GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2008 🢒 TopKick C-Series (Conventional) C4/C5 🢒 Engine 🢒 Engine Controls and Fuel - 7.8L (LF8) 🢒 DTC P0299

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provide an overview of each diagnostic category.

DTC Descriptor

DTC P0299: Turbocharger Engine Underboost

Circuit/System Description

The boost pressure sensor is located in the air induction tubing. The sensor is a transducer that varies voltage according to changes in the air pressure inside the air tubing. The ECM monitors the boost pressure sensor signal for abnormal values. If the ECM detects that the sensor signal is excessively low, this DTC will set. This indicates excessive low boost pressure.

Conditions for Running the DTC
    • DTC P0016, P0089, P0091, P0092, P0101, P0102, P0103, P0112, P0113, P0117, P0118, P0126, P0201, P0202, P0203, P0204, P0205, P0206, P0237, P0238, P0261, P0264, P0270, P0273, P0276, P0335, P0336, P0401, P0403, P0404, P042E, P0602, P0642, P0643, P0652, P0653, P1125, P1293, P2146, P2147, P2148, P2149, P2150, P2151, P2227, P2228, P2229, and P268A are not set.
    • The key switch is ON for 2 seconds.
    • Accumulation fuel quantity is more than 200 mm³/st.
    • The target mass air flow is less than 600 mg/cyl.
    • The engine speed is between 1,250-2,250 RPM.
    • The absolute value of change of engine speed is less than 100 RPM.
    • The fuel quantity is less than 300 mm³/st.
    • The absolute value of change of fuel quantity is less than 10 mm³/st.
    • The IAT is more than -7°C (+19°F).
    • The ECT is more than 60°C (140°F).
    • The battery voltage is between 10-16 volts.

Conditions for Setting the DTC

The difference of expected under boost pressure and actual boost pressure is more than a calibrated value for more than 5 seconds.

Action Taken When the DTC Sets

DTC P0299 is a Type A DTC.

Conditions for Clearing the MIL/DTC

DTC P0299 is a Type A DTC.

Diagnostic Aids

A boost sensor that is stuck at BARO, or is 40 kPa lower than desired while under boost, will cause this DTC to set. Use a scan tool and compare the Boost Sensor parameters to a known good vehicle. Operate both vehicles under various conditions.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

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 for scan tool information

Circuit/System Verification
  1. If DTCs P003A, P0045, P0698, P0699, P2563, P2564 or P2565 are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.
  2. Verify the integrity of the entire air induction and turbocharger pipes and hoses by inspecting for the following conditions:
    3. • Any damaged components
    • Loose or improper installation
    • Wet or restricted air filter
    • Exhaust leaks before the turbocharger
    • Exhaust restrictions after the turbocharger
  3. Engine running, use a scan tool to command the TC Vane Pos. Ctrl. Solenoid ON.
  4. Perform a snapshot of the engine data while slowly increasing the engine speed to 1,700 RPM, and then back to idle. Refer to Scan Tool Snapshot Procedure .
  5. Plot the snapshot and verify that the Boost Sensor parameter increases smoothly as the engine speed is increased and decreased smoothly as the engine speed is returned to idle.
    6. ⇒If the Boost parameter plot slope is not a straight line with even RPM change, replace the boost sensor.

Circuit/System Testing
  1. Test the charge air cooler for leaks.
  2. Test the turbocharger oil supply feed.
  3. Inspect the turbocharger oil supply tube for restrictions.
    4. ⇒If a restriction is found, clean or replace the oil supply tube as necessary.
  4. Inspect the turbocharger for debris, worn impeller and turbine blades, or damage .
    5. ⇒If worn or damaged turbocharger blades or vanes are found, replace the turbocharger assembly.
    ⇒If the charge air cooler system, the turbocharger and oil supply test normal, replace the boost sensor.

Repair Instructions

Perform the Repair Verification after completing the diagnostic procedure. If any service has been performed on the turbocharger or on the turbocharger components, a turbocharger relearn must be performed. Failure to perform the turbocharger learn procedure may cause this DTC to set. Refer to Turbocharger Learn .

    •  Manifold Absolute Pressure Sensor Replacement
    •  Turbocharger Vane Position Sensor Replacement

Repair Verification

Engine running, command the TC Vane Pos. Ctrl. Solenoid ON with a scan tool. Take a snapshot of the engine data while slowly increasing the engine speed to 1,700 RPM, and then back to idle. Plot the snapshot and verify that the Boost Sensor parameter increases smoothly as the engine speed is increased and decreased smoothly as the engine speed is returned to idle.