P121B
DTC P121B or P121C
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 P121B
- Mass Air Flow (MAF) Sensor Supply Voltage Control Circuit Low
DTC P121C
- Mass Air Flow (MAF) Sensor Supply Voltage Control Circuit High
Diagnostic Fault Information
Typical Scan Tool Data
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. A small quantity of air entering the engine indicates a low load or idle 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 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 control circuit DTC is set.
Conditions for Running the DTC
P121B
* The ignition is ON and the engine is not running.
* The DTC runs continuously when the above conditions are met for greater than 1 s.
P121C
* The ignition is ON and the engine is running.
* The DTC runs continuously when the above conditions are met for greater than 1 s.
Conditions for Setting the DTC
P121B
The ECM detects commanded and actual states of the MAF control circuit do not match for greater than 4 s.
P121C
The ECM detects commanded and actual states of the MAF control circuit do not match for greater than 4 s.
Action Taken When the DTC Sets
DTCs P121B and P121C are Type B DTCs.
Conditions for Clearing the MIL/DTC
DTCs P121B and P121C are Type B DTCs.
Reference Information
Schematic Reference
Engine Controls Schematics [1][2]Electrical Diagrams
Connector End View Reference
Component Connector End Views Hybrid/EV Battery Pack X1 A4
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 Control Module References for scan tool information
Circuit/System Verification
1. Service Mode active, 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. Ignition ON, 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.
Circuit/System Testing
1. Ignition OFF, disconnect the harness connector at the B75B mass air flow/intake air temperature sensor.
2. Service Mode active, 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. Service Mode active. Command the MAF Sensor Supply Voltage 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/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 Control Module References for ECM replacement, setup, and programming