P0756 on 2019-2020 Ram 3500: UD Solenoid Circuit Fault Causes and Fixes

What's Unique About the 2019-2020 Ram 3500

The key difference for this platform is the choice of two distinct automatic transmissions, which dictates the likely cause of a P0756 code. Ram 3500s with the standard-output 6.7L Cummins use the Chrysler 68RFE, while High-Output (H.O.) models use the Aisin AS69RC. 🎬 Watch: A detailed comparison of the Aisin vs 68RFE transmissions. The TSB (S2121000008) linking P0756 (UD Solenoid) and P0755 (2C Solenoid) is most relevant to the 68RFE transmission, where failure of the entire solenoid pack is a well-documented issue. For Aisin-equipped trucks, the problem is more likely an individual solenoid or a wiring issue, as they do not use the same integrated solenoid pack.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on
• Transmission warning light illuminated
• Harsh or delayed shifting
• Transmission slipping between gears
• Vehicle stuck in one gear (limp mode).
• Failure to shift into or out of certain gears
• Reduced fuel economy
• Transmission overheating.

Most Likely Causes

1. Faulty Solenoid Pack (68RFE Transmission) 🔴 High Probability The 68RFE transmission is known for failures of the integrated solenoid pack (or block). The TSB linking P0756 and P0755 points directly to this common failure pattern. The 2019+ models use a pack with a distinct blue electrical connector.
   How to confirm: A technician will use a scan tool to confirm communication failure with the solenoid. The ultimate confirmation is replacing the pack, which is located inside the transmission pan, attached to the valve body.
   Typical fix: Replace the entire solenoid pack assembly, along with the transmission fluid and filters.
   Est. part cost: $300-$500 for an OEM Mopar part.
2. Faulty Individual Shift Solenoid (Aisin AS69RC Transmission) 🟡 Medium Probability → Shop Transmission Valve Body Unlike the 68RFE, the Aisin transmission uses several individual solenoids. While robust, a single solenoid can fail electrically or mechanically. The P0756 code would point to the specific 'UD' solenoid within the valve body.
   How to confirm: A technician can use a high-end scan tool to command the solenoid on and off. The solenoid's electrical resistance can also be tested with a multimeter after dropping the transmission pan.
   Typical fix: Replace the single faulty solenoid. This also requires a fluid and filter change.
   Est. part cost: $50-$200 per solenoid.
3. Low or Contaminated Transmission Fluid 🟡 Medium Probability → Shop Transmission Assembly Fluid is the medium for both hydraulic pressure and cooling. If it's low, pressure drops; if it's dirty, it can clog the small passages in the solenoid and valve body. One owner on a forum reported this code after the transmission dipstick worked itself loose, causing fluid loss.
   How to confirm: Check the transmission fluid level and condition according to the owner's manual. The fluid should be at the correct level, pink/red in color, and not smell burnt. Ensure the dipstick is fully seated.
   Typical fix: Top off the fluid or perform a complete fluid and filter change using the correct specified fluid (Mopar ATF+4 for 68RFE, Mopar ASRC for Aisin).
   Est. part cost: $100-$250
4. Wiring or Connector Issues 🟡 Medium Probability The wiring harness leading to the transmission is exposed to heat, vibration, and road debris. A known issue on some Ram HD trucks is the harness chafing against a sharp metal bracket near the back of the engine, eventually cutting wires and causing solenoid circuit faults.
   How to confirm: Visually inspect the entire wiring harness from the TCM to the transmission, paying close attention to areas where it contacts brackets or the engine block. Check connectors for corrosion or loose pins.
   Typical fix: Repair the damaged section of wire or replace the faulty connector. Protect the harness from future damage with loom or by re-routing.
   Est. part cost: $20-$150
5. Faulty Transmission Control Module (TCM) ⚪ Low Probability → Shop Transmission Assembly While rare, the control module itself can fail, preventing it from sending the correct signal to the solenoid.
   How to confirm: This is typically a last resort after all other possibilities (solenoid, wiring, fluid) have been ruled out. Requires advanced diagnostics by a professional.
   Typical fix: Replace and reprogram the TCM.
   Est. part cost: $500-$1500

Rare But Worth Checking

• Internal Transmission Hydraulic Blockage: → Shop Transmission Assembly Debris from normal wear can sometimes clog the small fluid passages within the transmission valve body, mimicking a solenoid failure. A transmission flush might clear this, but it can also indicate a more serious internal problem requiring a valve body or transmission rebuild.

Diagnosis Steps

1. CRITICAL FIRST STEP: Identify the transmission. Use the VIN, build sheet, or look at the transmission pan shape. The 68RFE pan is rectangular with angled corners; the Aisin pan is larger and more square.
2. Verify the transmission fluid level and condition. Ensure the dipstick is secure. Top off or replace as necessary with the correct fluid type.
3. Use an OBD-II scanner to check for any other related trouble codes, paying special attention to P0755 and P0871 on 68RFE models.
4. Perform a thorough visual inspection of the transmission wiring harness and connectors. Specifically check for chafing on brackets near the back of the engine and top of the transmission.
5. Using a professional scan tool, monitor the live data for the UD solenoid and attempt to command it on and off to check for a response.
6. If the solenoid does not respond and wiring is intact, drop the transmission pan to access the valve body.
7. For 68RFE: Replace the entire solenoid pack (Mopar P/N 68456067AA).
8. For Aisin AS69RC: Test the resistance of the individual UD solenoid with a multimeter. Compare the reading to the manufacturer's specifications. Replace only the faulty solenoid.
9. If all other components are confirmed to be working correctly, the TCM may be at fault.

Parts You'll Likely Need

• Transmission Solenoid Pack (for 68RFE) (OEM #68456067AA) — This is the most common failure for this code on 68RFE-equipped trucks. It's an integrated module with a blue connector for 2019+ models.
  Trusted brands: Mopar, ATS Diesel, Suncoast, RevMax
  OEM price range: $300-$500
  Aftermarket price range: $250-$450
• Transmission Fluid and Filter Kit — Service is required whenever the transmission pan is removed. Using the correct fluid is critical: Mopar ATF+4 for the 68RFE, and Mopar ASRC or equivalent for the Aisin AS69RC.
  Trusted brands: Mopar
  OEM price range: $100-$200
  Aftermarket price range: $70-$150
• Individual Shift Solenoid (for Aisin AS69RC) (OEM #68019702AA (cross-reference)) — For trucks with the Aisin transmission, the failure is typically an individual solenoid rather than a complete pack.
  Trusted brands: Aisin, Rostra
  OEM price range: $100-$200
  Aftermarket price range: $70-$150

Related Codes That Often Appear With This One

• P0755 — This code for the '2C Solenoid Circuit' is explicitly mentioned alongside P0756 in TSB S2121000008, indicating they are part of a common fault pattern on 68RFE-equipped trucks, often pointing to a failed solenoid pack.
• P0700 — This is a generic code that simply means the Transmission Control Module (TCM) has detected a fault and has requested the Check Engine Light to be turned on. It will almost always be present with any specific transmission code like P0756.
• P0871 — A revision of the TSB (S2121000008 REV. C) also includes this code for 'Transmission Fluid Pressure Sensor/Switch “C” Circuit Performance', further strengthening the diagnosis of an internal electronic/hydraulic circuit fault within the 68RFE.

Technical Service Bulletins (TSBs) & Recalls

• S2121000008: Malfunction Indicator Lamp (MIL) Illumination. Diagnostic Trouble Codes (DTCs) P0756 – UD Solenoid Circuit, and/or P0755 – 2C Solenoid Circuit, Set. A later revision (REV. C) also adds DTC P0871 – Transmission Fluid Pressure Sensor/Switch “C” Circuit Performance.

Platform-Specific Known Issues

• TSB S2121000008 specifically targets this code (P0756) along with P0755, primarily affecting the 68RFE transmission and pointing towards a faulty solenoid pack.
• Owners have reported the transmission dipstick working itself loose, leading to fluid loss and triggering transmission fault codes.
• A potential wiring harness chafing point exists on a sharp bracket near the back of the engine/top of the transmission bellhousing, which can sever wires to the solenoids.

Mechanic-Grade Diagnostic Values

• 68RFE Underdrive (UD) Solenoid Resistance — expected: ~1.3 Ohms. Failure: An open circuit (infinite resistance) or a significant deviation from the expected value indicates a failed solenoid coil.
• Aisin AS69RC Pulse Width Modulated (PWM) Solenoid Resistance — expected: 4-6 Ohms. Failure: A reading outside this range suggests a faulty solenoid. These are typically the larger, black-connector solenoids.
• Aisin AS69RC On/Off Solenoid Resistance — expected: 11-21 Ohms. Failure: A reading outside this range indicates a failure. These are typically the green-connector solenoids.
• 68RFE UD Solenoid Circuit Continuity — expected: < 5.0 Ohms. Failure: High resistance indicates a problem in the wiring between the TCM and the transmission connector.

Scan Tool Commands That Help

• wiTECH (OEM FCA/Stellantis Tool): TCM Actuator Tests / Solenoid Test — This bidirectional command allows a technician to individually activate the UD solenoid to verify its mechanical and electrical operation without having to remove the transmission pan. If the solenoid clicks and the circuit shows a response, the issue is likely hydraulic or intermittent.
• wiTECH (OEM FCA/Stellantis Tool): VIN Re-learn / Module Replacement Programming — For 2019+ 68RFE models, if the solenoid pack or the entire transmission is replaced, the PCM must be programmed with the flow-rate information from the new TCC solenoid. Failure to do so will result in improper transmission performance.

Wiring & Ground Locations

• 68RFE Transmission Connector (23-pin) — Driver's side of the 68RFE transmission case.. This is the main electrical connection point for the entire solenoid pack. The UD Solenoid Control wire is located at Pin 17.
• TCM Connector C1 — On the Transmission Control Module (TCM), located on the passenger side firewall in the engine bay.. This is the starting point of the control circuit. The UD Solenoid Control wire originates at Pin 8 of this connector.
• UD Solenoid Control Wire (T59) — Runs from TCM Connector C1 (Pin 8) to the Transmission Connector (Pin 17). It is typically a 20-gauge yellow wire with a light blue stripe.. This specific wire carries the command signal to the UD solenoid. It is a known weak point for chafing on brackets near the transmission, which can cause a short to ground and trigger P0756.

OEM Part Supersession History

• 68456067AA → 68456067AB — Standard part revision by the manufacturer.
  Heads up: The parts are interchangeable, but 68456067AB is the latest revision. Both are for 2019+ 68RFE transmissions with the blue connector.
• 68367309AA → 68456067AA, then 68456067AB — Older part number that was replaced by the current series for 2019+ models.
  Heads up: 68367309AA should not be used; always use the latest part number (68456067AB) for a 2019-2020 Ram 3500.

Model Year Variations Within This Range

• 2019+: For the 68RFE transmission, 2019 and newer models feature an auxiliary valve body and a dedicated, flow-rated TCC (Torque Converter Clutch) solenoid. If the solenoid pack, transmission assembly, or PCM is replaced, the PCM must be programmed with the new solenoid's specific flow-rate data using a tool like wiTECH.