P121A
DTC P121A
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptors
DTC P121A
- Mass Air Flow (MAF) Sensor Supply Voltage Control Circuit
Diagnostic Fault Information
Circuit/System Description
The mass air flow (MAF) sensor is integrated with the intake 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.
Conditions for Running the DTC
P121A
* The ECM commands the MAF ON.
* The ECM commands the powertrain relay ON.
* The ignition voltage is at least 11 V.
* The DTC runs continuously when the above conditions are met.
Conditions for Setting the DTC
P121A
The ECM detects the commanded and actual states of the MAF control circuit do not match for greater than 5 s.
Action Taken When the DTC Sets
DTC P121A is a Type B DTC.
Conditions for Clearing the MIL/DTC
DTC P121A is a Type B DTC.
Reference Information
Schematic Reference
Engine Controls Schematics [1][2]Electrical Diagrams
Connector End View Reference
Component Connector End Views Hybrid/EV Battery Pack X1
Description and Operation
Hybrid Modes of Operation Description Hybrid Modes of Operation Description
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Control Module References Programming and Relearning for scan tool information
Circuit/System Verification
1. Vehicle in Service Mode:
* MAF Sensor Supply Voltage commanded OFF with a scan tool, the following parameters should display:
- MAF Sensor Supply Voltage Control Circuit Low Voltage Test Status-OK
- MAF Sensor Supply Voltage Control Circuit Open Test Status-OK
- MAF Sensor Supply Voltage Control Circuit High Voltage Test Status-Not Run
* MAF Sensor Supply Voltage commanded ON with a scan tool, the following parameters should be displayed:
- MAF Sensor Supply Voltage Control Circuit Low Voltage Test Status-Not Run
- MAF Sensor Supply Voltage Control Circuit Open Test Status-Not Run
- MAF Sensor Supply Voltage Control Circuit High Voltage Test Status-OK
2. Engine running, the following parameters should display:
* MAF Sensor Supply Voltage Command-ON
* MAF Sensor Supply Voltage Control Circuit Low Voltage Test Status-Not Run
* MAF Sensor Supply Voltage Control Circuit Open Test Status-Not Run
* MAF Sensor Supply Voltage Control Circuit High Voltage Test Status-OK
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.
Circuit/System Testing
1. Vehicle OFF, disconnect the harness connector at the B75B Mass Air Flow/Intake 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 circuit for a short to ground or an open/high resistance.
• If the circuit tests normal and the fuse is open, test all the components connected to the circuit and repair or replace as necessary.
3. Vehicle OFF, 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 Engine Control Module.
• 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 Engine Control Module.
5. If all circuits test normal, test or replace the B75B Mass Air Flow/Intake Air Temperature Sensor.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* Mass Airflow Sensor with Intake Air Temperature Sensor Replacement Service and Repair
* Control Module References Programming and Relearning for ECM replacement, setup, and programming