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DTC 121: MAF Signal

  1. If signal from MAF sensor is less than .2 volt or greater than 2.2 volts at idle, or is considered incorrect according to RPM and throttle angle, this is interpreted by ECM as a fault and DTC 121 is set. If fault is permanent and signal is too low, go to step 9). If fault is permanent and signal is too high, go to step 4). If fault is intermittent, go to next step.
  2. If fault is intermittent and signal is too high, check all wiring and connectors for loose contacts and open circuits. Check wiring between MAF sensor connector terminal No. 1 and ECM connector terminal A3 for an intermittent open circuit. Check wiring between MAF sensor terminal No. 2 and ECM connector terminal A5 for an intermittent open circuit. Also, check wiring between MAF sensor terminal No. 4 and ECM connector terminal A4 for an intermittent short to voltage. Repair as necessary.
  3. If fault is intermittent and signal is low, ensure fresh air intake is intact, and upper and lower charge air cooler pipes are correctly attached. Check wiring between main relay connector terminal No. 3 and MAF sensor connector terminal No. 3 for an intermittent open circuit. Also check wiring between MAF sensor connector terminal No. 4 and ECM connector terminal A4 for an intermittent open or short circuit to ground.
  4. Turn ignition off. Wait about 150 seconds and disconnect MAF sensor. Connect ohmmeter between MAF sensor connector terminal No. 1 and ground. If ohmmeter reads about zero ohms, go to next step. If ohmmeter does not indicate about zero ohms, check ECM connector for good terminal contact. Repair as necessary. Repeat test. If ECM connector terminal contact is okay, and/or ohmmeter still does not read about zero ohms, check wiring between MAF sensor connector terminal No. 1 and ECM connector terminal A3 for an open circuit.
  5. Ensure ignition is off and MAF sensor is disconnected. Connect ohmmeter between MAF sensor connector terminal No. 2 and ground. If ohmmeter indicates about zero ohms, go to step 7). If ohmmeter does not indicate about zero ohms, go to next step.
  6. Ensure ignition is off and MAF sensor is disconnected. Ensure ECM connector terminals are not oxidized and make good contact. Reconnect ECM. Connect ohmmeter between MAF sensor connector terminal No. 2 and ground. If ohmmeter indicates about zero ohms, go to next step. If ohmmeter does not indicate about zero ohms, check wiring between MAF sensor connector terminal No. 2 and ECM connector terminal A5 for an open circuit.
  7. Ensure ignition is on and MAF sensor is disconnected. Connect voltmeter between MAF sensor connector terminal No. 4 and ground. If voltmeter indicates 0-1 volt, go to next step. If voltmeter does not indicate 0-1 volt, check wiring between MAF sensor connector terminal No. 4 and ECM connector terminal A4 for a short to voltage. If voltage reading is still incorrect after repair, or short to voltage is not found, replace MAF sensor.
  8. Turn ignition off and reconnect MAF sensor. Turn ignition on. Connect voltmeter between MAF sensor connector terminals No. 2 and 4. If voltmeter indicates about .1-.2 volt, DTC is setting because of poor contact in MAF sensor connector and/or ECM connector. If voltmeter does not indicate about .1-.2 volt, replace MAF sensor.
  9. Ensure ignition is off. Ensure fresh air hoses and upper and lower charge air pipes are undamaged and securely connected. If hoses and pipes are okay, disconnect MAF sensor. Turn ignition on. Connect voltmeter between MAF sensor connector terminal No. 3 and ground. If battery voltage is present, go to next step. If battery voltage is not present, check wiring between MAF sensor connector terminal No. 3 and main relay connector terminal No. 3 for an open circuit.
  10. Ensure ignition is off. Wait 150 seconds, then disconnect MAF sensor. Connect ohmmeter between MAF sensor connector terminal No. 4 and ground. If ohmmeter indicates 9-11 k/ohms, go to next step. If ohmmeter shows about zero ohms, check wiring between MAF sensor connector terminal No. 4 and ECM connector terminal A4 for a short to ground. If ohmmeter shows infinite resistance, check wiring between MAF sensor connector terminal No. 4 and ECM connector terminal A4 for an open circuit.
  11. Turn ignition off. Reconnect MAF sensor connector. Turn ignition on. Connect voltmeter between MAF sensor connector terminals No. 4 and 2. If voltmeter shows .1-.2 volt, check for poor contact in MAF sensor connector and/or ECM connector. If voltmeter does not show .1-.2 volt, test system using a known-good MAF sensor.
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