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Home >> Chevrolet >> 2018 >> Trax Premier, AWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 10 (Engine Controls And Fuel - 1.4L (LE2) - Diagnostic Information And Procedures (P0325-P2635) And Symptoms) >> Diagnostic Information and Procedures >> DTC P121B or P121C >> Circuit/System Testing

Circuit/System Testing

WARNING: This page is about a different car, the 2018 Buick Encore. However, it is still accessible from the selected car via links, so may be relevant.
  1. Ignition OFF, disconnect the B75C Multifunction Intake Air sensor harness.
  2. Ignition ON, verify that a test lamp illuminates between the ignition circuit terminal 5 and ground.
    • If the test lamp does not illuminate and the circuit fuse is good 
    1. Ignition OFF, remove the test lamp and remove the fuse for the ignition voltage circuit.
    2. Test for less than 2 Ω in the ignition voltage circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, verify the fuse is not open and there is voltage at the fuse.
    • If the test lamp does not illuminate and the circuit fuse is open 
    1. Ignition OFF, remove the test lamp and remove the fuse for the ignition voltage circuit.
    2. Test for infinite resistance between the ignition voltage circuit and ground.
      • If less than infinite resistance, repair the short to ground on the circuit.
      • If infinite resistance, test all components connected to the ignition circuit, replace as necessary.
    • Go to next step: If a test lamp illuminates 
  3. Ignition OFF, connect the test lamp between the control circuit terminal 4 and the ignition voltage circuit terminal 5.
  4. Ignition ON. Command the MAF Sensor Supply Voltage Low Side Driver ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
    • If the test lamp is always ON 
    1. Ignition OFF, remove the test lamp.
    2. Test for infinite resistance between the control circuit and ground.
      • If less than infinite resistance, repair the short to ground on the circuit.
      • If infinite resistance, replace the K20 Engine Control Module.
    • If the test lamp is always OFF 
    1. Ignition OFF, disconnect the harness connector at the K20 Engine Control Module.
    2. Test for 2 Ω resistance between the control circuit and ground.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, replace the K20 Engine Control Module.
    • Go to next step: If the test lamp turns ON and OFF 
  5. Test or replace the B75C Multifunction Intake Air sensor.
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