P0128 Dodge RAM 1500 - Thermostat Rationality | Step-by-Step Diagnosis

P0128 (Thermostat Rationality) on a Dodge RAM 1500? This video walks through the complete 18-step factory diagnostic procedure with the wiring diagram and connector views - no guesswork, no parts cannon. When monitored: Engine running. Set condition: This diagnostic will fail when the model temperature reaches 91° C before the ECT. reaches 80° C. This monitor only runs after a cold start condition ECT, ACT, AMB are all within 10° C. of one another. Two Trip Fault. Three good trips to turn off the MIL. In this video: 1. Active DTC 2. Low Coolant Level 3. Thermostat Operation 4. Ambient Air Temp Sensor Operation 5. ECT Sensor Operation 6. Other Possible Causes 7. (K2) ECT Signal Circuit Shorted To Battery Voltage 8. (G31) Aat Signal Circuit Shorted To Battery Voltage 9. Temperature Sensor 10. (K2) ECT Signal Circuit Open 11. (G31) Aat Signal Circuit Open 12. (K900) Sensor Ground Circuit Open 13. (G930) Sensor Return Circuit Open 14. (K2) ECT Signal Circuit Shorted To Ground 15. (G31) Aat Signal Circuit Shorted To Ground 16. (K2) ECT Signal Circuit Shorted To The (K900) Sensor Ground Circuit 17. (G31) Aat Signal Circuit Shorted To The (G930) Sensor Ground Circuit 18. PCM Possible causes covered: Low Coolant Level; Thermostat Operation; Signal Circuit Shorted To Battery Voltage; Temperature Sensor; Signal Circuit Open; (K900) Sensor Ground Circuit Open; (G930) Aat Sensor Return Circuit Open; Signal Circuit Shorted To Ground. Applies to the Dodge RAM 1500 third generation (2002-2008) with the 3.7L V6, 4.7L V8 and 5.7L HEMI V8. Similar logic applies to RAM 2500/3500 gas models of the same generation. This is general information for educational purposes - always confirm against the service information for your exact vehicle.