Diagnostic Procedure

WARNING: This page is about a different car, the 2005 Jeep Liberty. However, it is still accessible from the selected car via links, so may be relevant.

P0128-THERMOSTAT RATIONALITY DIAGNOSTIC TEST REFERENCE

TEST	ACTION	APPLICABILITY
1	NOTE: If any ECT, AAT, CMP or CKP sensor DTCs have set along with P0128, diagnose them before continuing. NOTE: Ensure that Pinion Factor has been programmed correctly into the PCM. Ignition on, engine not running.With the DRBIII®, read DTCs and record the related Freeze Frame data.Is the Good Trip Counter displayed and equal to zero?Yes --> Go To 2No --> Refer to the INTERMITTENT CONDITION SYMPTOM .Perform POWERTRAIN VERIFICATION TEST VER - 5 .	All
2	NOTE: If a Engine Coolant Temperature (ECT) DTC is set along with this code, diagnose the ECT DTC first. NOTE: Inspect the ECT terminals and related PCM terminals. Make sure the terminals are free from corrosion and damage. NOTE: The best way to diagnose this DTC is to allow the vehicle to sit overnight outside in order to have a totally cold soaked engine. NOTE: Extremely cold outside ambient temperatures may have caused this DTC to set. WARNING: Never open the cooling system when the engine is hot. The system is under pressure. Extreme burns or scalding may result. Allow the engine to cool before opening the cooling system. Check the coolant system to make sure that the coolant is in good condition and at the proper level.Is the coolant level and condition OK?Yes --> Go To 3No --> Inspect the vehicle for a coolant leak and add the necessary amount of coolant.Perform POWERTRAIN VERIFICATION TEST VER - 5 .	All
3	NOTE: This test works best if performed on a cold engine (cold soak). Ignition on, engine not running.With the DRBIII®, read the ECT Deg value. If the engine was allowed to sit overnight (cold soak), the temperature value should be a sensible value that is somewhere close to the ambient temperature. NOTE: If engine coolant temperature is above 82°C (180°F), allow the engine to cool until 65°C (150°F) is reached. Start the Engine.During engine warm-up, monitor the ECT Deg value. The temp deg value change should be a smooth transition from start up to normal operating temp 82°C (180°F).Also monitor the actual coolant temperature with a thermometer. NOTE: As the engine warms up to operating temperature, the actual coolant temperature (thermometer reading) and the ECT Deg on the DRBIII® screen should stay relatively close to each other. Using the appropriate service information, determine the proper opening temperature of the thermostat.Did the thermostat open at the proper temperature?Yes --> Go To 4No --> Replace the thermostat.Perform POWERTRAIN VERIFICATION TEST VER - 5 .	All
4	Ignition on, engine not running. With the DRBIII®, read and record the AAT Sensor Temperature value. Using the DRB Temperature Probe #CH7050, measure the ambient air temperature near the AAT Sensor. Is the AAT Sensor value with -15°C (5°F) of the temperature probe reading? Yes --> Go To 5 No --> Go To 7	All
5	WARNING: MAKE SURE THE ENGINE COOLING SYSTEM IS COOL BEFORE REMOVING THE PRESSURE CAP OR ANY HOSE. SEVERE PERSONAL INJURY MAY RESULT FROM ESCAPING HOT COOLANT. THE COOLING SYSTEM IS PRESSURIZED WHEN HOT. With the DRBIII®, read and record the ECT Sensor Temperature value.Using the DRB Temperature Probe #CH7050, measure the engine coolant temperature.Is the ECT Sensor value with -15°C (5°F) of the temperature probe reading?Yes --> Go To 6No --> Go To 7	All
6	Inspect the Temperature sensors for any physical damage. Inspect the engine coolant. Make sure the coolant is at the proper level. Refer to COOLING . Make sure the Temperature sensors are properly installed. Make sure the CMP and CKP sensors are mounted properly. Check the connectors for any signs of damage. WARNING: WHEN THE ENGINE IS OPERATING, DO NOT STAND IN A DIRECT LINE WITH THE FAN. DO NOT PUT YOUR HANDS NEAR THE PULLEYS, BELTS OR FAN. DO NOT WEAR LOOSE CLOTHING. Refer to any Technical Service Bulletins (TSBs) that may apply. With the engine running at normal operating temperature, monitor the Temperature sensor parameters while wiggling the wire harness. Look for parameter values to change. Visually inspect the related wire harness. Look for any chafed, pierced, pinched, partially broken wires and broken, bent, pushed out, or corroded terminals. CAUTION: NEVER PROBE THE PCM HARNESS CONNECTORS. PROBING THE PCM HARNESS CONNECTORS WILL DAMAGE THE PCM TERMINALS RESULTING IN POOR TERMINAL TO PIN CONNECTION. INSTALL MILLER SPECIAL TOOL #8815 TO PERFORM DIAGNOSIS. Inspect and clean all PCM, engine, and chassis grounds. Were any problems found during the above inspections? Yes --> Repair as necessary Perform POWERTRAIN VERIFICATION TEST VER - 5 . No --> Test Complete.	All
7	NOTE: Visually inspect both the component and the PCM connectors. Look for damage, partially broken wires and backed out or corroded terminals Turn the ignition off. Disconnect the applicable Temperature Sensor harness connector.Disconnect the PCM harness connectors.Ignition on, engine not running.Measure the voltage of the Signal circuit in the appropriate Temperature Sensor harness connector.Is the voltage above 5.2 volts?Yes --> Repair the short to battery voltage in the Signal circuit.Perform POWERTRAIN VERIFICATION TEST VER - 5 .No --> Go To 8	All
8	Connect a jumper wire across the Temperature Sensor harness connector. With the DRBIII®, read the Temperature voltage. Does the voltage start at 5.0 volts and drop below 1.0 volt? Yes --> Replace the suspected Temperature Sensor. Perform POWERTRAIN VERIFICATION TEST VER - 5 . No --> Go To 9	All
9	Turn the ignition off. CAUTION: DO NOT PROBE THE PCM HARNESS CONNECTORS. PROBING THE PCM HARNESS CONNECTORS WILL DAMAGE THE PCM TERMINALS RESULTING IN POOR TERMINAL TO PIN CONNECTION. INSTALL MILLER SPECIAL TOOL #8815 TO PERFORM DIAGNOSIS. Measure the resistance of the Sensor Signal circuit from the Temperature Sensor harness connector to the appropriate terminal of special tool #8815. Is the resistance below 5.0 ohms? Yes --> Go To 10 No --> Repair the open in the Sensor Signal circuit. Perform POWERTRAIN VERIFICATION TEST VER - 5 .	All
10	CAUTION: DO NOT PROBE THE PCM HARNESS CONNECTORS. PROBING THE PCM HARNESS CONNECTORS WILL DAMAGE THE PCM TERMINALS RESULTING IN POOR TERMINAL TO PIN CONNECTION. INSTALL MILLER SPECIAL TOOL #8815 TO PERFORM DIAGNOSIS. Measure the resistance of the (K900) Sensor ground circuit from the appropriate Temperature Sensor harness connector to the appropriate terminal of special tool #8815.Is the resistance below 5.0 ohms?Yes --> Go To 11No --> Repair the open in the (K900) Sensor ground circuit.Perform POWERTRAIN VERIFICATION TEST VER - 5 .	All
11	Measure the resistance between ground and the Sensor Signal circuit in the Temperature Sensor harness connector. Is the resistance below 100 ohms? Yes --> Repair the short to ground in the Signal circuit. Perform POWERTRAIN VERIFICATION TEST VER - 5 . No --> Go To 12	All
12	Measure the resistance between the Signal circuit and the (K900) Sensor ground circuit at the Temperature Sensor harness connector. Is the resistance below 100 ohms? Yes --> Repair the short between the (K900) Sensor ground circuit and the Sensor Signal circuit. Perform POWERTRAIN VERIFICATION TEST VER - 5 . No --> Go To 13	All
13	NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal push out. Repair as necessary. Using the schematics as a guide, inspect the wire harness and connectors. Pay particular attention to all Power and Ground circuits.If there are no possible causes remaining, view repair.RepairReplace and program the Powertrain Control Module per Service Information.Perform POWERTRAIN VERIFICATION TEST VER - 5 .	All