P0734 on 2015-2020 Ram 1500: Gear 4 Incorrect Ratio Causes & Fixes

What's Unique About the 2015-2020 Ram 1500

The 2015-2020 Ram 1500, which spans two generations (DS and DT), primarily uses the excellent ZF-designed 8-speed automatic transmission (or a Chrysler-built variant like the 845RE/8HP70). While very reliable, these are complex, electronically controlled units. As shown by multiple Technical Service Bulletins (TSBs), many early shifting issues, including those triggering P0734, were related to software logic. [TSB #21-025-15, 29, 33] FCA (now Stellantis) released several updates to improve shift quality and correct the premature setting of error codes like P0734, making a software check a crucial first step for this specific platform. [TSB #21-008-20, TSB #21-036-16]

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Generation note: This range covers the late fourth-generation Ram 1500 'DS' (including the 'Classic' model sold through 2020) and the early fifth-generation 'DT' (starting in 2019). Both generations commonly use a version of the 8-speed automatic transmission (8HP70, 8HP75), but specific internal components, software, and TSBs can differ. It's important to verify which generation you have when sourcing parts or referencing service information.

Symptoms You May Notice

• Check Engine Light is on
• Transmission gets stuck in one gear (limp mode).
• Harsh, delayed, or erratic shifting.
• A bump or clunk felt during upshifts. [TSB #21-008-20]
• Vehicle will not shift into 4th gear.
• Engine RPMs are higher than usual at cruising speeds
• Decreased fuel economy.
• Transmission slipping.

Most Likely Causes

1. Low or Dirty Transmission Fluid 🔴 High Probability The 8-speed transmissions in these trucks do not have a traditional dipstick, making it difficult for owners to check the fluid level. 🎬 Watch: How to check fluid levels without a dipstick. An incorrect level or fluid contaminated with clutch material can cause hydraulic pressure issues, leading to slippage.
   How to confirm: The fluid level and condition must be checked by a qualified technician. The procedure requires the vehicle to be level, running, and the transmission fluid temperature to be between 30°C (86°F) and 50°C (122°F) for an accurate reading.
   Typical fix: Perform a transmission fluid drain and fill, or a full fluid exchange, using the correct Mopar ZF 8&9 Speed ATF or equivalent (like ZF Lifeguard 8). The integrated pan/filter assembly should also be replaced. 🎬 See this 8HP70 transmission fluid and filter change walkthrough.
   Est. part cost: $150-$300
2. Faulty Transmission Control Module (TCM) Software 🔴 High Probability Multiple TSBs were issued by Ram to address shifting irregularities and incorrect diagnostic trouble code triggers. TSB 21-008-20 specifically notes improvements for prematurely setting P0734. [TSB #21-008-20, TSB #21-036-16, TSB #21-007-20, TSB #21-025-15 REV-A, 29, 33]
   How to confirm: A Ram dealership or a well-equipped independent shop using a wiTECH scan tool can check the current software version of the TCM against the latest available updates for the vehicle's VIN.
   Typical fix: The TCM needs to be reprogrammed (flashed) with the latest software version. This is a common first step for dealers.
   Est. part cost: $100-$250
3. Failing Shift Solenoid or Solenoid Pack 🟡 Medium Probability Solenoids are electro-mechanical valves that control fluid flow to engage gears. The solenoid responsible for the 4th gear clutch pack can fail electrically or become clogged with debris over time, preventing proper engagement.
   How to confirm: A diagnostic scan tool can command the solenoids to activate and monitor the response. Resistance checks can also be performed on the solenoid's electrical circuit. Often, this is diagnosed after fluid/filter service and software updates fail to resolve the issue.
   Typical fix: Requires dropping the transmission pan and valve body to replace the individual solenoid or the entire solenoid pack. 🎬 Watch: Step-by-step guide to replacing the transmission solenoid pack.
   Est. part cost: $200-$600
4. Faulty Input/Output Speed Sensors ⚪ Low Probability The TCM relies on these sensors to calculate the gear ratio. If one provides an erratic or incorrect signal, the TCM can misinterpret the data and set a P0734 code. These sensors are relatively inexpensive and easy to access.
   How to confirm: A scan tool can monitor the live data from both sensors while driving. If one is dropping out or showing erratic readings, it is likely faulty. A user on YouTube documented fixing their P0734/P0735 codes by replacing both sensors.
   Typical fix: The sensors are typically located on the outside of the transmission case (driver's side) and are held in by a single 8mm bolt, making them relatively easy to replace.
   Est. part cost: $40-$100

Rare But Worth Checking

• Internal Transmission Failure: This could be a worn 4th gear clutch pack, damaged seals, or other mechanical issues inside the transmission. This is the most expensive repair and should only be considered after all other possibilities have been exhausted. A common failure point in the 8HP70 is the 'A' clutch pressure plate, which is prone to cracking. A service manual also suggests checking the 4C accumulator piston and seals.
• Valve Body Malfunction: The valve body is a complex maze of channels and valves. A stuck valve, worn bore, or blockage can prevent proper fluid pressure from reaching the 4th gear clutch pack. In some cases, the entire valve body may need to be replaced.
• Faulty Powertrain Control Module (PCM) or TCM: While rare, the control module itself can fail. This is usually a last resort after wiring and all other components have been verified as good. One owner on YouTube ultimately fixed P0734 and P0735 codes by replacing the PCM after changing speed sensors and the solenoid pack did not fix the issue.

Diagnosis Steps

1. Check and verify the transmission fluid level and condition. This should be the absolute first step, following the specific procedure for the 8-speed transmission (vehicle level, running, and fluid temp between 30-50°C or 86-122°F).
2. Scan the TCM for any other stored trouble codes to see if the problem is isolated or part of a larger issue. Note any solenoid, speed sensor, or pressure-related codes.
3. Check with a dealer or use a service database (like ALLDATA) to see if any TSBs related to software updates (e.g., 21-008-20, 21-036-16) apply to your vehicle's VIN.
4. Use a professional scan tool to monitor live data from the input and output speed sensors while driving to check for dropouts or erratic signals.
5. If fluid and software are good, perform diagnostic tests on the shift solenoids and their wiring, including resistance checks and commanded activation with a scan tool.
6. If solenoids test good, a more invasive inspection of the valve body for stuck valves or blockages may be necessary.
7. As a final step, if all else fails, inspect internal transmission components like the 4C clutch pack for wear or damage.

Parts You'll Likely Need

• Automatic Transmission Fluid and Filter Kit (OEM #68218925AB (Fluid), 68225344AA (Pan/Filter for 8HP70)) — Low or degraded fluid is a primary cause of shifting issues. The 8-speed transmission uses an integrated pan and filter assembly that must be replaced during a fluid service.
  Trusted brands: Mopar, ZF (Lifeguard 8 Fluid)
  OEM price range: $150-$250
  Aftermarket price range: $100-$200
• Transmission Shift Solenoid — A specific solenoid controls the shift to 4th gear. If it fails, the gear cannot be engaged correctly. Often replaced as a complete solenoid pack integrated with the valve body.
  Trusted brands: Mopar, Bosch, ZF
  OEM price range: $300-$600
  Aftermarket price range: $200-$450

Related Codes That Often Appear With This One

• P0731 — This is an incorrect ratio code for 1st gear. Its presence with P0734 is specifically mentioned in TSB #21-008-20, suggesting a more widespread hydraulic or electronic issue that can often be fixed with a software update.
• P0733 — This is an incorrect ratio code for 3rd gear. Its presence with P0734 points towards a problem that is not isolated to a single gear circuit and is also mentioned in TSB #21-007-20.
• P0735 — Incorrect ratio for 5th gear. A user on YouTube reported getting P0734 and P0735 together, which ultimately required a PCM replacement after other parts failed to fix it.
• P1DCD — This code indicates a TCM processor performance issue. It often appears alongside gear ratio codes when a software update is needed, as cited in TSB #21-036-16.

Technical Service Bulletins (TSBs) & Recalls

• 21-008-20
• 21-036-16
• 21-007-20
• 21-025-15 REV-A

Platform-Specific Known Issues

• Multiple Technical Service Bulletins (TSBs) have been issued by Ram for the 2015-2020 model years to address harsh shifting, clunking, and incorrect gear ratio codes via software updates to the Transmission Control Module (TCM).
• TSB 21-008-20 specifically addresses P0734 on 2020 Ram 1500s with the 5.7L eTorque engine and 8HP75 transmission, recommending a TCM flash to fix premature code setting and shift bumps.
• The sealed nature of the 8-speed transmission (no dipstick) makes owner-level fluid checks impossible, often leading to neglected fluid maintenance which can cause this code.

Mechanic-Grade Diagnostic Values

• Transmission Shift Solenoid Resistance — expected: Generally between 5 and 30 ohms. Specifically, regulating solenoids (like those in the 8HP) are typically under 10 ohms, while on/off solenoids are over 10 ohms.. Failure: A reading of infinite resistance (OL) indicates an open coil, while a reading of zero indicates a short. Either requires replacement.
• TCM Clutch Adaptation Values (Pressure) — expected: Normal adapted values should not exceed +/- 350 mbar.. Failure: A value greater than 350 mbar or less than -350 mbar suggests significant clutch wear or a hydraulic problem, potentially requiring a rebuild.

Hidden / Shadow Codes Worth Checking

• P0700 - Transmission Control System Malfunction: This is a generic, informational code set by the PCM to indicate that the TCM has stored a more specific fault. It almost always accompanies codes like P0734. A basic scanner might only show P0700, while a more advanced tool is needed to read the underlying TCM codes. (see via A professional scan tool that can communicate directly with the Transmission Control Module (TCM), not just the main ECU/PCM.)

Scan Tool Commands That Help

• wiTECH (or equivalent professional scanner): RESET ADAPTIVE VALUES / QUICK LEARN — This function should be performed after internal transmission repairs or if shift quality is poor. It forces the TCM to relearn clutch fill times and pressures from a baseline. Note: TSBs often advise NOT to clear adaptives for a simple software flash, as it can cause temporarily poor shifting.
• wiTECH (or equivalent professional scanner): Direct Gear Clutch Test — This bidirectional test allows a technician to command specific clutches to engage and disengage to isolate a hydraulic circuit or solenoid problem without a test drive.
• wiTECH (or equivalent professional scanner): Security Gateway Module (SGM) Bypass — On 2018 and newer Ram 1500s, a Security Gateway Module blocks bidirectional commands from the OBD-II port. A bypass cable or device is required to connect directly to the vehicle's internal network to perform functions like resetting adaptations or running active tests on the TCM.

Wiring & Ground Locations

• PCM/TCM Connector — On the firewall of the engine bay. A user on a Ram forum reported that a loose connection or short in this main connector caused intermittent transmission service messages and shifting issues when hitting bumps.. A poor connection at the main module connector can interrupt power, ground, or sensor data (like speed sensor signals) to the TCM, causing it to set false gear ratio codes.
• Transmission Harness to Starter — This harness section connects the main transmission harness to the starter motor area.. Damage to this harness could affect power feeds or communication lines running to the transmission, potentially causing various electrical faults including gear ratio errors.
• TCM Sensor Ground (Internal) — Inside the TCM, various pins provide a ground reference for sensors. For example, on some Dodge trucks, pin 53 is the ground for the line pressure sensor.. A failing ground circuit inside the TCM itself can cause high resistance, leading to incorrect sensor readings and false codes. One technician diagnosed a bad TCM by finding high, fluctuating resistance on an internal ground pin.

Real Owner Repair Stories

• YouTube user 'Fix It Frank' (2010 Dodge Ram 1500 (Note: Pre-dates the specified range but demonstrates a relevant failure pattern)) — Check engine light with codes P0734 and P0735 (Gear 5 Incorrect Ratio). Truck would randomly get stuck in 4th gear. The issue was intermittent.
  ❌ Tried (didn't work) Replaced the input speed sensor, Replaced the output speed sensor, Replaced the entire transmission solenoid pack (valve body)
  ✅ What actually fixed it Replacing the Powertrain Control Module (PCM/ECM). After spending approximately $600 on sensors and solenoids with no success, a refurbished PCM (costing around $560) resolved the codes and shifting issues.
• YouTube user 'SVCRUZA' (Vehicle year not specified, but experiencing P0734.) — Check engine light for P0734.
  ❌ Tried (didn't work) Initially planned to replace the solenoid pack if sensors didn't work.
  ✅ What actually fixed it Replacing both the input and output speed sensors on the transmission. The problem went away after replacement, making it an inexpensive first step that was successful.
• RamForum.com user 'BCRPrez' (2017 Ram 1500 5.7L with 6-speed (Note: 6-speed, not 8-speed, but shows a physical trigger)) — After hitting a large pothole on a gravel road, the check engine light came on with code P0734. The transmission would shift 1-2-3-4, but then high-rev/slip when trying for 5th, and get stuck in 4th gear.
  ❌ Tried (didn't work) Clearing the code and disconnecting the battery did not resolve the issue.
  ✅ What actually fixed it The user was still in the diagnostic phase, but the critical detail is that a significant physical impact immediately preceded the P0734 code, suggesting a potential wiring or connector issue was triggered.

When the Usual Fixes Don't Work

• While faulty speed sensors are listed as a 'low' probability cause, there are multiple owner-documented cases where replacing the input and/or output speed sensors was the final, successful fix for a P0734 code after other diagnostics were inconclusive. This suggests that intermittent sensor failure, which may not be obvious during live data monitoring, could be a more common cause than service literature implies and is a relatively inexpensive repair to attempt before moving to the valve body or PCM.

OEM Part Supersession History

• 68142478AB → 68225344AA — This is the OEM Mopar transmission pan with integrated filter for the 8HP70. Part numbers are often updated over time for minor revisions or supplier changes.
  Heads up: Both part numbers refer to the same application for the 8HP70 transmission pan/filter kit.

Model Year Variations Within This Range

• 2015-2018 (DS Generation): Primarily used the first-generation ZF 8HP70 transmission. Many early software-related issues and TSBs for P0734 apply to these years. [TSB #21-036-16, TSB #21-025-15 REV-A]
• 2019-2020 (DT Generation): Began using the second-generation 8HP75 transmission, which features an optimized oil pump and other internal improvements for efficiency and durability. While mechanically similar, the TCM software and some internal parts are not interchangeable with the 8HP70.
• 2018+: Introduction of the Security Gateway Module (SGM). This requires a special bypass cable or an authorized shop tool with an active subscription to perform bidirectional controls on the TCM, such as resetting adaptations or running solenoid tests.