Circuit Description
To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. While in closed loop, the PCM monitors the HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0%; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, indicating that the PCM is reducing the amount of fuel delivered. If an excessively rich condition is detected, the PCM will set DTC P0172.
The PCM's maximum authority to control long term fuel trim allows a range between -23% and +16%. The PCM's maximum authority to control short term fuel trim allows a range between -11% and +20%. The PCM monitors fuel trim under various operating conditions (fuel trim cells) before determining the status of the fuel trim diagnostic. The fuel trim cells used to determine fuel trim are as follows:
| • | Idle, purge commanded (Cell 0). |
| • | Deceleration, purge commanded (Cell 1). |
| • | Normal driving, purge commanded (Cell 2). |
| • | Deceleration, no purge commanded (Cell 6). |
| • | Normal driving, no purge commanded (Cell 7). |
The vehicle may have to be operated in all of the above conditions before the PCM determines fuel trim status. The amount that fuel trim deviates from 0% in each cell and the amount that each cell is weighted directly affects whether or not the vehicle must be operated in all of the cells described above to complete the diagnostic.
Conditions for Setting the DTC
| • | No VSS, TP sensor, EVAP system, HO2S, Misfire, IAT sensor, MAP sensor, IAC, Fuel Injector circuit, IC/Bypass line, EGR Pintle, EGR Flow, ECT sensor, CKP sensor, CMP sensor, MAF sensor DTC(s) set. |
| • | Engine coolant temperature between 20°C (68°F) and 65°C (149°F). |
| • | Intake air temperature is between -18°C (0°F) and 65°C (149°F). |
| • | Manifold absolute pressure is between 18 kPa and 85 kPa. |
| • | Baro is greater than 75 kPa. |
| • | Throttle angle is steady below 90%. |
| • | Vehicle speed is below 70 mph. |
| • | Engine speed between 500 and 4000 rpm. |
| • | Long term fuel trim is at or near maximum authority of -23%. |
| • | Short term fuel trim is at or near maximum authority of -11%. |
| • | All conditions have been met in Fuel Trim Cells 2, 5, 6, and/or 7. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed. |
| • | The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using the scan tool. |
Diagnostic Aids
Check for the following conditions:
| • | Fuel Pressure. The system will go rich if pressure is too high. The PCM can compensate for some increase. However, if fuel pressure is too high, a DTC P0172 may be set. Refer to Fuel System Pressure Test . |
| • | Fuel injector(s). Refer to Fuel Injector Balance Test . |
| • | Check the EVAP Canister for fuel saturation. Refer to Evaporative Emission Canister Replacement . |
| If full of fuel, check canister control and hoses. Refer to Evaporative Emission Control System . |
| • | Disconnect the MAF sensor and see if rich condition is corrected. If so, replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . |
| • | Check for a leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for the presence of fuel. |
| • | An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating. |
| • | Poor connection at PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. |
| • | Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving connectors and wiring harnesses related to the engine harness. A change in the display will indicate the location of the fault. |
Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
-
DTCs other than P0172 may indicate a condition present which may cause a lean condition. If this is the case, repairing the condition which caused the other DTC will most likely correct the DTC P0172.
-
If the DTC P0172 test passes while the Fail Records conditions are being duplicated, the rich condition is intermittent. Refer to Diagnostic Aids for additional information on diagnosing intermittent conditions.
Step | Action | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||
Are any DTCs set other than P0172? | -- | Diagnose the applicable DTCs before proceeding with this table. Refer to Powertrain Control Module Diagnosis . | ||||||
3 |
Are the displayed values at or near the specified values?
| -23% -11% | ||||||
Does the scan tool indicate DTC P0172 failed this ign? | -- | Refer to Diagnostic Aids. | ||||||
5 | Visually and physically inspect the air filter element and replace it if necessary. Refer to Air Cleaner Element . Did the air filter require replacement? | -- | ||||||
6 | Visually and physically inspect the air intake duct for being collapsed or restricted and repair if necessary. Refer to Air Induction System . Did your inspection reveal a condition requiring repair? | -- | ||||||
7 |
Was a problem found? | -- | ||||||
8 | Start the engine and note the idle quality. Is a low or unsteady idle being experienced? | -- | ||||||
9 | With the engine idling, observe the Idle Air Control display on the scan tool. Is Idle Air Control value less than the specified value? (Value is displayed in counts). | 100 | ||||||
10 |
Was a problem found? | -- | ||||||
11 |
Was a problem found? | -- | ||||||
12 |
Did the fuel pressure regulator require replacement? | -- | ||||||
13 |
Does the TP Angle display increase steadily and evenly from minimum value at closed throttle to maximum value at wide-open throttle? | 0%-100% | ||||||
14 |
Did both values change to near the specified value? | 0% | ||||||
15 |
Was problem found? | -- | ||||||
16 |
Was a problem found? | -- | ||||||
17 |
Was a problem found? | -- | ||||||
18 |
Did the sensor require replacement? | -- | Refer to Diagnostic Aids. | |||||
19 |
Is action complete? | -- | -- | |||||
20 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is action complete? | -- | -- | |||||
21 |
Does the scan tool indicate DTC P0172 failed this ign? | -- | System OK |
