P0174

DTC P0174

CIRCUIT DESCRIPTION
The powertrain control module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. Compressed natural gas (CNG) fuel delivery is controlled differently during Open and Closed Loop. During Open Loop, the PCM determines fuel delivery based on sensor signals without oxygen sensor (O2S) input. During Closed Loop, the O2S inputs are added and used by the PCM to calculate short and long term fuel trim fuel delivery adjustments. If the O2S indicate a lean condition, fuel trim values will be above 0 percent. If the O2S indicate a rich condition, fuel trim values will be below 0 percent. Short term fuel trim values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. Long term fuel trim makes coarse adjustments in order to maintain an air/fuel ratio of 16.5:1 while operating on CNG. If the PCM detects an excessively lean condition, DTC P0171 or P0174 sets.

DTC DESCRIPTORs
This diagnostic procedure supports the following DTCs:
- DTC P0171 Fuel Trim System Lean Bank 1
- DTC P0174 Fuel Trim System Lean Bank 2

CONDITIONS FOR RUNNING THE DTC
- DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0200, P0300, P0327, P0332, P0335, P0336, P0351-P0358, P0401, P0402, P0403, P0443, P0446, P0449, P0496, P0502, P0503, P1020-P1028, or P1258 are not set.
- The engine coolant temperature (ECT) is between 50-115°C (122-239°F).
- The intake air temperature (IAT) is between -20 to +90°C (4-194°F).
- The manifold absolute pressure (MAP) is between 26-90 kPa (3.7-13 psi).
- The vehicle speed is less than 137 km/h (85 mph).
- The engine speed is between 400-3,000 RPM.
- The barometric pressure (BARO) is more than 74 kPa (10.7 psi).
- The mass air flow (MAF) is between 5-90 g/s.
- The fuel level is more than 10 percent.
- The throttle position is less than 90 percent.

CONDITIONS FOR SETTING THE DTC
- The average long term fuel trim cell value is above 28 percent.
- All of the above conditions exist for at least 2.5 seconds.

ACTION TAKEN WHEN THE DTC SETS
- The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
- The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
- The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
- Clear the MIL and the DTC with a scan tool.

DIAGNOSTIC AIDS
- The system will go lean if an injector is not suppling enough fuel.
- A lean condition could be present during high fuel demand. Refer to Fuel System Diagnosis in order to test for proper CNG fuel pressure. Component Tests and General Diagnostics
- Use a scan tool in order to review Failure Records. If an intermittent condition is suspected, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Step 1 - Step 6:

Step 7 - Step 13:

The numbers below refer to the step numbers on the diagnostic table.
5. The PCM will adjust fuel trim based upon fuel rail pressure (FRP) sensor values. A false high pressure FRP value will cause the PCM to inaccurately lean the fuel trim.
8. If conditions were not corrected, a worn cam, worn intake or exhaust valves, or other engine mechanical failure may be the problem.
9. The fuel injector control module (FICM) uses the Injector Monitor circuits in order to calculate CNG fuel injector amperage. An inaccurate even bank circuit amperage reading will cause a cylinder #6 misfire, bank 1 sensor 1 voltage to be near 900 mV and bank 2 sensor 1 voltage to be near 0 mV. An inaccurate odd bank circuit amperage reading will cause a cylinder #1 misfire, bank 1 sensor 1 voltage to be near 0 mV and bank 2 sensor 1 voltage to be near 900 mV.