harsh (abrupt drop in RPM).

Lexus IS 300 P0846 Causes + Diagnosis

Diagnosing P0846: Lexus IS 300

Diagnosing P0846 in the Lexus IS 300 can be challenging due to the location of the TFPS sensor and the wiring associated with it.

1. Check for Other Codes and TSB’s

The first thing you want to do is check for other diagnostic codes (such as P0841).  They can provide valuable clues to what’s going on inside your transmission. 

TSB’s are technical service bulletins.  Lexus uses them out to deal with common and well-known issues.  Check to see if there are any related to the TFPS.  You can get them from the NHTSA website, and they can make diagnosing P0846 a lot easier.

2. Inspect the TFPS Wiring

The next thing you’re going to want to do is inspect the wiring going to your IS 300’s TFPS.  If it’s burnt or damaged, you’ll need to replace/repair it.

After that, clear your IS 300’s trouble codes and see if P0846 comes back.

Whether or not you’re going to be able to access the TFPS will determine whether most people can continue diagnosing P0846.  Sometimes the TFPS can be accessed from the outside of the transmission.  You may need to open the transmission.

Wiring issues are the most common cause of P0846.  Take your time and make sure that the harness is in good shape.

3. Other Causes

  • Bad TFPS Sensor
  • Internal transmission issue
  • Low transmission fluid
  • Bad TCM/PCM

While a bad TFPS sensor is highly likely, make sure that the voltage is correct and doesn’t change when wiggling the harness before replacing it.

Conclusion

Diagnosing P0846 is pretty straightforward.  The real issue is getting to the wiring and TFPS.  Good luck fixing your IS 300!