Symptoms
Symptoms of a P2177 engine code may include:
- Malfunction Indicator Lamp (MIL) illuminated
- Lack of power
- Occasional misfires
- Poor fuel economy
Possible causes for this code definition are:
- Faulty Air/Fuel Ratio sensor / Oxygen sensor (AFR/O2)
- Faulty Mass Air Flow sensor (MAF)
- Rarely – Faulty Powertrain Control Module (PCM)
Diagnostic and Repair Procedures
A good starting point is always to check the Technical Service Bulletins (TSBs) for your specific vehicle. Your issue might be a known problem with a published fix from the manufacturer and could save you time and money during diagnosis.
First, note if there are any other Diagnostic Trouble Codes. If any are related to the fuel/fuel system, diagnose those first. Misdiagnosis is known to occur if a technician diagnoses this code before codes related to the fuel system have been thoroughly diagnosed and dismissed. Ensure there are no intake or exhaust leaks. An intake leak, or vacuum leak, causes the engine to idle roughly. An exhaust leak makes the engine appear to be running lean to the AFR/O2 sensor.
Once located, visually inspect the connectors and wiring. Look for chafing, rubbing, bare wires, burns, or melted plastic. Separate the connectors and carefully inspect the terminals (the metal parts) inside the connectors. See if they look corroded, burnt, or perhaps greenish compared to the normal metallic color you are probably used to seeing. You can get electrical contact cleaner at any auto parts store if terminal cleaning is needed. If that’s not possible, find 91% isopropyl alcohol and a soft plastic-bristled brush to clean them. Then, let them air dry, get some dielectric silicone compound (the same kind used for bulb sockets and spark plug wires), and apply a small amount where the terminals make contact.
If you have a scan tool, clear the Diagnostic Trouble Codes from the memory and see if this code returns. If it does not, the connections were likely your problem.
If the code returns, we will need to test the voltage signal from the MAF sensor to the PCM. Monitor the MAF sensor voltage on your scan tool. If a scan tool is not available, test the signal from the MAF sensor with a Digital Volt Ohm Meter (DVOM). With the sensor connected, the red lead of the voltmeter should be attached to the MAF sensor signal wire and the black voltmeter lead connected to ground. Start the engine and monitor the MAF sensor input. As the engine RPM increases, the MAF sensor signal should increase. Check the manufacturer’s specifications, as there may be a chart informing you of the voltage at a given RPM. If it fails this test, replace the MAF sensor and retest.
If the previous tests passed and the code is still present, monitor the Air/Fuel Ratio sensor / Oxygen sensor (AFR/O2). If it continuously indicates the engine is running lean, locate all possibilities that could cause a lean running engine. These include:
- Intake or exhaust leaks
- Fuel system, including fuel pressure regulator / fuel pressure.
- Fuel pressure sensor
- Fuel injectors
- Post-catalytic converter O2 sensor
- EVAP system, to include the canister purge regulation valve.
- If the AFR/O2 sensor indicates the engine is running normally or even rich, a PCM may be suspected once all other issues have been ruled out.
Again, it cannot be overemphasized that all other codes must be diagnosed before this one, as problems causing other codes to set can also cause this one.