P0128 on 2015-2017 Ram ProMaster City: Thermostat Rationality Fixes

What's Unique About the 2015-2017 Ram PROMASTER CITY

For the 2.4L MultiAir engine in the ProMaster City and its platform mates, the thermostat, its plastic housing, and the coolant temperature sensor are sold as a single integrated unit; you cannot replace just the thermostat valve itself. Furthermore, Ram has issued several Technical Service Bulletins (TSBs) that address this code with a software update to the vehicle's computer, not a hardware replacement. This means that unlike many other vehicles, a trip to the dealer for a reflash could be the correct fix, as the original software logic was often too sensitive for cold climates.

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
• Temperature gauge on the dashboard reads lower than normal or takes a very long time to reach the middle
• Temperature gauge drops towards cold when driving at highway speeds or coasting downhill
• Heater blows cool or lukewarm air, especially on the highway
• Slightly decreased fuel economy
• Cooling fans may run constantly as a fail-safe, even when the engine is cold

Most Likely Causes

1. Faulty Thermostat Assembly (Stuck Open) 🔴 High Probability The original plastic thermostat housings can become brittle, and the internal thermostat mechanism frequently fails by sticking in the open position. This allows coolant to circulate through the radiator constantly, preventing the engine from warming up properly.
   How to confirm: Monitor the engine coolant temperature with a scan tool. If the temperature fails to reach ~195-205°F during normal driving or drops significantly at highway speeds, the thermostat is likely stuck open. You can also feel the upper radiator hose shortly after a cold start; if it gets warm almost immediately, coolant is flowing 🎬 Watch: Two simple DIY methods to diagnose a P0128 code. when it shouldn't be.
   Typical fix: Replace the entire thermostat housing assembly, which includes a new thermostat and coolant temperature sensor. This is a single, integrated part.
   Est. part cost: $50-$120
2. PCM Software Update Required 🟡 Medium Probability Ram issued TSBs #18-121-16, #18-063-17, and #1806417 which indicate that updated software calibrations are available to correct overly sensitive 'Thermostat Rationality' logic. This issue was particularly common in colder climates where the engine would not warm up as quickly as the original software expected, triggering a false P0128 code.
   How to confirm: A Ram dealership or a properly equipped independent shop can check the vehicle's current software version against the latest available version using their diagnostic tools. If the hardware (thermostat) tests good, a software issue is likely. This is often the case if the P0128 code appears alongside codes like U1424 or U0402.
   Typical fix: Flash the Powertrain Control Module (PCM) with the updated software calibration provided by the TSB.
   Est. part cost: $100-$250
3. Low Engine Coolant Level ⚪ Low Probability
   How to confirm: Visually inspect the coolant level in the overflow reservoir when the engine is cold. If it is below the 'MIN' line, there is a leak that needs to be found and repaired. A gurgling sound from the dashboard area can also indicate air in the system from a low coolant level.
   Typical fix: Top off the coolant with the correct Mopar OAT coolant (MS-12106 spec) and pressure test the cooling system to find and repair any leaks.
   Est. part cost: $20-$40

Rare But Worth Checking

• Faulty Engine Coolant Temperature (ECT) Sensor: This is rare because the sensor is typically replaced with the thermostat assembly on this vehicle. If the thermostat was replaced but the old sensor was somehow reused, or if the new sensor was faulty out of the box, it could be the problem. An erratic temperature gauge (e.g., reading -40°F) points to a sensor or wiring issue.
• Cooling Fan Stuck On: If the electric cooling fan runs constantly, even when the engine is cold, it can over-cool the engine and trigger P0128. This is usually caused by a faulty fan relay, a wiring issue, or as a fail-safe response to a bad ECT sensor reading.

Diagnosis Steps

1. Check for TSBs: Ask a dealer or use an online service to check if your vehicle's VIN has any open recalls or TSBs related to P0128 or PCM updates. TSBs #18-121-16, #18-063-17, and #1806417 are relevant.
2. Check Coolant Level: Ensure the coolant reservoir is filled to the proper level when the engine is cold. If low, top it off with Mopar OAT 10 Year/150,000 Mile coolant (or equivalent meeting MS-12106 spec) and check for leaks.
3. Verify the Symptom: Use a scan tool to monitor the live data for the Engine Coolant Temperature (ECT) sensor. Start the engine cold.
4. Monitor Warm-Up: Watch the ECT reading as the engine warms up. It should climb steadily to around 195-205°F (90-96°C) and stabilize. The upper radiator hose should remain cool until the engine is nearly at operating temperature.
5. Test Thermostat Function: If the temperature struggles to reach 190°F, or if it drops significantly when driving at highway speeds (e.g., below 175°F), the thermostat is almost certainly stuck open and needs to be replaced.
6. Test Sensor Accuracy: Before starting the cold engine, compare the ECT sensor reading to the Intake Air Temperature (IAT) sensor reading. They should be within a few degrees of each other. If the ECT reading is wildly different (e.g., -40°F), the sensor or its wiring is faulty.
7. Replace Assembly: If the thermostat is confirmed bad, replace the entire thermostat housing assembly. The job is moderately difficult due to tight access behind the engine. The PCM and a vacuum pump may need to be moved for access.
🎬 Watch: This step-by-step walkthrough shows you how to access the housing.
8. Bleed Cooling System: After replacement, the cooling system must be properly bled to remove air pockets, which can cause gurgling noises and erratic temperature readings. A spill-proof funnel kit is highly recommended.
9. Reflash PCM: If a new thermostat assembly does not resolve the code, the issue is likely the PCM software. The PCM will need to be reprogrammed by a dealer or qualified shop with the latest calibration.

Parts You'll Likely Need

• Engine Coolant Thermostat Assembly (OEM #5047861AD) — This is the most common cause of P0128. On the ProMaster City, the thermostat, housing, and sensor are sold as one complete unit that must be replaced together.
  Trusted brands: Mopar (OEM P/N: 5047861AD, supersedes 5047861AC, 5047861AB, etc.), Dorman 902-3113, Gates, Four Seasons
  OEM price range: $90-$150
  Aftermarket price range: $50-$110
• Engine Coolant / Antifreeze (OEM #Mopar 68163848AB (50/50 Pre-diluted)) — The cooling system must be drained to replace the thermostat. The vehicle requires a specific OAT coolant meeting Chrysler specification MS-12106 (or MS.90032). Total system capacity is 7.2 quarts.
  Trusted brands: Mopar OAT 10 Year/150,000 Mile, Valvoline Zerex G30/G40, PEAK OET for North American Vehicles (Purple)
  OEM price range: $25-$40
  Aftermarket price range: $20-$30

Related Codes That Often Appear With This One

• P0126 — P0126 is 'Insufficient Coolant Temperature for Stable Operation,' which is a very similar condition detected by the PCM and is often caused by the same faulty thermostat.
• U1424 — This code for 'Implausible Engine Torque Signal Received' is listed alongside P0128 in TSB #1806417 and #18-063-17 REV. A, suggesting a software calibration may fix them together.
• U0402 — This code for 'Implausible Data Received From TCM' is also listed with P0128 in TSBs, indicating a potential software link between engine and transmission control modules that is addressed by a PCM reflash.

Technical Service Bulletins (TSBs) & Recalls

• 18-121-16: Addresses Malfunction Indicator Lamp (MIL) illumination for P0128 - Thermostat Rationality.
• 18-063-17 (and Rev. A): Addresses MIL illumination for P0128 and a long list of other codes, with an available software enhancement. Also includes enhancements for catalytic converter operation and A/C clutch cycling.
• 1806417: Mentions P0128 - Thermostat Rationality as part of a group of codes that have an available calibration enhancement.

Platform-Specific Known Issues

• Multiple Technical Service Bulletins (TSBs) exist for this code, including #18-121-16, #18-063-17, and #1806417. These bulletins indicate that the fix may be a PCM software update rather than a hardware replacement.
• The thermostat is not sold separately from its housing and the coolant temperature sensor, requiring the replacement of the entire assembly.
• The radiator does not have a drain plug (petcock), so the system must be drained by removing the lower radiator hose, which can be messy.
• Access to the thermostat housing is tight. Mechanics often remove the PCM and the vacuum pump to gain better access.

Mechanic-Grade Diagnostic Values

• Engine Coolant Temperature (ECT) Sensor Resistance — expected: Approx. 2000-3000 Ω at 20°C (68°F) and 200-300 Ω at 90°C (194°F).. Failure: Resistance is significantly outside the expected range at a known temperature, or shows an open/short circuit.
• Engine Coolant Temperature (ECT) Sensor Signal Voltage — expected: Approx. 2.0V - 3.0V on a cold engine (key on, engine off), dropping to around 0.5V when the engine is fully warmed up.. Failure: Voltage is stuck high (near 5V) or low (near 0V), or does not decrease smoothly as the engine warms.
• ECT Sensor Reference Voltage at Connector — expected: A steady 5V supplied from the PCM to the sensor circuit.. Failure: Voltage is not present or is significantly lower than 5V, indicating a PCM or wiring issue upstream of the sensor.

Scan Tool Commands That Help

• wiTECH 2.0 (OEM Dealer Tool): View and graph live ECT sensor data; Check for PCM software updates (flashes). — This is the definitive tool for diagnosing software-related P0128 issues by checking for and applying TSB-related PCM updates. It also provides the most detailed live data for monitoring sensor performance.

Wiring & Ground Locations

• ECT Sensor Connector — Plugged directly into the Engine Coolant Temperature sensor, which is part of the thermostat housing assembly on the passenger side of the engine.. This 2-pin connector is a common point for wiring issues. One pin is the 5V reference from the PCM, and the other is the signal return. Corrosion, a loose connection, or damaged wires here will directly cause incorrect readings and can trigger P0128.
• G102 (Engine Ground) — A primary engine ground point located on the left side of the engine block.. A poor engine ground can create voltage offsets and noise on sensor circuits, including the ECT sensor, leading to inaccurate readings. While not a primary cause, checking major ground points is a valid step if sensor readings are erratic without a clear sensor or wiring fault.

Real Owner Repair Stories

• YouTube channel 'World Mechanics' (2015-2020 Ram ProMaster City with 2.4L MultiAir engine) — Check Engine Light with code P0128.
  ✅ What actually fixed it The video identifies the two most common causes as a bad thermostat or a bad coolant temperature sensor. It is strongly recommended to replace the entire thermostat housing assembly, which comes with a new thermostat and a new sensor, to fix the issue definitively.
• YouTube channel 'Mailman Mechanic' (2016 Ram ProMaster City) — Intermediate overheating issue and likely P0128 (implied).
  ✅ What actually fixed it Replaced the entire thermostat housing assembly. The video provides a detailed walkthrough, noting the difficulty of accessing the bolts and the fact that the radiator has no drain petcock, requiring removal of the lower hose to drain the system.
• YouTube channel 'd-ray's shop' (Dodge ProMaster City with 2.4L engine) — Check Engine Light with code P0128.
  ✅ What actually fixed it The mechanic confirmed with a BlueDriver scan tool that the engine was not reaching temperature. He replaced the entire thermostat housing assembly. The video details the difficult process, including removing the battery, overflow tank, and computer to access the housing, and the need to siphon coolant because there is no radiator drain plug.

OEM Part Supersession History

• 5047263AB, 5047263AC, 5047772AB, 5047861AA, 5047861AB, 5047861AC → 5047861AD — Standard part revision and consolidation by the manufacturer to improve reliability or manufacturing.