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

What's Unique About the 2015-2015 Dodge CHALLENGER

For the 2015 Dodge Challenger equipped with the 8-speed 8HP45 or 845RE automatic transmission, this code is frequently a software issue rather than a mechanical one. Chrysler (FCA) released multiple Technical Service Bulletins (TSBs) acknowledging that the original TCM software could incorrectly trigger P0734, often accompanied by harsh shifting. One TSB specifically notes that a Flash Over The Air (FOTA) radio update could even cause the transmission shift points to default to incorrect settings, leading to this code. Therefore, unlike many other vehicles where this code points directly to internal wear, the first diagnostic step for this specific Challenger is to check for a software update.

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 (Malfunction Indicator Lamp) is on
• Harsh or erratic shifting, particularly on downshifts like 8-6, 6-4, or 6-3.
• Transmission slipping while in or shifting to 4th gear, felt as a flare in RPMs without an increase in speed.
• Inability to shift into 4th gear, with the engine revving high at highway speeds.
• Vehicle may enter 'limp mode', limiting available gears to prevent damage.
• Decreased fuel efficiency, especially on the highway.
• A noticeable "bump" or jolt during low-speed 2-1 downshifts.

Most Likely Causes

1. Transmission Control Module (TCM) Software Issue 🔴 High Probability → Shop Transmission Assembly This is a well-documented issue by Dodge for the 2015 Challenger with the 3.6L engine and 8HP45/845RE transmission. Multiple TSBs were issued to correct this specific problem, which can cause an 'erroneous' MIL illumination for P0734.
   How to confirm: Contact a Dodge dealership with your Vehicle Identification Number (VIN) and ask them to check for open campaigns or applicable TSBs related to transmission software updates. Specifically reference TSBs #21-041-17, #21-019-16, or #21-006-15 REV-B. The diagnostic process involves using a wiTECH scan tool to check for the code and then performing the re-flash if applicable.
   Typical fix: Reprogramming (flashing) the TCM with the latest manufacturer software. The TSBs explicitly state this is the remedy. The procedure takes approximately 30-60 minutes of labor.
   Est. part cost: $0-$300
2. Low or Dirty Transmission Fluid 🟡 Medium Probability → Shop Transmission Assembly While the ZF 8-speed is a 'fill for life' transmission under normal conditions, fluid can still become low due to leaks or degraded from heat, causing low hydraulic pressure needed for clutch engagement.
   How to confirm: The ZF 8-speed transmission does not have a dipstick. The fluid level must be checked via a fill plug on the side of the transmission while the vehicle is level and the fluid is within a specific temperature range (30-50°C), monitored with a scan tool. The fluid should be a clear, honey-like color (for ZF Lifeguard 8); if it's dark, burnt, or contains metallic debris, it indicates a problem.
   Typical fix: Perform a transmission fluid and filter service. The filter is integrated into the transmission pan, so the entire pan assembly must be replaced. 🎬 See this step-by-step 845RE transmission fluid and filter service. This is a complex procedure due to the temperature-sensitive fill process.
   Est. part cost: $150-$250
3. Faulty Shift Solenoid or Valve Body ⚪ Low Probability → Shop Transmission Valve Body With mileage and heat, solenoids can fail electrically or valves within the valve body can stick or wear, causing hydraulic pressure loss to the 4th gear clutch pack. Debris from normal wear can accelerate this issue.
   How to confirm: A professional-grade scan tool can command the shift solenoids and monitor their response. Pressure testing specific ports on the transmission case may also be necessary. This requires accessing the valve body inside the transmission by removing the pan.
   Typical fix: Replace the specific faulty shift solenoid or the entire valve body assembly. A remanufactured valve body is often a cost-effective option.
   Est. part cost: $400-$1000

Rare But Worth Checking

• Internal Transmission Mechanical Failure: → Shop Transmission Assembly This includes worn or damaged 4th gear clutch packs or other internal components like planetary gears. This is a costly repair and should be considered a last resort after all software, hydraulic, and electronic issues have been ruled out.
• Failing Torque Converter: → Shop Automatic Transmission Torque Converter While less likely to cause a specific gear ratio code, a failing torque converter lock-up clutch can shed debris that contaminates the fluid and clogs the valve body, indirectly leading to shifting issues and codes like P0734.

Diagnosis Steps

1. Scan the vehicle's computer system to confirm P0734 is present and document any other stored codes, especially P1DCD and P1B13.
2. CRITICAL: Contact a Dodge dealership with your VIN to determine if TSB #21-041-17, #21-019-16, or #21-006-15 REV-B apply. If a software update is available, this is the first and most likely fix. The dealer will use the wiTECH tool to perform the flash.
3. During the flash, the technician must ensure the battery voltage stays between 13.2 and 13.5 volts to prevent failure. The TSBs also note NOT to clear the adaptation memory cells after the flash, as this can cause poor shifting until the transmission re-learns.
🎬 Watch: Three methods to reset your Dodge transmission control module.
4. If no software update is available or the issue persists, check the transmission fluid level and condition. This requires the vehicle to be level and the fluid to be at a specific temperature (30-50°C), monitored by a scan tool.
5. If the fluid is low, top it off with the correct fluid (Mopar ZF 8&9 Speed ATF or equivalent like ZF Lifeguard 8) and inspect for leaks. If the fluid is dark or smells burnt, a full fluid and filter (pan) service is required.
6. If the fluid and software are correct, use an advanced scan tool to monitor the input and output speed sensors for erratic readings while driving. A discrepancy between the two can point to sensor failure or internal slippage.
7. Further diagnosis involves testing shift solenoid operation and hydraulic pressures within the valve body, which should be performed by a qualified transmission specialist.
8. If all other steps fail to identify the cause, an internal mechanical failure of the 4th gear clutch pack is likely, requiring a transmission rebuild or replacement.

Parts You'll Likely Need

• TCM Software Update — This is not a physical part but a service. It is the most common and manufacturer-recommended fix documented in multiple TSBs for the P0734 code on this vehicle.
  Trusted brands: Mopar (Dealer Service)
  OEM price range: $150-$300
  Aftermarket price range: N/A
• Transmission Pan and Filter Kit (OEM #52854834AB) — The filter for the 8HP45 transmission is integrated into the plastic oil pan. A fluid service requires replacing the entire pan assembly. This part number supersedes previous versions like 68233701AA.
  Trusted brands: Mopar, ZF, Filtran, ATP
  OEM price range: $120-$270
  Aftermarket price range: $80-$150
• Mopar ZF 8&9 Speed Automatic Transmission Fluid (OEM #68218925AB) — This is the manufacturer-specified fluid required for a fluid service. Using the correct fluid is critical for the performance and longevity of the ZF 8-speed transmission. ZF's own branded fluid is ZF Lifeguard 8.
  Trusted brands: Mopar, ZF Lifeguard 8, Liqui Moly Top Tec 1800, Red Line D6 ATF
  OEM price range: $25-$45 per quart
  Aftermarket price range: $15-$30 per quart

Related Codes That Often Appear With This One

• P1DCD — This code, 'TCM Monitoring Processor Performance Multiple Clutches Locked Up', is listed in the exact same TSBs as P0734, indicating they are often triggered together by the same software fault.
• P1B13 — This code, 'Park-By-Wire Unintended Out Of Park Position', is also mentioned in TSBs #21-041-17 and #21-023-15 REV. B alongside P0734 as part of the same software-related issue.

Technical Service Bulletins (TSBs) & Recalls

• 21-041-17: Supersedes 21-019-16. Involves reprogramming the TCM for P0734, P1DCD, P1B13 and notes that a radio FOTA update can cause the issue. Applies to Challenger, Charger, and 300 with 3.6L/8HP45.
• 21-019-16: Mentions P0734 as a code resolved by a software update to improve shift quality.
• 21-006-15 REV-B: Supersedes REV-A. Lists P0734 as a primary DTC that can be set due to software, along with harsh downshifting symptoms (8-6, 6-4, 6-3). Applies to Challenger with 3.6L/845RE.
• 21-006-15 REVA: Identifies the Malfunction Indicator Lamp (MIL) illuminating with DTC P0734 as a primary customer concern addressed by a software update.

Platform-Specific Known Issues

• The 2015 Dodge Challenger with the 3.6L engine and 8HP45 transmission is highly susceptible to setting a P0734 code due to a software issue within the Transmission Control

Mechanic-Grade Diagnostic Values

• Shift and TCC Solenoid Resistance — expected: Approximately 5.0 - 5.5 ohms at 68°F (20°C).. Failure: A reading significantly higher or lower than 5.5 ohms, or an open circuit, indicates a failed solenoid coil.
• Solenoid Mechanical Seal Vacuum Test — expected: A good solenoid should hold over 20 in-Hg of vacuum.. Failure: Any reading below 20 in-Hg indicates a compromised internal seal, meaning the solenoid is leaking pressure and should be replaced, even if its electrical resistance is correct.
• Transmission Fluid Temperature for Adaptation — expected: The adaptation learning procedure must be performed when the transmission oil temperature is between 50°C (122°F) and 100°C (212°F).. Failure: Attempting the learn procedure outside this temperature window will cause it to abort or fail.

Scan Tool Commands That Help

• wiTECH or AlfaOBD: Quick Learn — This procedure performs an adaptation of the clutches and should be run after replacing the valve body, solenoids, or the entire transmission to establish baseline shift parameters. It is also recommended by TSBs after a software flash.
• wiTECH or AlfaOBD: Reset Adaptive Values — This function clears the learned shift adaptation memory cells. It is a necessary step before performing a new adaptation drive learn procedure, but TSBs for the P0734 software flash specifically warn *not* to clear the memory cells as it can cause poor shifting until the transmission relearns on its own.
• wiTECH or equivalent professional scanner: Solenoid Control Test — This bidirectional command allows a technician to activate a specific gear's solenoid (e.g., the 4th gear solenoid) with the engine running to verify its mechanical and hydraulic function. A click should be heard, and a change in pressure should be observed on the live data screen.

Wiring & Ground Locations

• Fuse F29 (Front Fuse Box) — In the under-hood fuse box, located on the passenger side of the engine bay.. Fuse F29 is a key power supply for the Transmission Control Module (TCM). Loss of power from this fuse can cause various transmission codes, including P0734, and a complete loss of shifting.
• Fuse F37 (Front Fuse Box) — In the under-hood fuse box, located on the passenger side of the engine bay.. Fuse F37 is another critical power source for the TCM. Checking this 10A fuse is an essential first step in diagnosing any electrical issue with the transmission.
• Fuse F26 (Rear Fuse Box) — In the trunk, next to the battery. Note that not all vehicles will have this fuse, depending on the specific transmission options.. This fuse is also listed as a power source for the TCU/TCM. If present, it should be checked along with the front fuses.
• TCM Location — For the 8-speed transmission, the TCM is integrated into the valve body assembly (mechatronic unit) inside the transmission oil pan.. Unlike older models with an external TCM, diagnosing this requires dropping the transmission pan. A failure here necessitates replacing the entire valve body/TCM assembly, which is a significant repair.

Real Owner Repair Stories

• YouTube user Jamarrel J. (2015 Ram 2500 with 68RFE (Note: While not a Challenger, the diagnostic process for a persistent P0734 after fluid service is relevant)) — Check engine light with code P0734, transmission issues.
  ❌ Tried (didn't work) Initial diagnostics pointed towards common causes.
  ✅ What actually fixed it The ultimate fix was a valve body rebuild. The shop dropped the transmission pan, removed the valve body, and rebuilt it with a thicker separator plate, upgraded springs, and new seals to handle pressure more effectively.