P0128 on 2015 Chrysler 200: Thermostat Rationality Causes and Fixes

What's Unique About the 2015-2015 Chrysler 200

The 2015 Chrysler 200, with either the 2.4L or 3.6L engine, is known for setting the P0128 code. Owner complaints and mechanic reports confirm that the entire plastic thermostat housing assembly is a very common failure point. Unlike some vehicles where this code can have many causes, on the 200 it points very strongly to the thermostat itself. The repair on the 2.4L engine is particularly known for being labor-intensive due to the thermostat's location, requiring the removal of the battery, PCM, and throttle body for access.

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
• Engine temperature gauge reads lower than normal or takes a very long time to warm up
• Poor heater performance
• Decreased fuel economy
• "SERVICE ELECTRONIC THROTTLE CONTROL" message may appear on the dash.
• In some cases, the temperature gauge may suddenly spike to hot or drop to an illogical reading like -41°F, even if the engine isn't overheating, as a computer response to the irrational data from a failed sensor.
• Cooling fans running constantly, even when the engine is cold.

Most Likely Causes

1. Stuck-Open Thermostat 🔴 High Probability The integrated plastic thermostat and housing assembly, particularly on the 2.4L and 3.6L engines, is a very common failure point leading to this specific code. Mechanics report replacing them frequently.
   How to confirm: Monitor the engine temperature with a scan tool. If it warms up extremely slowly or never reaches the normal operating temperature (approx. 195-205°F), the thermostat is likely stuck open. You can also feel the upper radiator hose; it should stay cool for the first few minutes after a cold start and then get hot quickly when the thermostat opens. If it gets warm gradually right away, it's stuck open.
   Typical fix: Replace the entire thermostat and housing assembly. On these engines, the thermostat is integrated into a plastic housing that often includes the ECT sensor. It is highly recommended to replace the whole unit.
   Est. part cost: $50-$120
2. Low Engine Coolant 🟡 Medium Probability
   How to confirm: Visually inspect the coolant reservoir when the engine is cold. If the level is below the 'MIN' or 'ADD' line, it's low. Also, check for any visible signs of coolant leaks around hoses, the radiator, and the water pump. A leak from the thermostat housing itself is also a possibility.
   Typical fix: Top off the coolant with the correct type (Mopar OAT 10 Year/150,000 Mile coolant, part #68163848AB) and bleed any air from the system. If the level drops again, you must find and repair the leak.
   Est. part cost: $25-$35 for coolant
3. Faulty Engine Coolant Temperature (ECT) Sensor ⚪ Low Probability While the sensor can fail, it's much less common than the thermostat for this code. Often, people replace the sensor first, only to find the code returns. Since the sensor is typically included with the new thermostat housing assembly, it gets replaced anyway during the proper repair.
   How to confirm: Use a scan tool to watch the live ECT sensor data. If the reading is erratic, jumps around, or shows an illogical temperature (e.g., -40°F on a warm day) before starting, the sensor is likely bad. You can also compare the sensor reading to the actual coolant temperature with an infrared thermometer. 🎬 See how to diagnose and repair the P0128 code on V6 models.
   Typical fix: Replace the ECT sensor. However, the recommended repair is to replace the entire thermostat housing assembly which includes a new sensor.
   Est. part cost: $20-$50

Rare But Worth Checking

• PCM Software Issue: A Technical Service Bulletin (TSB #18-022-17 REV. A) mentions P0128 in a list of codes that could be resolved with a PCM software update. While the thermostat is the far more likely mechanical failure, it's worth checking if your vehicle's software is up to date, especially if other electrical codes are present or if a new thermostat doesn't fix the issue.

Diagnosis Steps

1. Check for other DTCs. Pay special attention if P2181 or P0126 is also present, as this reinforces a cooling system performance issue.
2. Inspect the coolant level in the reservoir (when the engine is cold). Ensure it is at the proper level.
3. Use an OBD-II scanner to monitor live data from the Engine Coolant Temperature (ECT) sensor from a cold start.
4. Observe the warm-up process. The temperature should rise steadily to around 195-205°F and then stabilize. If it rises very slowly, barely reaches operating temperature, or drops significantly at highway speeds, the thermostat is almost certainly stuck open.
🎬 Watch: A step-by-step walkthrough for replacing the 3.6L thermostat housing.
5. If the temperature reading is erratic, nonsensical (like -40°F), or doesn't change at all, suspect a faulty ECT sensor or wiring.
6. If the coolant level is correct and the sensor provides a logical reading, but the engine fails to warm up properly, the thermostat is the primary suspect and should be replaced as a complete assembly.
7. Before replacing parts, check with a dealer if any PCM software updates related to TSB #18-022-17 REV. A are available for your vehicle's VIN.

Parts You'll Likely Need

• Thermostat and Housing Assembly (OEM #68253514AF (for 2.4L), 5184977AE (for 3.6L)) — This is the most common cause of P0128 on this vehicle. The original thermostat fails by sticking open. The part is sold as a complete assembly with the housing and often a new ECT sensor.
  Trusted brands: Mopar, Dorman, Gates, Motorad
  OEM price range: $75-$120
  Aftermarket price range: $45-$80
• Engine Coolant Temperature (ECT) Sensor — Replaced if diagnosis points to an erratic or failed sensor, though this is a less common fix than the thermostat. On most quality thermostat assemblies for this car, the sensor is included.
  Trusted brands: Mopar, Delphi, NGK
  OEM price range: $30-$50
  Aftermarket price range: $15-$30
• Engine Coolant (OAT) (OEM #68163848AB (1 Gallon, Concentrated)) — The cooling system will need to be drained and refilled when replacing the thermostat. The 2015 Chrysler 200 requires a specific OAT (Organic Acid Technology) coolant to prevent corrosion.
  Trusted brands: Mopar OAT 10 Year/150,000 Mile
  OEM price range: $25-$35 per gallon
  Aftermarket price range: $20-$30 per gallon

Related Codes That Often Appear With This One

• P2181 — P2181 is 'Cooling System Performance'. This code is often set alongside P0128 because they both relate to the engine's inability to reach or maintain its correct operating temperature. NHTSA complaints confirm these two codes frequently appear together.
• P0126 — P0126 is 'Insufficient Coolant Temperature for Stable Operation'. This is another closely related code that points to the engine running too cool, reinforcing the diagnosis of a stuck-open thermostat.

Technical Service Bulletins (TSBs) & Recalls

• 18-022-17 REV. A: Mentions P0128 - Thermostat Rationality as one of several codes that may be resolved by a PCM software update.

Platform-Specific Known Issues

• A technician noted in NHTSA complaint #11054658 that he had seen 'a lot of this issue with the 200 models', indicating a high failure rate for the thermostat.
• NHTSA complaint #11206259 links codes P0128 and P2181 with a 'SERVICE ELECTRONIC THROTTLE CONTROL' message, which can be a confusing secondary symptom.
• TSB #18-022-17 REV. A includes P0128 in a list of DTCs that may be addressed by a PCM software update, suggesting a potential, though less common, electronic cause.
• A DIY video for the 2.4L engine confirms the difficulty, showing the need to remove the engine cover, battery, PCM connectors, and throttle body to access the thermostat housing, which is buried low in the engine bay.

Scan Tool Commands That Help

• Generic OBD-II Scanner: Live Data - Engine Coolant Temperature — Monitor from a cold start to verify if the engine warms up slowly or fails to reach the target temperature (approx. 195-205°F), which is the primary confirmation method for a stuck-open thermostat.
• Professional Scan Tool (e.g., wiTECH): Cooling Fan Actuation/Bidirectional Control — To test if the cooling fans are functional and not stuck on, which could cause over-cooling and contribute to a P0128 code, although this is a less common cause.

Wiring & Ground Locations

• ECT Sensor Connector (2.4L) — On the passenger side of the engine, plugged directly into the thermostat housing assembly.. This is the primary data source for the PCM to determine coolant temperature. A poor connection, corrosion, or damaged wiring at this connector can cause erratic or incorrect readings, potentially mimicking a thermostat issue. However, for P0128, a mechanical thermostat failure is far more common.
• ECT Sensor Connector (3.6L) — On the front of the engine, driver's side, threaded into the cylinder head near the thermostat housing.. This sensor provides the temperature data to the PCM. A bad connection or damaged wires can lead to incorrect readings. A video for the 3.6L shows a red locking tab on the connector that must be pulled back before the connector can be unplugged.
• PCM Connector C2, Pin 11 (2.4L) — At the Powertrain Control Module (PCM). The ECT signal wire is typically Violet/Brown.. For advanced diagnosis, this pin can be tested for continuity back to the sensor connector to rule out a break in the signal wire. This is typically only done after confirming the sensor itself is good and the thermostat has been replaced.

Real Owner Repair Stories

• YouTube video by 'T_ Z' (2015 Chrysler 200C 2.4L with ~79,000 miles) — Check Engine Light with code P0128.
  ❌ Tried (didn't work) Replacing only the Engine Coolant Temperature (ECT) sensor.
  ✅ What actually fixed it Replacing the entire thermostat and housing assembly finally resolved the P0128 code.
• Reddit user 'OTFJunkie92' (2015 Chrysler 200) — Check Engine Light with code P0128. The light later turned off on its own but the code was stored.
  ✅ What actually fixed it The user was quoted $700+ for a thermostat replacement due to high labor costs, confirming the difficulty of the job. The thread highlights that the code can sometimes clear itself, but the underlying issue (likely a failing thermostat) remains.

When the Usual Fixes Don't Work

• While extremely rare for this specific code, if a new thermostat assembly does not fix the issue, the problem could lie with a wiring harness issue (e.g., a loose connector pin or chafed wire) or a PCM software glitch. A forum user with a different vehicle reported a P0128 code returned after a thermostat replacement because the sensor connector was not fully seated and had a loose clip. Additionally, TSB 18-022-17 REV. A for the 2015 2.4L model does list a PCM update as a potential fix for P0128, indicating a possible, though unlikely, software-based cause.

OEM Part Supersession History

• 5184977AE (for 3.6L V6) → 05184977AJ — Part has been updated by the manufacturer. The new part number replaces several previous versions (5184977AD, 5184977AE, 5184977AG, 5184977AH, 5184977AI).