P0734 on 2015 Dodge Charger: Gear 4 Incorrect Ratio Causes and Fixes

What's Unique About the 2015-2015 Dodge CHARGER

The 2015 Dodge Charger uses a ZF-designed 8-speed automatic transmission (the 845RE or 8HP45). For this specific model year, Dodge released several Technical Service Bulletins (TSBs) acknowledging that the original TCM software could incorrectly trigger the P0734 code, sometimes accompanied by harsh shifting. TSB #21-041-17 specifically mentions that a Firmware-Over-The-Air (FOTA) radio update could disrupt transmission settings and cause this code to appear. This makes a software check with a dealer a critical first step, as the problem is often not a mechanical 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 is on
• Vehicle cannot shift into 4th gear
• Harsh or delayed shifting
• Transmission slipping, may feel like a high-revving engine before dropping into gear.
• Vehicle enters "limp mode" (stuck in a single gear, often 4th).
• Decreased fuel economy
• Hesitation or difficulty accelerating from a stop.
🎬 Watch: A breakdown of common causes for the P0734 code

Most Likely Causes

1. Outdated Transmission Control Module (TCM) Software 🔴 High Probability → Shop Transmission Assembly Dodge issued multiple TSBs for the 2015 model year to correct software logic that could erroneously trigger P0734. TSB 21-041-17 explicitly states a FOTA radio update can cause the issue.
   How to confirm: Contact a Dodge dealership with your VIN. They can check for any open campaigns or applicable TSBs (like #21-041-17, #21-019-16, or #21-023-15) related to transmission software updates. A wiTECH scan tool is used for diagnosis.
   Typical fix: The dealership will reflash (reprogram) the TCM with the updated software. This is often a one-hour labor charge. One owner on ChargerForums.com confirmed a dealer flash resolved their P0734 code.
   Est. part cost: $0-$250
2. Low or Degraded Transmission Fluid 🟡 Medium Probability → Shop Transmission Assembly The ZF 8-speed is considered 'filled for life' by Chrysler, but ZF recommends service every 60,000 miles. Low fluid from a leak causes low hydraulic pressure, leading to slipping. The fluid level cannot be easily checked as there is no dipstick.
   How to confirm: This must be done by a professional or experienced DIYer. The vehicle must be level, and with a scan tool monitoring fluid temperature (must be between 30-50°C), a fill plug on the side of the transmission is removed to check the level. The fluid should be honey-colored (ZF Lifeguard 8) or light red (Mopar); if it's dark, burnt, or contains metal, there is likely internal damage.
   Typical fix: Perform a transmission fluid and filter service. The filter is integrated into the plastic transmission pan, so the entire pan assembly must be replaced. 🎬 See this step-by-step guide for a fluid and filter service Use only the specified fluid (Mopar ZF 8&9 Speed ATF P/N 68218925AA or 68218925AB).
   Est. part cost: $200-$400
3. Faulty Shift Solenoid 🟡 Medium Probability → Shop Transmission Valve Body
   How to confirm: A professional scan tool can command the 4th gear shift solenoid to operate and monitor its response. Resistance can also be checked with a multimeter, but this requires accessing the solenoid inside the transmission pan.
   Typical fix: Replace the faulty shift solenoid. On the 845RE, solenoids are part of the valve body/mechatronic unit, making replacement complex and expensive. It is usually done after dropping the transmission pan.
   Est. part cost: $100-$300 (if individual solenoid is sourced), $1000+ (for mechatronic assembly)
4. Faulty Valve Body ⚪ Low Probability → Shop Transmission Valve Body
   How to confirm: This is usually diagnosed after software, fluid, and solenoids have been ruled out. A technician may perform hydraulic pressure tests to check for internal leaks or sticking valves within the valve body.
   Typical fix: The valve body may need to be cleaned or replaced. Replacement is a major repair. A potential part number for the valve body and control unit for a 3.6L AWD Charger is 68259301AD.
   Est. part cost: $800-$1400

Rare But Worth Checking

• Internal Transmission Failure: → Shop Transmission Assembly This is the worst-case scenario. If the transmission fluid contains metal shavings or clutch material, it indicates worn or damaged internal components like the 4C clutch pack, which requires a full transmission rebuild or replacement.
• Faulty Input/Output Speed Sensor: While less common for this specific code in isolation, a failing input or output speed sensor can provide incorrect data to the TCM, leading to a gear ratio error code. Often, a failing sensor will trigger its own specific DTC (e.g., P0716, P0721).

Diagnosis Steps

1. Check for Technical Service Bulletins (TSBs) using your vehicle's VIN. This is the most critical first step for this specific vehicle and code.
2. Scan the TCM for any other stored trouble codes, paying close attention to related codes like P1DCD or speed sensor faults.
3. Check the transmission fluid level and condition. This is a complex procedure on the sealed 845RE transmission and requires a scan tool to monitor fluid temperature.
4. If the fluid is low, inspect for leaks from seals, the pan gasket, or cooler lines.
5. Use an advanced scan tool (like wiTECH) to monitor live data, including input and output speed sensor readings, commanded gear vs. actual gear, and to command individual shift solenoids to function.
6. If software and fluid issues are ruled out, test the electrical circuits for the shift solenoids and speed sensors for proper voltage and resistance.
7. If electrical tests pass, a transmission pressure test may be needed to diagnose internal hydraulic leaks or a failing pump.
8. Inspect the valve body for sticking valves or blockages if all other tests are inconclusive.

Parts You'll Likely Need

• Transmission Fluid and Filter Kit (OEM #68225344AA (Pan/Filter Kit), 68218925AA / 68218925AB (Fluid)) — Low or dirty fluid is a common cause of shifting problems. The filter is integrated into the pan and must be replaced as a unit during a fluid service.
  Trusted brands: Mopar, ZF
  OEM price range: $200-$350
  Aftermarket price range: $150-$250
• Shift Solenoid — The solenoid responsible for engaging 4th gear can fail electronically or mechanically, preventing the gear from engaging correctly. On this transmission, it is often part of the larger mechatronic/valve body assembly.
  Trusted brands: Mopar, Bosch, ZF
  OEM price range: $100-$200 (individual, if available)
  Aftermarket price range: $50-$150
• Valve Body Assembly (OEM #68259301AD (example for 3.6L AWD)) — A faulty valve body can cause hydraulic pressure loss. This is an expensive, less common fix and should only be considered after software and fluid issues are ruled out. 🎬 Watch: A real-world diagnostic and fix for Gear 4 ratio errors
  Trusted brands: Mopar
  OEM price range: $1200-$2000
  Aftermarket price range: $800-$1500

Related Codes That Often Appear With This One

• P1DCD — This code for 'TCM Monitoring Processor Performance Multiple Clutches Locked Up' is specifically mentioned alongside P0734 in Dodge TSBs #21-041-17 and #21-023-15 REV-B, indicating they are related to the same software issue.
• P1B13 — 'Park-By-Wire Unintended Out Of Park Position' is another code listed in the same TSBs as P0734, suggesting a common software-related root cause.

Technical Service Bulletins (TSBs) & Recalls

• 21-041-17: Supersedes 21-019-16. Involves reprogramming the TCM for P0734, P1DCD, and P1B13. Critically, it notes that a radio

Platform-Specific Known Issues

• Multiple TSBs (#21-041-17, #21-019-16, #21-023-15 REV-B) were issued by Dodge for the 2015 Charger to address P0734 via a TCM software update. These bulletins explicitly state the update is to improve shift quality and resolve this specific code, which may be set without an actual mechanical fault.
• TSB #21-041-17 specifically notes that a Firmware-Over-The-Air (FOTA) update for the radio system could inadvertently cause the transmission to default to higher shift points, block 7th/8th gears, and set code P0734.

Mechanic-Grade Diagnostic Values

• Battery Voltage During TCM Flash — expected: Stable between 13.2 and 13.5 volts. Failure: Voltage dropping below 13.2V or climbing above 13.5V can cause the reflash procedure to fail.
• Transmission Fluid Temperature for Level Check — expected: 30°C to 50°C (86°F to 122°F). Failure: Checking the fluid level outside this temperature range will result in an inaccurate reading, leading to over or under-filling.
• Shift Solenoid Coil Resistance — expected: Generally between 5 and 30 ohms (vehicle-specific value should be confirmed in service manual). Failure: A reading of infinite resistance (open circuit) or zero ohms (short circuit) indicates a failed solenoid coil.

Scan Tool Commands That Help

• wiTECH, Autel, AlfaOBD: Quick Learn / Reset Adaptive Values — This procedure should be performed after replacing the transmission, valve body, or TCM, or after a software reflash. It forces the TCM to relearn clutch fill times and pressures to ensure smooth shifting. Note: Some TSBs specifically advise NOT to clear adaptations after a simple software update, as the transmission will relearn on its own.

Wiring & Ground Locations

• TCM Fuses (Front Fuse Box) — In the fuse box under the hood.. Fuse F29 (15A) and Fuse F37 (10A) provide power to the TCM. A loss of power from a blown fuse will cause transmission communication and operational faults.
• TCM Relay (Front Fuse Box) — In the fuse box under the hood.. Relay R44 is involved in the TCM power circuit. A faulty relay can cause intermittent or total loss of power to the transmission module.
• TCM Fuse (Trunk Fuse Box) — In the fuse box located in the trunk, near the battery.. Fuse F26 in the trunk fuse box is also part of the TCM power circuit and should be checked if power issues are suspected.
• Chassis Ground — A common ground point is a bolt on the chassis near the under-hood fuse box. The main battery ground is in the trunk.. A poor ground connection can cause a wide range of electrical issues, including erratic sensor readings and incorrect TCM behavior, potentially leading to false gear ratio codes.

Real Owner Repair Stories

• YouTube video (Dodge truck (model not specified) with P0734) — Check Engine Light with P0734 code.
  ❌ Tried (didn't work) Initial diagnosis was uncertain between sensors and a more expensive module.
  ✅ What actually fixed it Replacing both the input and output speed sensors on the transmission resolved the code. The repair was described as inexpensive and successful.

When the Usual Fixes Don't Work

• While the most common fix for P0734 on a 2015 Charger is a TCM software update, it is not the only possible cause. A documented repair showed that replacing the input and output speed sensors on the transmission was sufficient to fix the problem, representing a much cheaper and simpler hardware fix. This confirms that basic sensor diagnostics should not be skipped in favor of assuming a software-only issue. In other cases involving different transmissions but the same code, a complete valve body rebuild was required after fluid service and software checks failed to resolve the issue, indicating that mechanical wear remains a potential, albeit less common, root cause.