P0734 on 2015 Chrysler 300: Gear 4 Incorrect Ratio Causes and Fixes

What's Unique About the 2015-2015 Chrysler 300

For the 2015 Chrysler 300 equipped with the 8-speed 8HP45 or 845RE automatic transmission, this code is frequently a software issue rather than a major 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 even notes that a Flash Over The Air (FOTA) radio update could 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 car 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.
• Vehicle may not shift into 4th gear.
• Harsh or delayed shifting, especially into or out of 4th gear.
• Transmission slipping (engine RPMs rise without an increase in speed).
• Vehicle may enter 'limp mode', locking the transmission in a single gear (e.g., stuck in 4th).
• Reduced fuel economy.
• A bump or jolt felt during downshifts, particularly the 2-1 shift.

Most Likely Causes

1. Transmission Control Module (TCM) Software Requires Update 🔴 High Probability → Shop Transmission Assembly Multiple Manufacturer TSBs confirm that outdated software can cause this code, sometimes triggered by unrelated events like a radio update. This is the most common and well-documented cause for this specific vehicle.
   How to confirm: A Chrysler dealership or a shop with a factory-level scan tool (like wiTECH) can check the current software version against available updates for your VIN.
   Typical fix: Flashing the TCM with the latest software version. The TSBs note that a stable power supply (13.2-13.5 volts) is critical during the flash, and that adaptation memory cells should not be cleared.
   Est. part cost: $150-$300 (Labor for flashing at a dealer or specialty shop).
2. Low or Dirty Transmission Fluid 🟡 Medium Probability → Shop Transmission Assembly The ZF 8-speed transmission is 'sealed for life' but fluid can still degrade from heat or leak, causing hydraulic pressure issues that lead to slipping. Contaminated fluid can also clog narrow passages in the valve body.
   How to confirm: The fluid level and condition must be checked. This is difficult as there is no dipstick; it requires a special procedure using a fill plug on the transmission while monitoring the fluid temperature with a scan tool (target is 30-50°C).
   Typical fix: Perform a transmission fluid and filter (which is integrated into the pan) service. Use only the specified Mopar 8&9 Speed ATF (ZF Lifeguardfluid 8).
   Est. part cost: $250-$450.
3. Faulty Shift Solenoid ⚪ Low Probability → Shop Transmission Valve Body A solenoid responsible for activating the 4th gear clutch pack can fail electronically or become clogged with debris, preventing proper hydraulic fluid flow.
   How to confirm: A technician can command the solenoid on and off with a bi-directional scan tool and check its resistance. This requires dropping the transmission pan. Often, a solenoid-specific code will also be present.
   Typical fix: Replace the faulty solenoid. This is often done as part of a valve body service.
   Est. part cost: $50-$150 per solenoid
4. Failing Valve Body ⚪ Low Probability → Shop Transmission Valve Body The valve body is the hydraulic control center. Debris from normal wear can clog its small passages or cause valves (like the main pressure regulator valve) to stick, leading to incorrect pressure for the 4th gear shift. This is a known failure point on ZF 8HP transmissions.
   How to confirm: This is usually diagnosed after ruling out software, fluid, and solenoids. A technician may perform hydraulic pressure tests or inspect the valve body for worn bores and sticking valves after removal.
   Typical fix: The valve body may be cleaned, rebuilt with oversized valves, or replaced. The TCM is integrated into the valve body on this transmission.
   Est. part cost: $500-$1200 for a remanufactured unit.

Rare But Worth Checking

• Internal Mechanical Failure: This is the worst-case scenario. If the fluid is full of metal shavings, it indicates a worn clutch pack (e.g., the OD clutch), broken pressure plate, or other internal damage, requiring a full transmission rebuild or replacement.
• Faulty Input/Output Speed Sensor: While less common for this specific platform's software issues, a failing input or output speed sensor can send incorrect data to the TCM, causing it to miscalculate the gear ratio. Usually, a failing sensor will set its own specific trouble code (e.g., P0716, P0721).

Diagnosis Steps

1. Scan the TCM for all stored trouble codes. Note any codes that appear alongside P0734, especially P1DCD and P1B13.
2. CRITICAL: Check with a Chrysler dealer or authorized service center for any open recalls or available TSBs related to transmission software for your vehicle's VIN. This is the most likely fix. Specifically reference TSBs #21-041-17, 🎬 Watch: A breakdown of common causes for the P0734 code #21-019-16, and #21-023-15 REV-B.
3. If a software update is available and applicable, have the TCM flashed. This service costs approximately $150-$300 and is the most probable solution.
4. If no software update is needed or the problem persists, the next step is to check the transmission fluid level and condition. This requires lifting the vehicle level, bringing the transmission to a specific temperature (30-50°C) monitored by a scan tool, and using the fill port. This is best left to a professional.
5. If the fluid is low, top it off with the correct fluid (Mopar 8&9 Speed ATF / ZF Lifeguard 8) and check for leaks.
6. If the fluid is dark, burnt, or contains excessive metallic debris, a transmission fluid and filter (pan) service is necessary. Significant metal debris points to internal failure requiring a rebuild or replacement.
7. If fluid and software are good, a technician will need to test the shift solenoids and inspect the valve body. This requires removing the transmission pan and may involve pressure testing.
8. If all other possibilities are exhausted, internal mechanical failure (such as a worn OD clutch pack) is suspected, which would require transmission removal and teardown.

Parts You'll Likely Need

• Transmission Pan and Filter Kit (OEM #52854834AB) — If the cause is old fluid, the filter (which is integrated into the plastic pan on the ZF 8-speed) must be replaced during the service.
  Trusted brands: Mopar, ZF
  OEM price range: $200-$300
  Aftermarket price range: $150-$250
• Mopar 8&9 Speed Automatic Transmission Fluid (OEM #68218925AB) — This is the required fluid for the ZF 8-speed transmission. Using the wrong fluid can cause severe damage. The equivalent is ZF Lifeguardfluid 8.
  Trusted brands: Mopar, ZF
  OEM price range: $25-$40 per quart
• Transmission Valve Body (OEM #ZF Part: 1090127305 (Verify application)) — If solenoids or hydraulic passages are faulty, replacing the entire valve body assembly is a common repair. The TCM is integrated with this part.
  Trusted brands: Mopar, ZF (remanufactured)
  OEM price range: $500-$1200 for a remanufactured unit.

Related Codes That Often Appear With This One

• P1DCD — This code for 'TCM Monitoring Processor Performance' is listed alongside P0734 in multiple TSBs for the 2015 Chrysler 300, indicating a software-related fault.
• P1B13 — This 'Park-By-Wire Unintended Out Of Park Position' code also appears with P0734 in TSBs, suggesting a broader TCM software issue.
• P0730 — This is a general 'Incorrect Gear Ratio' code. P0734 is the specific version for 4th gear. Seeing P0730 alongside it points to a more significant transmission problem that may not be isolated to a single gear.
• P0735 — Incorrect Gear Ratio for 5th gear. If seen with P0734, it suggests a wider problem than just the 4th gear circuit, potentially pointing towards a failing clutch pack shared between gears, a valve body issue, or low fluid pressure.

Technical Service Bulletins (TSBs) & Recalls

• Bulletin #21-041-17: Mentions P0734 - Gear 4 Shift Incorrect Ratio and links it to FOTA radio updates and improved shift quality. It supersedes TSB 21-019-16.
• Bulletin #21-019-16: Cites P0734 - GEAR 4 SHIFT INCORRECT RATIO as a code addressed by a software update for improved shift quality for the 3.6L engine with 845RE or 8HP45 transmissions.
• Bulletin #21-023-15 REV-B: Lists P0734 - Gear 4 Shift Incorrect Ratio as a code fixed by a software update, also covering the 3.0L engine with the 845RE.
• Bulletin #21-023-15: Notes that a Malfunction Indicator Lamp may illuminate with DTC P0734 stored in the TCM memory, addressable by a software flash.

Platform-Specific Known Issues

• Multiple TSBs (#21-041-17, #21-019-16, #21-023-15 REV-B, #21-023-15) were issued by Chrysler for the 2015 model year specifically mentioning P0734 as a condition that can be resolved with a TCM software update to improve shift quality.
• TSB #21-041-17 explicitly states that a Flash Over The Air (FOTA) radio update could cause the transmission to default to higher shift points and trigger codes like P0734.

Mechanic-Grade Diagnostic Values

• Shift Solenoid Resistance — expected: Approximately 5.0 - 6.0 Ohms at 68°F (20°C). Failure: A reading significantly higher or lower than the expected range, or an open circuit (infinite resistance), indicates a failed solenoid coil.
• Live Data: Gear Ratio Calculation — expected: The TCM expects a gear ratio of approximately 1.285:1 for 4th gear in the 8HP45. A scan tool should show the calculated ratio (Input RPM / Output RPM) matching this target.. Failure: If the calculated ratio deviates from the target by more than a few percent while in 4th gear, it confirms slippage and triggers the code.
• Clutch Apply Pressure (4th Gear) — expected: While specific values vary, a scan tool with hydraulic test capabilities should show a target pressure of around 200-240 psi for clutch engagement.. Failure: Pressure that is significantly below the specified range indicates a hydraulic leak (bad seal, worn bushing, valve body issue), while excessively high pressure could mean a stuck regulator valve.

Hidden / Shadow Codes Worth Checking

• Event Data / Freeze Frame: While not a 'shadow code' in the typical sense, a dealer-level tool like wiTECH captures a detailed 'Event Data' snapshot at the moment DTC P0734 is set. This includes all sensor readings, clutch pressures, temperatures, and solenoid command states, providing much deeper insight than standard freeze-frame data. (see via Chrysler wiTECH Diagnostic Application.)

Scan Tool Commands That Help

• wiTECH or equivalent professional scanner (Autel, Launch, etc.): Quick Learn / Clutch Volume Relearn — This procedure is mandatory after replacing the valve body/TCM, transmission, or sometimes even after a fluid service. It forces the TCM to relearn the hydraulic volume and timing needed to apply each clutch pack, preventing harsh shifts and incorrect ratio codes.
• wiTECH or equivalent professional scanner: Reset Adaptive Shifting — This function clears the learned shift adaptations based on driving style. While sometimes helpful for shift quality issues, the TSBs for P0734 specifically warn *against* clearing adaptations during the software flash. It should be used with caution and followed by a proper drive cycle.
• wiTECH or equivalent bi-directional scanner: Solenoid Actuation Test — To diagnose a specific solenoid, a technician can command it on and off while monitoring for an audible click or a change in hydraulic pressure. This helps isolate a faulty solenoid without immediate removal.

Wiring & Ground Locations

• Main Transmission Harness Connector — A large, 16-pin round connector located on the passenger side of the transmission case.. This is the sole electrical interface for the entire mechatronic unit (TCM, solenoids, sensors). Corrosion or a loose connection here can interrupt power, ground, or CAN bus communication, leading to various transmission codes, including P0734.
• TCM/Transmission Ground — The primary ground for the internally-mounted TCM is through the transmission case itself. The case is then grounded to the chassis via a braided ground strap, typically running from the engine block or bell housing to the vehicle's frame rail.. A corroded or loose main ground strap can cause floating voltages and erratic behavior from the TCM and solenoids, potentially leading to incorrect ratio calculations and other electronic faults.
• TCM Power Fuses — Power for the TCM is supplied through fuses located in the front fuse box (under the hood) and potentially the rear fuse box (in the trunk). Specific locations are F29 and F37 in the front box.. Loss of power to the TCM will cause a total loss of shifting and communication, but an intermittent power supply due to a poor fuse connection could cause unpredictable behavior and fault codes.

Real Owner Repair Stories

• 300cforums.com user report (2015 Chrysler 300) — Check engine light with code P0734 and harsh shifting.
  ❌ Tried (didn't work) Dealer performed TSB software update for the TCM., Transmission fluid and filter service.
  ✅ What actually fixed it The problem was ultimately resolved by replacing the entire transmission valve body (mechatronic unit).
• lxforums.com user report (2015 Chrysler 300 3.6L) — P0734 code, vehicle would sometimes feel like it was slipping or shifting harshly.
  ❌ Tried (didn't work) TCM software flashed by the dealership.
  ✅ What actually fixed it After the software update failed to fix the issue, the technician found excessive metal debris in the transmission pan. The final repair was a complete transmission replacement due to internal mechanical failure.

When the Usual Fixes Don't Work

• While TSBs strongly point to a software fix, multiple owner-reported repairs show that this is not always the case. In several documented instances, the P0734 code persisted after the dealer performed the software flash. The subsequent, successful repair often involved replacing the valve body (mechatronic unit). In more severe cases, where metal was found in the pan, a full transmission replacement was required. This indicates that while software should always be the first check, it is not a guaranteed fix, and underlying hydraulic or mechanical issues can present with the exact same code.

OEM Part Supersession History

• 68218925AA (Quart), 68157995AA → 68218925AB (Quart) — Part number consolidation and potential minor formula updates over time.
  Heads up: The Mopar 8&9 Speed ATF (part no. 68218925AB) is specified. The direct equivalent is ZF Lifeguard 8. While other fluids may claim compatibility, using the OEM/ZF fluid is critical to prevent shift quality issues and further codes, especially when addressing a pre-existing fault.