Volt

DTC P121B or P121C

The mass air flow (MAF) sensor is integrated with the inlet air temperature (IAT) sensor. The MAF sensor is an air flow meter that measures the amount of air entering the engine. The engine control module (ECM) uses the MAF sensor signal to provide the correct fuel delivery for all engine speeds and loads. The air flow and engine speed are very dependent on the battery pack state of charge. A small quantity of air entering the engine indicates a low load condition. A large quantity of air entering the engine indicates a high load condition.

The ECM applies 5 volts to the MAF sensor on the MAF sensor signal circuit. The sensor uses the voltage to produce a frequency based on the inlet air flow through the sensor bore.

The MAF sensor is supplied power through the powertrain relay and a fuse. The ECM enables the MAF sensor only during engine run mode by grounding the MAF control circuit via a solid state devise called a driver. The driver is equipped with a feedback circuit that is pulled-up to a voltage. The ECM monitors the status of the driver. If the ECM detects an incorrect voltage for the commanded state of the driver, a MAF sensor supply voltage low side driver control circuit DTC is set.

P121B

P121C

P121B

The ECM detects the commanded and actual states of the MAF control circuit do not match for greater than 4 s.

P121C

The ECM detects the commanded and actual states of the MAF control circuit do not match for greater than 4 s.

DTCs P121B and P121C are Type B DTCs.

DTCs P121B and P121C are Type B DTCs.

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Hybrid Modes of Operation Description

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Vehicle in Service Mode, observe the control circuit status parameters listed below with a scan tool:

•	MAF Sensor Supply Voltage Control Circuit Low Voltage Test Status

•	MAF Sensor Supply Voltage Control Circuit Open Test Status

•	MAF Sensor Supply Voltage Control Circuit High Voltage Test Status

Each parameter should display OK or Not Run.

2. Engine running, observe the control circuit status parameters listed below with a scan tool:

•	MAF Sensor Supply Voltage Control Circuit Low Voltage Test Status

•	MAF Sensor Supply Voltage Control Circuit Open Test Status

•	MAF Sensor Supply Voltage Control Circuit High Voltage Test Status

Each parameter should display OK or Not Run.

3. Operate the vehicle within the conditions for running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the freeze frame/failure records data.

1. Vehicle OFF, disconnect the harness connector at the B75B mass air flow/inlet air temperature sensor.
2. Vehicle in Service Mode, verify that a test lamp illuminates between the ignition voltage circuit terminal 4 and ground.

⇒	If the test lamp does not illuminate, test the ignition voltage circuit for a short to ground or an open/high resistance. If the circuit tests normal and the ignition voltage circuit fuse is open, test or replace the components on the ignition voltage circuit.

3. Connect the test lamp between the control circuit terminal 2 and the ignition voltage circuit terminal 4.
4. Vehicle in Service Mode. 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, test the control circuit for a short to ground. If the circuit tests normal, replace the K20 ECM.

⇒	If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the K20 ECM.

5. If all circuits test normal, test or replace the B75B mass air flow/inlet air temperature sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.