Fix P0845: Mechanic’s solutions that WORKED

The P0845 code signals an issue with the Transmission Fluid Pressure Sensor/Switch “B” Circuit. This code is crucial because it relates to the transmission’s ability to monitor fluid pressure accurately, which is essential for proper gear shifting and overall transmission performance. A malfunction in this sensor or switch can lead to transmission issues such as erratic shifting, slipping, or even the vehicle unexpectedly going into neutral, especially under load.

P0845 quick overview

Meaning	P0845: Transmission Fluid Pressure Sensor/Switch “B” Circuit
Is it serious?	Yes, it can lead to transmission performance issues, potentially causing safety concerns while driving.
Possible causes	– Faulty pressure sensor/switch – Wiring or connector issues in the pressure sensor circuit – Hydraulic blockages or malfunctions within the transmission – Transmission control module (TCM) problems
How to diagnose?	Inspect the wiring and connectors for damage, corrosion, or poor connections Check the pressure sensor/switch for proper operation and replace if necessary Examine the transmission fluid for quality and level, replacing or refilling as needed Perform a transmission scan to check for other related codes and ensure proper communication with the TCM Consider a TCM reset or reprogramming if other fixes do not resolve the issue

How to fix P0845 – learn from mechanics

I looked into P0845 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: GMC

Vehicle Information 2002 GMC Sierra 2500 HD 6.6L

Issue + Repair History

• Truck intermittently goes into neutral under load around 80kph at or near full throttle.
• Initially came in with OBD2 code P0845 after a transmission service at the dealer.
• Replaced shift control solenoid D as per TSB for code P0845.
• Despite replacement, code intermittently returns during test driving.
• Checked pressure switches, TCM, and wiring for proper signals and connections.
• Replaced both filters and oil, as well as the range switch on the transmission.
• Inspected all wiring and grounds for chaffing/corrosion.

Other OBD2 Codes + Test Results

• P0845

Mechanic Suggestions

• Inspect wiring for poor electrical connections at the TCM.
• Check for chafed wires or broken wires inside the insulation.
• Consider the possibility of a short to ground in the pressure switch circuit or a stuck shift valve.
• Investigate the transfer case as a potential source of the issue.

Final Fix

• Discovered a bad connection in the harness in the pan.

Case #2: Chrysler

Vehicle Information 2003 Chrysler 300M 3.5L

Issue + Repair History

• Initially came in with an Output Speed Sensor code and harsh shifting.
• Transmission shop replaced output speed sensor and valve body.
• Started setting a code P0845 2/4 Hydraulic pressure test failure code.
• Found that the code resets immediately after a 1-2 shift despite the 2-4 pressure switch working properly.
• Snapshot revealed the 2/4 responding the moment a shift occurs.
• Occasionally shifts through all gears properly when cold.

Other OBD2 Codes + Test Results

• P0845

Mechanic Suggestions

• Verify if the solenoid pack was replaced along with the valve body, as a faulty solenoid pack might be the issue.
• Run a quick learn in the scan tool and then clear codes to reset transmission adaptations.

Final Fix

• Replacing the solenoid pack with a new one resolved the issue, fixing the P0845 code.

Case #3: Nissan

Vehicle Information 2007 Nissan Altima S 2.5L

Issue + Repair History

• Customer complaint of rough idle only, drives fine otherwise.
• Pulled codes: P0075 (intake valve timing circuit bank 1), P0300 (random misfire), P0845 (trans sensor B).
• VVT is fully on at 42 degrees, unplugging solenoid resolves rough idle.
• Test resistance on solenoid reads 8.2 ohms on warm engine, indicating functionality.
• Voltage on ground side at 1.68 volts when connected, suggesting a possible PCM internally shorting issue.
• Used PCM showed only a trans pressure sensor code, P0075 was no longer present.

Other OBD2 Codes + Test Results

• P0075 (intake valve timing circuit bank 1)
• P0300 (random misfire)
• P0845 (trans sensor B)

Mechanic Suggestions

• Check for oil on the dipstick, as low oil levels could impact VVT functionality.
• Verify if the Red/Blue control wire for the solenoid has a short to ground.
• Consider the possibility of the solenoid not dwelling at the correct rate, necessitating scope analysis of the on-time.
• Check camshaft rotation and timing condition.
• Explore the option of reprogramming the VVT.

Final Fix

• Diagnosis points to a bad PCM, awaiting customer to provide a replacement module.

Diagnostic Step	Brief Explanation
Inspect wiring and connectors	Identifies and corrects electrical issues affecting sensor operation
Replace pressure sensor/switch if faulty	Directly addresses malfunction in the sensor/switch itself
Check and replace transmission fluid and filters	Ensures transmission operates with clean fluid at correct pressure
Reset or reprogram the TCM	Corrects any software issues affecting transmission operation
Inspect and repair transmission internals	Addresses mechanical issues inside the transmission causing pressure problems

In each service case, addressing the P0845 code required specific actions, from fixing a bad connection in the wiring harness for a GMC Sierra to replacing the solenoid pack in a Chrysler 300M, and diagnosing a potential PCM issue in a Nissan Altima. These examples highlight the importance of a thorough diagnostic process to identify and fix the root cause of the transmission fluid pressure sensor/switch issue, ensuring reliable transmission performance.