P0979 on 2019 Ram 1500: Shift Solenoid 'C' Circuit Low Causes and Fixes

What's Unique About the 2019-2019 Ram 1500

The 2019 model year is unique because it includes two different Ram 1500 generations: the all-new 5th generation (DT) and the carry-over 4th generation, sold as the Ram 1500 Classic (DS). Both typically use a ZF-designed 8-speed automatic transmission (like the 8HP75 or 850RE) where the solenoids are integrated into the valve body, often called a mechatronic unit. A 'circuit low' code on this platform often points to an internal wiring short or a failing solenoid within the sealed valve body assembly. TSB 21-009-20 specifically notes that a TCM software update may resolve a cluster of diagnostic codes, including P0979, on the 2019 DT models with the 5.7L Hemi and 8HP75 transmission, making a software flash an important early diagnostic step before replacing expensive hardware.

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 includes the new 5th generation Ram 1500 (DT) and the 4th generation Ram 1500 Classic (DS). While both use 8-speed transmissions (e.g., 8HP75, 850RE), the diagnostic and repair procedures may have slight variations. The provided TSB #21-009-20 applies to the 2019 (DT) Ram 1500 with the 5.7L Hemi and 8HP75 transmission, recommending a TCM software update. The underlying cause (solenoid circuit fault) is common across platforms using the ZF 8-speed family.

Symptoms You May Notice

• Check Engine Light is on
• Transmission warning light is on
• Harsh or delayed shifting
• Transmission is stuck in one gear (limp mode)
• Failure to shift into or out of certain gears
• Vehicle may not shift into third gear
• Poor acceleration
• Increased fuel consumption
🎬 See this video for a breakdown of bad shift solenoid symptoms.

Most Likely Causes

1. Faulty Shift Solenoid 'C' / Valve Body Assembly 🔴 High Probability → Shop Transmission Valve Body The shift solenoids are integrated into the valve body assembly (mechatronic unit) on the ZF 8-speed transmissions. A failure of one solenoid often requires replacement of the entire unit, which is a known, though not frequent, repair item for these transmissions.
   How to confirm: A technician will use a scan tool to command the solenoid on and off while monitoring voltage. If the solenoid doesn't respond or the voltage remains low, and the wiring is confirmed to be good, the solenoid is faulty. Resistance of the solenoid circuit can be checked and should be between 5-10 Ω for this transmission family.
   Typical fix: Replace the entire valve body/mechatronic unit, which contains all the solenoids. This is done by dropping the transmission pan.
   Est. part cost: $600-$1500
2. Wiring Harness Damage or Short Circuit 🟡 Medium Probability The wiring harness inside the transmission is exposed to heat and fluid, which can cause insulation to become brittle. External wiring, particularly where it routes around the back of the engine and down to the transmission, can chafe against the engine block or other components, causing a short to ground.
   How to confirm: Visually inspect the external wiring harness for any signs of damage, chafing, or corrosion. To check the internal harness, the transmission pan must be removed. A multimeter is used to check for a short to ground on the control circuit wire between the TCM and the solenoid connector.
   Typical fix: Repair the damaged section of the wire or replace the affected wiring harness (internal or external). In some cases, the intake manifold may need to be removed to access the chafed area of the harness.
   Est. part cost: $50-$400
3. Needs Transmission Control Module (TCM) Software Update 🟡 Medium Probability → Shop Transmission Assembly FCA has issued TSBs for the 2019 Ram 1500 that address clusters of transmission-related fault codes, including P0979, with a software update. The logic within the TCM can sometimes misinterpret data, leading to false codes.
   How to confirm: A dealership or qualified shop can check the current software version of the TCM against the latest version available from FCA. If an update is available as specified in TSB 21-009-20, it should be performed.
   Typical fix: Reprogram or 'flash' the TCM with the latest available software using a wiTECH diagnostic tool. This should be done before replacing any hardware.
   Est. part cost: $0-$250 (Labor for flashing)
4. Low or Contaminated Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While less likely to cause a specific 'circuit low' code, extremely dirty or low fluid can cause erratic solenoid operation, overheating, and debris that can cause a solenoid to stick or fail electrically.
   How to confirm: Check the transmission fluid level and condition. On these sealed transmissions, this must be done by a technician at a specific temperature.
   Typical fix: Perform a transmission fluid and filter service. This is often done as a first step before more expensive repairs.
   Est. part cost: $200-$400
5. Faulty Transmission Control Module (TCM) ⚪ Low Probability → Shop Transmission Assembly While TCM hardware failure is not common, it can happen. The TCM is the computer that controls the transmission and sends the voltage signals to the solenoids. On this platform, it is integrated with the valve body.
   How to confirm: This is usually the last item to be diagnosed. If the software has been updated, and the solenoid and wiring have been tested and are confirmed to be good, the TCM is the likely culprit.
   Typical fix: Replace and program the Transmission Control Module, which requires replacing the entire valve body assembly. 🎬 Watch: Step-by-step guide for installing the 8-speed valve body and computer.
   Est. part cost: $500-$1000

Diagnosis Steps

1. Verify the code with a professional OBD-II scan tool.
2. Check for applicable Technical Service Bulletins (TSBs), especially TSB 21-009-20, which recommends a TCM software update for a group of codes including P0979 on certain 2019 models. Perform the flash update if applicable.
3. Check the transmission fluid level and condition. Note that 2019 Ram 1500s have a sealed transmission, and this check must be performed by a technician at a specific temperature.
4. Inspect the external transmission wiring harness and connectors for any visible damage, corrosion, or loose connections, paying close attention to the area behind the intake manifold where chafing can occur.
🎬 Watch: How to find and fix a shredded wiring harness.
5. Drop the transmission pan to gain access to the internal components.
6. Visually inspect the internal wiring harness leading to the valve body.
7. Using a multimeter and the vehicle's wiring diagram, test the resistance of the Shift Solenoid 'C' circuit. A normal reading is typically between 5 and 10 ohms for this transmission family.
8. Check the control circuit wire for a short to ground between the TCM connector and the transmission connector.
9. If wiring and connectors are good, and the TCM software is up to date, the fault is likely with the solenoid itself, which is part of the valve body assembly.
10. If the solenoid and wiring test good, the final step is to test the Transmission Control Module (TCM).

Parts You'll Likely Need

• Transmission Valve Body Assembly (Mechatronic Unit) (OEM #R8402284AA (Remanufactured, supersedes others)) — The shift solenoids are integrated into this unit. A single solenoid failure often requires replacing the entire assembly.
  Trusted brands: Mopar, ZF
  OEM price range: $800-$1200
  Aftermarket price range: $600-$900
• Transmission Pan Gasket and Filter Kit (OEM #52854834AB (Supersedes 68233701AB, 68233701AC)) — The plastic pan has an integrated filter and must be replaced whenever the pan is dropped for service. It includes a new gasket and bolts.
  Trusted brands: Mopar, ZF, Dorman
  OEM price range: $150-$250
  Aftermarket price range: $80-$150
• ZF Lifeguard 8 Transmission Fluid (OEM #68218925AB (Mopar 8&9 Speed ATF)) — The correct fluid must be used when refilling the transmission after a valve body or pan replacement. Using incorrect fluid can cause severe damage.
  Trusted brands: Mopar, ZF
  OEM price range: $30-$50 per liter
  Aftermarket price range: $20-$40 per liter

Related Codes That Often Appear With This One

• P0731 — As seen in TSB #21-009-20, an incorrect gear ratio code can accompany the solenoid code because the faulty solenoid prevents the transmission from achieving the correct gear.
• P1DCD — Also cited in TSB #21-009-20, this indicates a performance issue with the TCM's monitoring processor, which can be triggered by persistent faults like the P0979.
• P0978 — This code is for 'Shift Solenoid C Control Circuit Range/Performance', indicating a related but different electrical issue with the same solenoid.
• P0980 — This code is for 'Shift Solenoid C Control Circuit High', the opposite of P0979, but could appear if wiring is intermittently shorting to power and ground.

Technical Service Bulletins (TSBs) & Recalls

• 21-009-20: Recommends reprogramming the Transmission Control Module (TCM) with the latest software to address a list of potential DTCs, including P0979, on 2019 DT Ram 1500s with the 5.7L Hemi and 8HP75 transmission.

Platform-Specific Known Issues

• TSB #21-009-20, released on February 11, 2020, lists P0979-00 - Shift Solenoid 3 Control Circuit Low as a potential fault code on 2019 (DT) Ram 1500s with the 5.7L Hemi and 8HP75 transmission. The prescribed fix is to reprogram the TCM with updated software.
• Owners on forums have reported experiencing limp mode that can sometimes be temporarily resolved by turning the vehicle off and on again, which points towards an electronic or intermittent fault.
• Wiring harness chafing behind the engine is a known issue on some Ram trucks, which can cause a short circuit and trigger this code. The repair may involve removing the intake manifold for access.

Mechanic-Grade Diagnostic Values

• Shift Solenoid Coil Resistance (ZF 8HP VFS Solenoid) — expected: ~5.5 Ohms. Failure: A reading that is significantly higher (open circuit) or lower (short circuit) than the specification.
• Shift Solenoid Coil Resistance (General) — expected: 5.0 to 10.0 Ohms at 68°F (20°C). Failure: A reading outside of this range, or showing OL (open) or near zero (short). Resistance will vary with temperature.

Scan Tool Commands That Help

• AlfaOBD / wiTECH: Reset adaptive values / Quick learn — This function should be performed after replacing the valve body/mechatronic unit. It clears the TCM's learned shift patterns and forces it to relearn based on the new hardware, which is critical for resolving harsh or incorrect shifting after the repair.

Wiring & Ground Locations

• Transmission Control Module (TCM) — Integrated into the mechatronic (valve body) assembly, located inside the transmission oil pan.. Since the TCM is internal, a suspected TCM failure requires removal of the transmission pan and valve body. It cannot be swapped as a simple external module.
• Main Transmission Ground Strap — Typically a braided metal strap connecting the transmission case to the vehicle's frame or chassis, often near one of the transmission mounts.. A corroded or loose ground strap can cause fluctuating voltage references for all transmission sensors and solenoids, leading to intermittent and difficult-to-diagnose electrical fault codes like P0979.
• Potential Harness Chafe Point — Inspect the engine/transmission wiring harness where it routes near sharp metal edges, such as the corners of cylinder heads, accessory brackets, and the upper control arm mounts.. Engine vibration can cause the harness to rub against these points, eventually wearing through the insulation and causing a short to ground, which is a direct cause for a 'Circuit Low' code.

OEM Part Supersession History

• 68371273AB → 68371273AC — Part revision or update by the manufacturer for the engine/transmission wiring harness on the 2019 Ram 1500 Classic.

Model Year Variations Within This Range

• 2019: The 2019 model year includes the 4th Gen Ram 1500 Classic (DS) and the 5th Gen Ram 1500 (DT). While both use ZF 8-speed transmissions, advanced diagnostic tools like AlfaOBD have separate vehicle selections ('Ram 1500' for DS vs 'Ram 1500 (DT)' for 2019+), which may alter the menu path for accessing TCM functions.