P1720 on 2015-2016 Ram ProMaster City: Output Speed vs. Wheel Speed Fault

What's Unique About the 2015-2016 Ram PROMASTER CITY

The 2015-2016 ProMaster City, equipped with the ZF-designed 948TE 9-speed automatic transmission, is known for transmission control issues that are primarily software-related. Ram released several Technical Service Bulletins (TSBs) specifically to address P1720 and other related transmission codes by reprogramming the TCM. This is different from many other vehicles where a similar code would more likely point to a faulty sensor. Additionally, this platform was subject to a specific safety recall (NHTSA 16V-202 / FCA S24) for a poorly-located CAN-C bus connector that is prone to moisture and corrosion, directly causing communication loss with the TCM and triggering this code.

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, MIL) is on.
• Transmission may enter limp mode (stuck in one gear).
• Harsh or erratic shifting.
• Potential loss of forward or reverse gear intermittently.
• Vehicle may not restart after being turned off.
• Unexpected shift to neutral while driving.
🎬 See common signs and symptoms of a failing TCM

Most Likely Causes

1. Outdated Transmission Control Module (TCM) Software 🔴 High Probability → Shop Transmission Assembly Ram issued multiple TSBs (e.g., 21-018-17 REV. C) specifically to correct this and other related codes via a software update for the 948TE transmission. These early model years of the 9-speed transmission had numerous software bugs that were later resolved with flashes.
   How to confirm: A dealer or a properly equipped independent shop can check the current software version against the latest available version using a wiTECH scan tool. The TSB requires the wiTECH software to be at the latest release before performing the procedure.
   Typical fix: Reprogramming (flashing) the TCM to the latest software version. The Powertrain Control Module (PCM) may also require an update simultaneously. The flash process takes approximately 4 minutes, but the total service time is longer.
   Est. part cost: $0
2. Corroded CAN-C Bus Connector 🟡 Medium Probability A recall (NHTSA ID: 16V-202, FCA Campaign S24) was issued for 2015-2016 models because a low-voltage CAN-C 'Star' harness connector located under the driver's seat was susceptible to water intrusion and corrosion, causing communication loss with the TCM. This applies to vehicles built through March 5, 2016.
   How to confirm: Inspect the CAN-C 'Star' connector for signs of water damage or green/white corrosion on the pins. Access involves removing the driver's side door sill plate and lifting the carpet in front of the seat riser.
   Typical fix: Performing the S24 recall procedure. A dealer will inspect the connector. If corroded, the connector is replaced. In all cases, the harness and connector are relocated to a safer position under the instrument panel. This is a free repair under the recall.
   Est. part cost: $0 (if recall applies)
3. Faulty Transmission Control Module (TCM) ⚪ Low Probability → Shop Transmission Assembly This code is often seen with other TCM internal performance codes (like P1634 and P1DCD), suggesting the module itself can fail.
   How to confirm: This is a diagnosis of exclusion. If the software is up-to-date, the S24 recall has been performed, and all wiring/sensors test good, the TCM is the likely culprit.
   Typical fix: Replacing the TCM and performing a 'Proxy Alignment' procedure to configure the new module to the vehicle. A used TCM part number may be P50046628 for a 2015 model. A new Mopar TCM could be part number 68225514AB, which fits multiple FCA vehicles with the 948TE transmission.
   Est. part cost: $400-$800
4. Wiring Harness or Connector Issue ⚪ Low Probability
   How to confirm: Visually inspect the wiring harness and connectors between the output speed sensor, wheel speed sensors, and the TCM for any chafing, breaks, or corrosion, especially if the S24 recall for the CAN-C connector was not performed.
   Typical fix: Repairing the damaged section of the wire or replacing the faulty connector.
   Est. part cost: $10-$100

Rare But Worth Checking

• Faulty Transmission Output Speed Sensor: → Shop Transmission Assembly While the code directly names the sensor, it's less commonly the root cause on this platform compared to software or TCM issues. It should only be replaced after other causes are ruled out. The input and output speed sensors are part of a single assembly located under the valve body.
• Faulty Wheel Speed (ABS) Sensor: → Shop ABS Wheel Speed Sensor An erratic signal from any of the four wheel speed sensors can cause the rationality check to fail. This would typically be accompanied by ABS or traction control warning lights and codes.

Diagnosis Steps

1. Scan the TCM for all diagnostic trouble codes. Note P1720 and any other codes present, as they are critical for diagnosis.
2. Check for applicable recalls and Technical Service Bulletins (TSBs) for your vehicle's VIN. Specifically, look for Safety Recall S24 (NHTSA 16V-202) and TSB 21-018-17 REV. C.
3. If the S24 recall is open, have a dealer perform the inspection and relocation of the CAN-C bus connector. This is a primary suspected cause.
4. Using a dealer-level scan tool (like wiTECH), verify if the TCM and PCM software are up-to-date. If not, perform the software update as described in TSB 21-018-17 REV. C. This is the most likely fix.
5. If software is current and the recall is complete, use the scan tool to monitor live data from the transmission output speed sensor and all four wheel speed sensors while driving. Look for any signal that is erratic, dropping out, or inconsistent with the others.
6. If a sensor signal is faulty, inspect its wiring and connector for damage. If the wiring is intact, test the sensor's resistance and signal output.
7. If all sensors and wiring test good and the software is updated, the fault is likely with the Transmission Control Module (TCM) itself. A proxy alignment will be required if the TCM is replaced.

Parts You'll Likely Need

• Transmission Control Module (TCM) Reprogramming — This is the most common fix, as specified by multiple manufacturer TSBs to resolve software glitches causing the code.
  OEM price range: $150-$300 (Labor for flash)
  Aftermarket price range: N/A
• Transmission Control Module (TCM) (OEM #68225514AB) — If a software update does not fix the issue and wiring is confirmed good, the module itself may have an internal fault, especially if other TCM-related codes are present.
  Trusted brands: Mopar
  OEM price range: $500-$800
  Aftermarket price range: $300-$600
• CAN-C Bus Connector Repair — Required if the connector under the driver's seat is found to be corroded. This repair is covered for free under recall S24.
  Trusted brands: Mopar
  OEM price range: $0
  Aftermarket price range: N/A

Related Codes That Often Appear With This One

• P0887 — TCM Power Control Circuit High. Listed in the same TSB as P1720.
• P1634 — TCM Internal Watchdog Performance. Listed in the same TSB as P1720.
• P1DCD — TCM Monitoring Processor Performance Multiple Clutches Locked Up. Listed in the same TSB as P1720.
• P0711 — Transmission Fluid Temperature Sensor A Circuit Range-Performance. Listed in the same TSB as P1720.
• P1DAD — Input Shaft-Output Shaft Direction Correlation. Also addressed by TSB 21-018-17 REV. C.

Technical Service Bulletins (TSBs) & Recalls

• 21-018-17 REV. C: Flash: Transmission Diagnostic And Shift Enhancements. This bulletin involves reprogramming the TCM with the latest software to fix P1720 and other codes. It also notes the PCM must be updated.
• 21-018-17 REV. A: Flash: Transmission Shift and Driveability Enhancements. An earlier version of the TSB to update TCM software.
• 2101817: Original TSB mentioning P1720 as a code fixed by a software update.

Platform-Specific Known Issues

• A recall (FCA S24 / NHTSA 16V-202) was issued for potential corrosion in the CAN-C bus connector under the driver's seat, which can cause a loss of communication with the TCM, leading to various transmission faults including unexpected shifts to neutral.
• Multiple TSBs (21-018-17 REV. C, 21-018-17 REV. A, 21-039-15 REV B) were released by Ram to address this code with a software update for the TCM and PCM.

Mechanic-Grade Diagnostic Values

• Resistance of (T14) Output Speed Sensor Signal circuit and (T72) Sensor Ground circuit — expected: Below 5.0 Ohms. Failure: Resistance above 5.0 Ohms indicates an open in the circuit.
• Resistance between (T14) Output Speed Sensor Signal circuit and ground — expected: Above 100k Ohms. Failure: Resistance below 100k Ohms indicates a short to ground.
• Battery voltage during TCM flash procedure — expected: Between 13.2 and 13.5 volts. Failure: Voltage dropping below 13.2V or climbing above 13.5V can cause the flash to fail or be aborted.

Scan Tool Commands That Help

• wiTECH: Reprogram the TCM — This is the primary repair step outlined in TSB 21-018-17 REV. C to fix software glitches causing P1720. The PCM must also be updated.
• wiTECH: Proxy Configuration Alignment (Proxy Alignment) — This procedure is required after replacing the Transmission Control Module (TCM) to ensure the new module is correctly configured to the vehicle's other systems. Failure to perform this can result in 'ECU Configuration Mismatch' codes and persistent shifting problems.
• wiTECH: Reset Adaptive Values — This function can be used to clear learned shift patterns. However, TSBs for the P1720 flash specifically advise NOT to clear the shift adaptives, as it may cause rough shifting until the transmission relearns.
• wiTECH: Quick Learn Procedure — This procedure, which takes 2-5 minutes, may be required after certain transmission repairs or software updates to help the TCM learn clutch engagement points. It should only be performed after ensuring no other DTCs are present.

Wiring & Ground Locations

• CAN-C 'Star' Connector — Under the driver's seat, beneath the carpet in front of the seat riser (on pre-recall models).. This connector is a central communication hub. It is highly susceptible to water damage and corrosion due to its original location, causing loss of communication with the TCM and triggering P1720. This was the subject of recall S24.
• Primary Engine/Transmission Ground Strap — A braided strap that connects from the transmission case (near the mount) to the chassis. On a ProMaster, it is located near the bottom of the transmission mount, accessible by removing the driver's side headlight.. While not a primary cause, a poor or corroded main ground can introduce electrical noise and voltage issues for all modules, including the TCM. This can lead to unpredictable behavior and rationality-check codes like P1720. Verifying this ground is clean and secure is a good practice during advanced diagnosis.

OEM Part Supersession History

• Unknown → 68225514AB — This is the Mopar part number for the TCM that fits the 2015-2016 ProMaster City with the 948TE transmission, as well as numerous other Chrysler, Jeep, and Fiat vehicles using the same transmission.
  Heads up: This part fits a wide range of vehicles and engines. It is critical to perform a 'Proxy Alignment' with a wiTECH tool after installation to configure the module for the specific vehicle it is in, otherwise configuration mismatch codes will be set.

Model Year Variations Within This Range

• 2015 - Early 2016: Vehicles built on or before March 5, 2016, were subject to Safety Recall S24 for the corroded CAN-C bus connector. Vehicles built after this date had the connector located in a different, safer position from the factory.