P0756 on 2019-2020 Ram 2500: UD/Shift Solenoid 'B' Circuit Causes and Fixes

What's Unique About the 2019-2020 Ram 2500

The 2019-2020 Ram 2500, particularly models with the 6.7L Cummins engine, uses the 68RFE automatic transmission. This transmission is known for having specific weak points, and the solenoid pack is one of the most frequently cited. While P0756 is a generic SAE code, on this specific platform it almost always points to an issue with the Underdrive (UD) solenoid within the main solenoid block. 🎬 Watch: Diagnosing common solenoid codes on 2019+ models. These solenoid packs are a known failure item, often due to internal wear, heat, or contamination from normal transmission wear. The circuit boards on the solenoids are also known to flex, causing false pressure switch codes. For 2019 and newer models, Mopar introduced an updated solenoid pack with a blue connector, part number 68456067AA, to address these issues.

Diagnostic Flowchart

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

Generation note: The 2019 model year was a transition from the fourth-generation (DS) to the fifth-generation (DT) Ram HD truck. However, both generations offered the 6.7L Cummins engine paired with the 68RFE transmission, making the information regarding this code applicable to both generations within the 2019-2020 year range. The 68RFE has been in use since 2007 in Ram HD trucks.

Symptoms You May Notice

• Check Engine Light is on
• Transmission stuck in 'limp mode' (usually 4th gear)
• Harsh, delayed, or erratic shifting
• Inability to shift into certain gears
🎬 Watch: How to fix a P0756 performance code.
• Transmission slipping
• Transmission overheating
• Decreased fuel economy
• Buzzing noise from the transmission area when in Park or Neutral, which can be caused by line pressure being maxed out during a limp-in condition

Most Likely Causes

1. Faulty Transmission Solenoid Pack 🔴 High Probability → Shop Transmission Assembly The solenoid pack on the 68RFE is a well-documented failure point due to internal wear, contamination from clutch material, and flexing of the internal circuit board. Aftermarket transmission builders note a failure rate of over 60% on original solenoid packs due to contamination.
   How to confirm: Use a high-end scan tool to command the UD solenoid and monitor its response. If it fails to actuate or the circuit shows incorrect electrical values (resistance), the solenoid is likely bad. The typical resistance for the UD solenoid should be around 1.3 Ohms. The ultimate confirmation is replacing the part.
   Typical fix: Replace the entire solenoid pack assembly with a new OEM Mopar unit (Part #68456067AA for 2019+ models). This part is located on the valve body inside the transmission and can be replaced without removing the entire transmission.
   Est. part cost: $350-$500
2. Low or Contaminated Transmission Fluid 🟡 Medium Probability → Shop Transmission Assembly Over time, transmission fluid (ATF) breaks down and can become contaminated with clutch material and metal shavings, which can clog the small passages in the solenoid and valve body. The recommended service interval is approximately every 25,000 miles for a fluid and filter change, especially under heavy use.
   How to confirm: Check the transmission fluid dipstick for level and condition. The fluid should be bright red and clear. If it is dark, brown, or smells burnt, it is contaminated and needs service.
   Typical fix: Perform a transmission fluid and filter change using Mopar ATF+4. If the fluid is heavily contaminated, this may be a symptom of a larger issue, but changing the fluid is a necessary first step.
   Est. part cost: $100-$200
3. Wiring Harness or Connector Issue ⚪ Low Probability The wiring harness leading to the transmission can be exposed to heat, debris, and vibration, potentially causing chafing or loose connections over time. The main connector at the transmission case is a key point of inspection.
   How to confirm: Visually inspect the wiring harness from the TCM to the transmission case connector for any signs of damage, melting, or corrosion. Test for continuity and shorts between the TCM and the solenoid connector. Check for an open circuit in the solenoid by testing resistance between the appropriate pins.
   Typical fix: Repair the damaged section of wire or replace the faulty connector.
   Est. part cost: $20-$100
4. Faulty Valve Body ⚪ Low Probability → Shop Transmission Valve Body The 68RFE valve body, made of soft aluminum, can warp or develop cross-leaks over time, which affects hydraulic pressure and can mimic a solenoid failure. This is a known issue, though less common than a direct solenoid pack failure.
   How to confirm: This is difficult to confirm without specialized equipment and is often diagnosed after a new solenoid pack fails to resolve the issue. A transmission specialist may use a vacuum tester on individual valve body channels to check for leaks; a reading below 15 inHg (inches of mercury) indicates a failure.
   Typical fix: Replace the valve body. Many owners and shops opt to install an upgraded aftermarket valve body (e.g., from RevMax or SunCoast) that addresses the factory design's weak points.
   Est. part cost: $400-$800

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly This is rare, but a failing TCM can stop sending the correct signals to the solenoid. On 2010+ models, the TCM function is integrated into the Powertrain Control Module (PCM). This should only be considered after all other possibilities (solenoid, wiring, fluid) have been thoroughly ruled out.

Diagnosis Steps

1. Check the transmission fluid level and condition. If the fluid is low, top it off. If it's dark, burnt, or contains debris, a fluid and filter change is necessary.
2. Use an OBD-II scanner to check for any other transmission-related codes that could provide more context, such as P0755, P0868, or P0871.
3. With a capable scan tool, monitor the live data for the Shift Solenoid 'B' (UD solenoid) and attempt to command it on and off. Observe if the solenoid responds and if the circuit voltage/resistance is within specification.
4. Inspect the main transmission wiring harness connector for corrosion, bent pins, or damage. Follow the harness as much as possible to look for chafing or melting, especially near exhaust components.
5. If the external wiring is okay, the next step is to drop the transmission pan to access the internal components.
6. Visually inspect the solenoid pack and its connector for any obvious damage. Note the color of the connector (2019+ models should have a blue connector).
7. Test the resistance of the UD solenoid directly at the solenoid pack connector. The resistance between the power feed pin and the UD solenoid control pin should be approximately 1.3 Ohms at room temperature. An open or shorted circuit indicates a failed solenoid.
8. If the solenoid tests okay electrically, the failure may be mechanical (stuck valve) or the issue may lie in the valve body or, rarely, the TCM itself.

Parts You'll Likely Need

• Transmission Solenoid Pack (Block) (OEM #68456067AA) — This is the most frequent cause of P0756 on the 68RFE transmission. The entire block, which contains multiple solenoids, is replaced as a single unit. This is the updated part for 2019+ models with a blue connector.
  Trusted brands: Mopar (OEM)
  OEM price range: $350-$500
  Aftermarket price range: $250-$400
• Transmission Filter Kit (OEM #68019688AA (Spin-on), 5013470AC (Sump)) — The two filters (spin-on cooler return filter and internal sump filter) must be replaced anytime the transmission pan is removed for service.
  Trusted brands: Mopar, Wix, ATP
  OEM price range: $50-$80
  Aftermarket price range: $30-$60
• Mopar ATF+4 Transmission Fluid (OEM #68218057AC (Quart), 68218058AC (Gallon)) — This is the required fluid for the 68RFE transmission. A typical pan drop and filter change requires 7-8 quarts.
  Trusted brands: Mopar
  OEM price range: $10-$15 per quart
  Aftermarket price range: $7-$12 per quart

Related Codes That Often Appear With This One

• P0755 — This code for the 2C Solenoid Circuit is mentioned alongside P0756 in TSB S2121000008, indicating they can be related issues stemming from the same root cause, such as a faulty solenoid block or wiring.
• P0868 — This code indicates low transmission line pressure, which can be a cause or a result of a solenoid malfunction. Forum users report this code appearing with solenoid/valve body issues.
• P0871 — This code relates to the Overdrive pressure switch. Problems in the hydraulic circuits, including solenoid issues, can cause a cascade of pressure-related fault codes.

Technical Service Bulletins (TSBs) & Recalls

• S2121000008: Mentions MIL illumination for DTCs P0756 (UD Solenoid Circuit) and/or P0755 (2C Solenoid Circuit).

Platform-Specific Known Issues

• TSB S2121000008: FCA issued this Technical Service Bulletin on May 3, 2021, for "Malfunction Indicator Lamp (MIL) Illumination. Diagnostic Trouble Codes (DTCs) P0756 – UD Solenoid Circuit, and/or P0755 – 2C Solenoid Circuit, Set." This bulletin acknowledges the issue and likely provides diagnostic guidance to technicians, confirming that these two codes often appear together and point towards a common fault, likely the solenoid pack.
• Recall 20V093000 (FCA W03): This safety recall was for a potential leak from the transmission dipstick tube, which could cause a fire if fluid contacted an ignition source. While not directly causing code P0756, a significant fluid leak could lead to low fluid levels, which is a potential cause of various transmission codes and drivability problems.

Mechanic-Grade Diagnostic Values

• Underdrive (UD) Solenoid Coil Resistance — expected: 1.3 Ohms. Failure: An open circuit (infinite resistance) or a short (near zero resistance) indicates a failed solenoid coil.
• Pressure Control Solenoid Resistance — expected: 4.9 Ohms. Failure: Significant deviation from this value suggests a fault in the pressure control solenoid, which is part of the same assembly.
• Valve Body Channel Vacuum Test — expected: 15 inHg (inches of mercury) or higher. Failure: A reading below 15 inHg indicates excessive wear in the valve body bore, causing a cross-leak that can mimic solenoid failure.

Scan Tool Commands That Help

• wiTECH (or equivalent high-end scanner): Solenoid Actuation Test / Solenoid Switch Test — This bidirectional command allows the technician to manually energize and de-energize each shift solenoid while monitoring circuit data. It's used to confirm if the solenoid is electrically responsive to commands from the TCM/PCM.
• wiTECH (or equivalent): Solenoid Pack Relearn / QR Code Scan — After replacing a solenoid pack on a 2019+ model, the new pack may have a QR code that needs to be scanned. The resulting 46-character code must be entered into the ECU via the scan tool to properly calibrate the new component. Failure to do so can result in the transmission not operating correctly.

Wiring & Ground Locations

• Transmission Solenoid Connector (C1 / Main Connector) — On the driver's side of the 68RFE transmission case, pointing upwards.. This is the main electrical interface for the entire valve body assembly. All voltage and resistance tests for the solenoid circuit are performed at this connector's pins.
• Pin 17 (UD Solenoid Control) — On the main transmission solenoid connector.. This is the specific pin for the Underdrive (UD) solenoid control circuit. A technician would test for continuity from this pin to the corresponding pin at the PCM/TCM (Pin 8) to check for wiring faults.
• Pin 10 (Transmission Control Relay Output) — On the main transmission solenoid connector.. This pin provides the power feed to the solenoid pack. Resistance of the UD solenoid coil is measured between Pin 17 (control) and Pin 10 (power feed) at the solenoid pack itself.

OEM Part Supersession History

• 68367309AA (and prior revisions with white/grey connectors) → 68456067AA (Blue Connector) — Revisions to address known failure points, including flexing of the internal circuit board which caused false pressure switch codes.
  Heads up: The blue connector solenoid pack (68456067AA) is for 2019+ models. Earlier models used white or grey connector packs (e.g., 68353383AB, 68376695AA). The part number 68456067AA has itself been superseded by 68456067AB.

Model Year Variations Within This Range

• 2019+: Starting in 2019, the 68RFE uses a solenoid pack with a blue connector (PN 68456067AA and newer). These may require a 'relearn' or calibration procedure using a scan tool where a QR code from the new part is entered into the vehicle's computer. Pre-2019 models used solenoid packs with white or grey connectors that did not have this requirement.