Fix P0841: Mechanic’s solutions that WORKED

The OBD2 code P0841 indicates a problem with the transmission fluid pressure sensor/switch “A” circuit, suggesting that the sensor’s signal is outside the expected range or performance. This sensor is crucial for monitoring the hydraulic pressure inside the transmission, which in turn controls gear shifting and operation efficiency.

P0841 quick overview

Meaning	P0841: Transmission Fluid Pressure Sensor/Switch “A” Circuit Range/Performance
Is it serious?	Yes, it can lead to transmission shifting issues, affecting the vehicle’s drivability and potentially causing further damage to the transmission system.
Possible causes	– Faulty transmission fluid pressure sensor/switch – Wiring or connector issues in the sensor circuit – Low transmission fluid level or contaminated fluid – Transmission control module (TCM) problems
How to diagnose?	Inspect the transmission fluid for proper level and condition; replace if necessary. Check wiring and connectors for damage or corrosion affecting the sensor circuit. Test the sensor’s operation using diagnostic tools to assess its performance. Evaluate the TCM for software updates, connectivity issues, or the need for replacement.

How to fix P0841 – learn from mechanics

I looked into P0841 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: Dodge

Vehicle information 2005 Dodge Ram 1500 ST 4.7L

Issue + repair history

• Check engine light came on suddenly, with symptoms of 3rd gear starts
• Code P0841: LR pressure switch incorrect position
• Reverse works fine, but stalls occasionally when coming to a stop
• Occasionally clunks into gear, resembling a converter drain back condition
• Cleared the code, but problem persisted

Other OBD2 Codes + Test Results

• P0841: LR pressure switch incorrect position

Mechanic suggestions

• Refer to service bulletin #21-007-06
• Check solenoid assembly for issues; known to have problems and may require replacement
• Consider the possibility of the converter draining back, which may be caused by trouble with the converter control valve in the pump
• Understand the functionality of the LR pressure switch: it should close immediately on startup, indicating pressure for low (1st) and reverse; failure to do so may result in limp-in mode
• Check for issues with split transmission filters and bad check valves in the lube filters; if the LR pressure switch doesn’t close within three seconds of startup, replace both filters
• Replace the solenoid pack with a new Dodge unit, service both transmission filters and fluid, clear codes, and perform a “quick learn” function with a scanner

Final fix

• Identified a rotted wire off the PCM (yellow/tan wire) about 5 inches from the PCM; repaired the wire, and the transmission worked properly afterward

Case #2: Mazda

Vehicle information 2010 Mazda 3 i 2.0L

Issue + repair history

• Sets a P0841 after half an hour of driving
• Also stores U0101 in every module
• Cannot communicate with TCM via Snap-On Solus Edge
• Verified wiring and continuity of 0.6 ohms on both CAN lines
• Swapped TCMs, but still no communication
• Checked A/T main relay and TCM fuse, both are okay
• Transmission fluid changed and filter replaced

Other OBD2 Codes + Test Results

• P0841: Transmission fluid pressure sensor/switch “A” circuit range/performance
• U0101: Lost communication with TCM

Mechanic suggestions + final fix

• Test all power and grounds to TCM using a headlamp
• Test powers and grounds to the TCM
• Verify the CAN bus resistance; normal resistance should be 60.0 ohms with a voltage of approximately 5.0 volts
• Ensure that a known good TCM was installed
• In this case, the issue was traced to a defective TCM losing its ground for the oil pressure sensor; a known good TCM was swapped in, and the problem was resolved

Final fix

• Found TCM losing its ground for the oil pressure sensor
• Swapped in a known good TCM
• Road tested and verified that the issue was resolved

Case #3: Dodge

Vehicle information

• 2007 Dodge Ram 3500 ST 6.7L

Issue + repair history

• Goes into limp mode at highway speeds (high gear only) intermittently
• No codes in memory except P0087 (low fuel rail pressure)
• Reflashed all control modules with the latest software versions
• Recurred with the same issue after driving
• Surges then goes into limp mode when coming to a stop
• Setting a P0841 (LR pressure switch rationality) code
• LR switch remained closed in all gear selections until the engine was shut off and restarted, then switch worked normally

Other OBD2 Codes + Test Results

• P0841: LR pressure switch rationality
• P0087: Low fuel rail pressure

Mechanic suggestions + final fix

• Two similar cases went into limp mode without setting codes, both needed transmission rebuilds
• Check the transmission harness for a rubbed-through wire in the LR switch, potentially causing a ground turning on the switch, more common due to diesel characteristics of vibration
• Check the connectors of a separate TCM for looseness
• Possibilities include a bad switch, bad TCM, wiring issue, or something internal to the transmission
• Perform a quick learn/trans adaptives reset to see if the issue behaves for a while
• Ensure adaptive percentages are within spec
• Inspect the transmission harness for chafing and damaged wires, as diesel engines vibrate a lot

Final fix

• After driving the truck and allowing it to set and idle for hours to maximize heat buildup, it turned out to have a bad transmission
• Installed a remanufactured unit, and the issue has not recurred for anything transmission-related since then

Diagnostic Step	Brief Explanation
Inspect transmission fluid	Check if the fluid is at the correct level and quality, as poor fluid condition can affect pressure readings.
Examine wiring and connectors	Look for any damage or corrosion that could impair the sensor’s ability to function.
Verify sensor functionality	Use a scan tool to test if the sensor is operating within its expected parameters.
Assess the TCM	Check for any necessary updates or signs of malfunction that could be affecting transmission operation.

These cases highlight the necessity of thorough diagnostics, encompassing both the mechanical and electrical aspects of the transmission system. Addressing a P0841 code involves ensuring the fluid pressure sensor is accurately monitoring and relaying information, which is vital for the transmission to operate correctly. This process may include inspecting and potentially replacing the sensor, examining related wiring and connectors for faults, and considering the condition and level of the transmission fluid.