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 oxygen sensor 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, the fuel trim values will be above 0 percent. If the O2S indicate a rich condition, the 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. The fuel trim diagnostic will conduct a test to determine if a rich failure actually exists, or if excessive vapor from the evaporative emission (EVAP) canister is causing a rich condition. The PCM does not perform the excess vapor test on the dedicated CNG fuel system. If the PCM detects an excessively rich condition, DTC P0172 or P0175 sets.
This diagnostic procedure supports the following DTCs:
• | DTC P0172 Fuel Trim System Rich Bank 1 |
• | DTC P0175 Fuel Trim System Rich Bank 2 |
• | 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 and +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 throttle position is less than 90 percent. |
• | The fuel level is more than 10 percent. |
• | The average long term fuel trim value is below -18 percent. |
• | All of the above conditions exist for at least 0.5 second. |
• | 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. |
• | 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. |
• | Fuel contamination, such as water will effect fuel trim. |
• | A malfunctioning MAF sensor can cause a rich condition and set this DTC. Refer to DTC P0101 in Engine Controls in the C/K Pickup Service Manual. |
• | Use a scan tool in order to review Failure Records. If an intermittent condition is suspected, refer to Intermittent Conditions . |
The numbers below refer to the step numbers on the diagnostic table.
If the cause of the DTC is not found and corrected, refer to Fuel System Diagnosis for a possible fuel related condition.
On KL6 equipped vehicles, an EVAP canister that is saturated will cause a rich condition. If the condition is not found, a worn cam, worn intake or exhaust valves, or other engine mechanical failure may be the cause of the DTC.
Step | Action | Values | Yes | No | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) | ||||||||||||
2 |
Important: If any DTCs other than P0172 or P0175 are set, refer to those DTCs before continuing.
Does the scan tool indicate that the long term fuel trim is less than the specified value? | -15% | Go to Step 3 | Go to Diagnostic Aids | ||||||||||||
3 |
Does the scan tool indicate that the values are within the specified range and fluctuating? | 200-800 mV | Go to Step 4 | Go to Step 5 | ||||||||||||
4 |
Did you find and correct the condition? | -- | Go to Step 7 | Go to Step 6 | ||||||||||||
Did you find and correct the condition? | -- | Go to Step 7 | Go to Fuel System Diagnosis in Engine Controls in the C/K Pickup Service Manual | |||||||||||||
Inspect for the following:
Did you find and correct the condition? | -- | Go to Step 7 | Go to Symptoms - Engine Mechanical in Engine Mechanical in the C/K Pickup Service Manual | |||||||||||||
7 |
Important: After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim.
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 8 | ||||||||||||
8 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK |