P0765 on 2019-2020 Ram 2500: Underdrive Solenoid Circuit Fault Causes and Fixes

What's Unique About the 2019-2020 Ram 2500

For the fifth-generation Ram 2500, this code is frequently linked to software glitches, especially on models equipped with the 6.7L Cummins engine (using the 68RFE or Aisin AS69RC transmission). Multiple Technical Service Bulletins (TSBs) from Ram, such as #18-094-22, indicate that P0765 can be triggered incorrectly along with engine codes like P0299 (Turbo Underboost) and P0401 (EGR Performance). In these cases, the primary fix is a software update for the PCM/TCM, not a hardware replacement. This is a 🎬 Watch: How to fix a Ram stuck in 4th gear. crucial distinction from older trucks where the solenoid pack itself was the most common point of failure.

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 or Transmission Warning Lamp is on
• Transmission is stuck in one gear (limp mode), often 4th gear
• Harsh, delayed, or failed shifts
• Inability to shift into higher gears (e.g., locked out of 5th and 6th)
• Reduced acceleration and poor fuel economy
• Sudden RPM flare-up while cruising, followed by a jolt into limp mode
• "Service Exhaust System" message may appear on diesel models

Most Likely Causes

1. Powertrain/Transmission Control Module (PCM/TCM) Software Issue 🔴 High Probability → Shop Transmission Assembly Multiple TSBs, including #18-094-22, confirm that software logic can incorrectly flag this code, sometimes in conjunction with unrelated engine codes like P0299 (Turbo Underboost) or P0401 (EGR Performance). This is a well-documented issue for 2019+ diesel models.
   How to confirm: A Ram dealer or a qualified shop must use an OEM-level scan tool (like wiTECH) to check the current software calibration against the latest available updates. The presence of codes P0299 or P0401 is a strong indicator of a software-related issue.
   Typical fix: Update the PCM/TCM to the latest software version. TSB #18-094-22 specifically outlines this procedure. For 68RFE transmissions, a 'Quicklearn' procedure is required after the flash.
   Est. part cost: $0
2. Faulty Transmission Solenoid Pack 🟡 Medium Probability → Shop Transmission Assembly The solenoid pack is an electromechanical component subject to failure from heat, vibration, and internal debris. It is the most common hardware failure for this code. TSB #9004380 specifically addresses this part for repeatable P0765 faults.
   How to confirm: After ruling out software, a technician will test the solenoid's electrical circuit. If the wiring is good, but the solenoid doesn't respond correctly to scan tool commands or its resistance is out of spec (should be approx. 1.3 ohms for the 68RFE UD solenoid), the pack is condemned. The presence of code P0755 almost guarantees the pack has failed.
   Typical fix: Replace the entire transmission solenoid pack, which is located inside the transmission oil pan. This also requires replacing the transmission fluid and filters.
   Est. part cost: $300-$500
3. Wiring or Connector Issue ⚪ Low Probability The wiring harness to the transmission is exposed to heat and road debris, which can cause chafing, corrosion, or broken wires over time. A YouTube video details a case where a wire's insulation was worn through, causing a short.
   How to confirm: A visual inspection of the wiring harness from the TCM to the transmission case is the first step. A continuity test using a multimeter on the UD solenoid control circuit (T59 wire, often yellow with a light blue stripe) can confirm an open or short. The resistance on the circuit should be very low, under 5.0 ohms.
   Typical fix: Repair the damaged section of wire or replace the corroded connector.
   Est. part cost: $10-$100
4. Low or Dirty Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While P0765 is an electrical code, extremely low or contaminated fluid can cause overheating and erratic hydraulic pressure, which may indirectly stress the solenoids and contribute to a fault.
   How to confirm: Check the transmission fluid level and condition. The fluid should be at the correct level, red in color, and not smell burnt.
   Typical fix: Perform a transmission fluid and filter change. Use only the specified fluid (e.g., Mopar ATF+4).
   Est. part cost: $100-$200

Rare But Worth Checking

• Faulty Transmission Control Module (TCM) / Engine Control Module (ECM): → Shop Transmission Assembly This is rare. The TCM/ECM is the computer that controls the transmission. TSB #18-072-23 (NHTSA ID 10240902) notes that for some 2023 models, an internal ECM capacitor leak can cause P0765 and other open circuit codes, requiring ECM replacement. It should only be considered after all other possibilities have been exhaustively ruled out.

Diagnosis Steps

1. Check for Technical Service Bulletins (TSBs). Given the high probability of a software issue, this should be the first step. Specifically ask a dealer to check for TSB #18-094-22.
2. Use a scan tool to check for other stored codes. Note if codes like P0755, P0871, P0299, or P0401 are present, as this strongly points towards either a software issue or a failed solenoid pack.
3. Check the transmission fluid level and condition. Ensure it is full, clean, and does not smell burnt.
4. Inspect the transmission wiring harness and main connector for any signs of damage, corrosion, or loose pins. Pay close attention to areas where the harness might rub against the chassis or engine components.
5. If no software updates are available and wiring is intact, use a high-level scan tool to command the UD solenoid on and off while monitoring its status.
6. If the solenoid does not respond, perform a circuit test. Check for proper voltage and ground at the solenoid connector and test the resistance of the solenoid coil itself. For the 68RFE, the UD solenoid resistance should be around 1.3 ohms.
7. If the circuit and wiring are good but the solenoid fails tests, the solenoid pack needs replacement. Per TSB #9004380, event data should be downloaded before swapping the module.

Parts You'll Likely Need

• Transmission Solenoid Pack / Assembly (68RFE) (OEM #68456067AB) — This is the most common hardware failure for this code on 68RFE-equipped trucks. It contains the Underdrive solenoid along with others in a single, non-serviceable module. This part number supersedes 68456067AA and 68367309AA.
  Trusted brands: Mopar, RevMax, Suncoast
  OEM price range: $350-$500
  Aftermarket price range: $250-$600
• Transmission Fluid and Filters — Required when replacing the solenoid pack, as the transmission pan must be removed for access. Always use Mopar ATF+4 or an equivalent licensed fluid.
  Trusted brands: Mopar
  OEM price range: $100-$150
  Aftermarket price range: $70-$120

Related Codes That Often Appear With This One

• P0755 — This code for the '2C Solenoid Circuit' is often triggered along with P0765. TSBs #S2121000008RevC and #18-094-22 list them together. Their simultaneous appearance strongly points to a failed solenoid pack, as both solenoids are part of the same integrated component.
• P0871 — This code for 'Transmission Fluid Pressure Sensor/Switch 'C' Circuit Performance' is also mentioned in TSB #S2121000008RevC. Its presence suggests a more widespread electrical or hydraulic issue within the valve body or solenoid pack.
• P0299 or P0401 — The appearance of engine-related codes like Turbo Underboost or EGR Performance alongside P0765 is a strong indicator of the software issue described in Ram TSB #18-094-22. The root cause is the PCM software, not multiple independent hardware failures.

Technical Service Bulletins (TSBs) & Recalls

• 9004380: Recommends downloading event data before replacing the solenoid module for a repeatable P0765.
• S2121000008RevC: Notes that P0765 can appear with P0755 and P0871.
• 18-094-22: Outlines a software fix for P0765 when it appears with engine codes like P0299 and P0401.

Platform-Specific Known Issues

• TSB #9004380 advises technicians to download event data before replacing the solenoid module on vehicles with a repeatable P0765, to assist with root cause analysis.
• TSB #S2121000008RevC notes that P0765 can set along with P0871 and P0755, indicating a potential multi-circuit fault within the transmission's control system.
• TSB #18-094-22 is a key document, outlining a software reflash to fix P0765 when it appears with P0755, P0299, and P0401 on 2022 models, a pattern also seen on 2019-2021 trucks.

Mechanic-Grade Diagnostic Values

• Underdrive (UD) Solenoid Coil Resistance — expected: Approximately 1.3 Ohms. Failure: An open circuit (infinite resistance) or a significant deviation from the expected value indicates a failed solenoid coil.
• UD Solenoid Control Circuit (T59) Resistance — expected: Below 5.0 Ohms. Failure: Resistance above 5.0 ohms indicates an open or excessive resistance in the wiring between the TCM and the solenoid pack.
• Continuity between TCM and Transmission Connector — expected: 0.05 to 0.20 Ohms. Failure: A higher reading indicates a problem in the wire between the two points.

Scan Tool Commands That Help

• wiTECH, Snap-On, Autel, or equivalent: Quick Learn — This procedure is required after replacing the transmission solenoid pack or valve body. It allows the TCM to learn the hydraulic and electrical characteristics (Clutch Volume Index) of the new components to ensure proper shift quality.
• wiTECH or equivalent: Actuate UD Solenoid — This bidirectional test is used to directly command the Underdrive solenoid on and off. It helps a technician determine if the fault is with the solenoid itself, the wiring, or the TCM driver, often by monitoring the circuit's response or seeing if the P0765 code immediately resets.
• Professional Scan Tool: Read Clutch Volume Index (CVI) Data — This allows a technician to view the learned values for clutch application volumes. Out-of-spec or high CVI values for the Underdrive clutch can indicate an internal hydraulic leak or mechanical wear, even if the solenoid's electrical circuit tests good.

Wiring & Ground Locations

• UD Solenoid Control Circuit (T59) — A yellow wire with a light blue stripe running from the TCM to the transmission.. This is the specific wire that carries the command signal for the Underdrive solenoid. It is known to chafe on brackets or other components, causing a short or open that directly triggers P0765.
• TCM Connector C1, Pin 8 — On the Transmission Control Module (TCM), located on the passenger side of the engine bay firewall.. This is the origin point of the UD Solenoid Control circuit. Voltage and resistance tests start here to diagnose the circuit.
• Transmission Connector, Pin 17 — On the main 23-pin connector located on the driver's side of the 68RFE transmission case.. This is the termination point of the UD solenoid control circuit before it enters the solenoid pack. Testing at this pin helps isolate the fault between the external harness and the internal solenoid pack.
• Main Body/Engine Ground Strap — A primary ground strap is typically located on the passenger side, connecting the battery negative terminal to the fender/body. Another braided strap often connects the transmission case to the chassis.. While not a direct cause, a poor or corroded main ground can introduce electrical noise and voltage drops, potentially causing unpredictable behavior from the TCM and other modules, which could contribute to setting false codes.

Real Owner Repair Stories

• YouTube channel 'Ovens Garage' (2009 Ram 3500 w/ 6.7L Cummins & 68RFE (Note: 3rd Gen, but the P0765 fault logic and wiring diagnosis are identical for 2019-2020 models)) — Truck went into limp mode (stuck in 4th gear) and stored code P0765.
  ❌ Tried (didn't work) The owner researched the issue and noted many people replaced the solenoid pack without fixing the problem, so he decided to diagnose the wiring first.
  ✅ What actually fixed it A thorough visual inspection of the wiring harness revealed a spot where the T59 (UD solenoid control) wire had chafed through its insulation on a bracket, causing an intermittent short. The owner repaired the damaged section of wire and protected it with a new loom, which permanently resolved the P0765 code without replacing any parts.

OEM Part Supersession History

• 68367309AA, 68456067AA → 68456067AB — Component revision and updates by the manufacturer for improved reliability or manufacturing changes.
  Heads up: The 68456067AB part, known as the 'Blue Connector' solenoid pack, is for 2019 and newer 68RFE transmissions. It is not compatible with pre-2019 models that used a grey or white connector solenoid pack.

Model Year Variations Within This Range

• 2019-2020: Starting in 2019, the 68RFE transmission uses an updated solenoid pack identified by a blue main connector. This is a key physical difference from prior generations. When sourcing a replacement, it is critical to get the correct 'blue connector' part (Mopar #68456067AB) for these model years.