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Tahoe / Yukon Service Manual

DTC P0128 or P01F0 (1)

Engine Controls and Fuel - 3.0L LM2 Diesel | Diagnostic Information and Procedures | Document ID: 5456255 DTC P0128 or P01F0 Diagnostic Instructions Perform the Diagnostic System Check prior to using this diagnostic procedure: Diagnostic System Check - Vehicle Review the description of Strategy Based Diagnosis: Strategy Based Diagnosis An overview of each diagnostic category can be found here: Diagnostic Procedure Instructions DTC Descriptor DTC P0128 Engine Coolant Temperature Below Thermostat Regulating Temperature DTC P01F0 Coolant Temperature Dropped Below Diagnostic Monitoring Temperature Diagnostic Fault Information

Circuit Short to Ground Open/High Resistance Short to Voltage Signal Performanc
Signal P0117, P0119 P0118, P0119 P0118, P0119 * P0116, P0119, P0128
Low Reference — P0118, P0119 ** * P0119, P0128
* Internal control module or component damage may occur if the circuit shorts to B+. ** Additional DTCs may set

Circuit/System Description For an overview of the component/system, refer to: Cooling System Description and Operation

Circuit Description
Signal The control module input circuit has an internal resistance connected to 5 V.
Low Reference Grounded through the control module.
Component Description
B34 Engine Coolant Temperature Sensor The sensor is a negative temperature coefficient thermistor. As the temperature of the air, fluid, or metal increases, the resistance of the sensor decreases, varying the voltage on the signal circuit.
Component Description
K20 Engine Control Module The control module contains a microprocessor used to process input data to control outputs. The control module controls a series of actuators to ensure optimal engine performance. The control module does this by reading values from a variety of sensors, interprets the data and adjusts the engine actuators accordingly.

Conditions for Running the DTC P0128 DTCs related to the following system/component = Not Set – B34 Engine Coolant Temperature Sensor – B66 Intake Air Temperature Sensor – B75C Multifunction Intake Air Sensor Engine Off Time = Greater than 30 min Engine Run Time = 10 s to 30 min Fuel Alcohol Content = Less than 87% Start-Up Engine Coolant Temperature Sensor = −40 to 52°C (−40 to 126°F) Frequency the DTC runs = Once — Per ignition cycle — After the running conditions are met P01F0 DTC P0116, P0117, P0118, P0119, P111E = Not set Ambient Air Temperature = Warmer than −9°C (21°F) Barometric Pressure = Greater than 55 kPa (8 PSI) Cylinder Deactivation System = Inactive Deceleration Fuel Cut-Off = Less than 23 s Engine Coolant Temperature = Warmer than 66°C (150°F) Engine Run Time = Greater than 30 min Engine Speed = Less than 8,200 RPM Vehicle Distance Traveled During Drive Cycle = Greater than 1.2 km (.8 mi) Frequency the DTC runs = Continuously — After the running conditions are met Conditions for Setting the DTC P0128 Engine Coolant Temperature = Less than a predefined threshold P01F0 Engine Coolant Temperature = Colder than 65°C (149°F) — For greater than 60 s Actions Taken When the DTC Sets DTCs listed in the DTC Descriptor category = Type B DTC Cooling Fan = On Conditions for Clearing the DTC DTCs listed in the DTC Descriptor category = Type B DTC Diagnostic Aids { P0128 } The parameters for intake air temperature and engine coolant temperature should be within 3°C (5°F) of each other if the ignition has been Off for greater than 8 h. High resistance in the circuits of the component may set DTC P0128, P01F0 without setting the DTC for the component: B34 Engine Coolant Temperature Sensor Engine Coolant Temperature Sensor — Description / Production Location

Component Location
B34A Engine Coolant Temperature Sensor 1 Engine Coolant Inlet Pipe
B34B Engine Coolant Temperature Sensor 2 Engine Block — Front
B34E Engine Coolant Temperature Sensor 5 Engine Block — Rear

Reference Information Schematic Reference Engine Controls Schematics Engine Heating/Cooling Schematics Connector End View Reference Master Electrical Component List Electrical Information Reference Circuit Testing Connector Repairs Testing for Intermittent Conditions and Poor Connections Wiring Repairs DTC Type Reference Powertrain Diagnostic Trouble Code (DTC) Type Definitions Scan Tool Reference Control Module References Circuit/System Verification

  1. Verify the condition does not exist: Incorrect fluid level or condition » Refer to: Loss of Coolant ⇒ If a condition exists Repair or replace as necessary ⇓ If no condition exists
  2. Verify there are no DTCs set related to the following component/system: B34 Engine Coolant Temperature Sensor ⇒ If a related DTC is set Refer to: Diagnostic Trouble Code (DTC) List - Vehicle ⇓ If a related DTC is not set
  3. Perform the scan tool control function: Engine Block Coolant Valve Position = The parameter changes as commanded with the scan tool. ⇒ If the value does not change Test or replace the component: Q97B Engine Coolant Flow Control Valve - Block ⇓ If the value changes
  4. Engine » Running @ 1,800 to 2,200 RPM Verify the scan tool parameter: Engine Coolant Temperature Sensor = Greater than 80°C (176°F) — Within 15 min ⇒ If 80°C (176°F) or colder Test or replace the component: Q97B Engine Coolant Flow Control Valve - Block ⇓ If warmer than 80°C (176°F)
  5. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data. Verify the DTC does not set. ⇒ If the DTC sets Refer to: Circuit/System Testing ⇓ If the DTC does not set
  6. All OK. Circuit/System Testing
  1. Ignition/Vehicle & All vehicle systems » Off
  2. Disconnect the appropriate electrical connector: B34 Engine Coolant Temperature Sensor
  3. Test for less than 10 Ω between the test points: Low Reference circuit terminal 2 & Ground ⇒ If 10 Ω or greater 3.1. Disconnect the appropriate electrical connector: K20 Engine Control Module 3.2. Test for less than 2 Ω between the test points: Low Reference circuit terminal 2 @ Component harness & The other end of the circuit @ Control module harness ⇒ If 2 Ω or greater » Repair the open/high resistance in the circuit. ⇒ If less than 2 Ω » Replace the component: K20 Engine Control Module ⇓ If less than 10 Ω
  4. Ignition » On / Vehicle » In Service Mode
  5. Verify the appropriate scan tool parameter: Engine Coolant Temperature Sensor = Colder than −39°C (−38°F) ⇒ If −39°C (−38°F) or warmer 5.1. Ignition/Vehicle » Off 5.2. Disconnect the appropriate electrical connector: K20 Engine Control Module 5.3. Test for infinite resistance between the test points: Signal circuit terminal 1 @ Component harness & Ground ⇒ If less than infinite resistance » Repair the short to ground on the circuit. ⇒ If infinite resistance » Replace the component: K20 Engine Control Module ⇓ If colder than −39°C (−38°F)
  6. Connect a 3 A fused jumper wire between the test points: Signal circuit terminal 1 & Low Reference circuit terminal 2
  7. Verify the appropriate scan tool parameter: Engine Coolant Temperature Sensor = Warmer than 149°C (300°F) ⇒ If 149°C (300°F) or colder 7.1. Ignition/Vehicle » Off & Remove » Jumper wire(s) 7.2. Disconnect the appropriate electrical connector: K20 Engine Control Module 7.3. Ignition » On / Vehicle » In Service Mode 7.4. Test for less than 1 V between the test points: Signal circuit terminal 1 @ Component harness & Ground ⇒ If 1 V or greater » Repair the short to voltage on the circuit. ⇓ If less than 1 V 7.5. Ignition/Vehicle » Off 7.6. Test for less than 2 Ω between the test points: Signal circuit terminal 1 @ Component harness & The other end of the circuit @ Control module harness ⇒ If 2 Ω or greater » Repair the open/high resistance in the circuit. ⇒ If less than 2 Ω » Replace the component: K20 Engine Control Module ⇓ If warmer than 149°C (300°F)
  8. Test or replace the component: B34 Engine Coolant Temperature Sensor Component Testing
  9. Ignition/Vehicle » Off
  10. Disconnect the appropriate electrical connector: B34 Engine Coolant Temperature Sensor
  11. Test the resistance between the test points: Signal circuit terminal 1 & Low Reference circuit terminal 2 @ Component
  12. Verify the value is within the range listed in the table: Temperature Versus Resistance ⇒ If not in the specified range Replace the appropriate component: B34 Engine Coolant Temperature Sensor ⇓ If in the specified range
  13. Test for infinite resistance between the test points: Each terminal of the component & The component’s housing ⇒ If less than infinite resistance Replace the appropriate component: B34 Engine Coolant Temperature Sensor ⇓ If infinite resistance
  14. All OK. Repair Instructions Perform the Diagnostic Repair Verification after completing the repair: Diagnostic Repair Verification Engine Coolant Temperature Sensor Replacement - Water Pump Inlet Pipe — B34A Engine Coolant Temperature Sensor 1 Engine Coolant Temperature Sensor Replacement - Engine Block - Front — B34B Engine Coolant Temperature Sensor 2 Engine Coolant Temperature Sensor Replacement - Engine Block - Rear — B34E Engine Coolant Temperature Sensor 5 For control module replacement, programming, and setup refer to: Control Module References