Circuit Description
A closed loop air/fuel metering system is used in order to provide the best possible combination of driveability, fuel economy, and emissions control. While in closed loop, the Powertrain Control Module (PCM) monitors the heated oxygen sensor (HO2S) 1 signal and adjusts the fuel delivery based upon the HO2S signal voltages. A change made to the 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 approximately 0 percent. If the HO2S signals indicate a lean condition the PCM will add fuel, resulting in fuel trim values that are more than 0 percent. If a rich condition is detected, the fuel trim values will be less than 0 percent, indicating that the PCM is reducing the amount of fuel that is delivered. If an excessively rich condition is detected, the PCM will set DTC P0172. The PCM's maximum authority to control the long term fuel trim allows a range between -30 percent and +22 percent. The PCM's maximum authority to control the short term fuel trim allows a range between -27 percent and +27 percent. The PCM monitors the fuel trim during various operating conditions and monitors the fuel trim cells before determining the status of the fuel trim diagnostic. The fuel trim cells are as follows:
Cell | Purge ON | Purge OFF |
|---|---|---|
Idle (Cell 0) | X | -- |
Decel (Cell 1) | X | -- |
Normal (Cell 2) | X | -- |
Accel (Cell 3) | -- | -- |
High Flow (Cell 4) | -- | -- |
Long Term FT average less than -22% | ||
Short Term FT less than 1% | ||
The vehicle may have to be operated in all of the above conditions marked by an X before the PCM determines the fuel trim status. The amount of fuel trim deviates from 0 percent in each cell, and the amount that each cell is used directly affects whether or not the vehicle must be operated during conditions described in all of the cells described above in order to complete the diagnostic.
Conditions for Running the DTC
| • | No MAF, ECT, TP, MAP, CMP, CKP, IAT sensor, EVAP, EGR, HO2S, Misfire, Injector circuit, Idle speed, VSS, IC circuit DTCs are set. |
| • | The engine coolant temperature is between 20°C (68°F) and 110°C (230°F). |
| • | The intake air temperature is between -18°C (0°F) and 70°C (158°F). |
| • | The manifold absolute pressure is between 18--80 kPa. |
| • | The throttle angle is steady and less than 90 percent. |
| • | The vehicle speed is less than 132 km/h (82 mph). |
| • | The engine speed is between 600--4000 RPM. |
| • | The baro is more than 70 kPa. |
| • | The air flow is between 3-- 150 gm/s. |
Conditions for Setting the DTC
| • | The long term FT average is less than -22 percent. |
| • | The short term FT is less than 1 percent. |
| • | All conditions have been met in the fuel trim cells. |
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/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has 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 a scan tool. |
Diagnostic Aids
Use the following table in order to identify the system or component that is suspected:
System/ Component | Condition |
|---|---|
Sensors | HO2S wiring for correct routing, and contamination. MAF sensor for fluctuation and correct operation, TP sensor for correct throttle response. ECT and IAT sensor for a skewed condition. Refer to the affected sensor performance DTC, Temperature Versus Resistance , and Harness Routing Views in Wiring Systems. |
Engine/PCM Grounds | Loose or corroded connections--Refer to Ground Distribution Schematics in Wiring Systems. |
Fuel System | High fuel pressure--Refer to Fuel System Pressure Test . |
Fuel Metering | Mechanically stuck or electrically inoperable fuel injectors--Refer to Fuel Injector Balance Test . |
Fuel Quality | Fuel for water, alcohol, or dirt contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis . |
EVAP/ORVR | Leaking or plugged EVAP Components--Refer to Evaporative Emission Control System Diagnosis . Leaking or plugged ORVR components--Refer to Poor Fuel Fill Quality . |
Ignition System | Inspect the Ignition System for fouled, damaged, or corroded ignition components. Refer to Electronic Ignition (EI) System Diagnosis . |
Vacuum leaks/ Unmetered air | Vacuum hoses, intake manifold, throttle body, IAC valve, EGR valve, crankcase ventilation, MAF sensor and air intake duct for correct routing and connection--Refer to Visual/Physical Inspection in Symptoms . |
Cooling System | Correct cooling system operation. Inspect the thermostat for correct operation. Refer to Thermostat Diagnosis in Engine Cooling. |
Exhaust System | Inspect the exhaust for leaks, dents, or a restriction. Refer to Exhaust System Inspection in Engine Exhaust. |
Engine | Inspect the engine for mechanical damage. Refer to Engine Mechanical. |
Also inspect for the following:
Many situations may lead to an intermittent condition. Perform each inspection or test as directed.
Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.
| • | Loose terminal connection |
| - | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| - | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| • | Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
| • | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
DTCs other than P0172 may indicate the presence of a condition which may cause a rich 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 Failure Records conditions are being duplicated, the rich condition is intermittent. Refer to Diagnostic Aids or Symptoms for additional information on diagnosing intermittent problems.
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||
Are any DTCs set other than P0172? | -- | Go to the applicable DTCs | ||||||
3 |
Are the displayed values at or near the specified values?
| -22 % +1 % | ||||||
Does the scan tool indicate that DTC P0172 failed this ignition? | -- | Go to Diagnostic Aids | ||||||
5 | Inspect the air filter element and replace it if necessary. Refer to Air Filter Element Replacement . Did the air filter require replacement? | -- | ||||||
6 | Inspect the air intake duct for being collapsed or restricted and repair the duct if necessary. Did your inspection reveal a condition that requires a repair? | -- | ||||||
7 | Inspect the throttle body inlet screen for damage or for the presence of foreign objects which may partially block airflow through the screen and correct any problem that you find. Did your inspection of the throttle body reveal a condition that requires a repair or replacement? | -- | ||||||
8 | Start the engine and note the idle quality. Is the idle low or unsteady? | -- | ||||||
9 | With the engine idling, observe the IAC display on the scan tool. Is IAC value less than the specified value? | 100 Counts | ||||||
10 |
Did your inspection reveal a condition that requires repair? | -- | ||||||
11 |
Did the System Check isolate a problem that requires a repair? | -- | ||||||
12 |
Did the fuel pressure regulator require replacement? | -- | ||||||
13 |
Does the TP Angle display increase steadily and evenly from the minimum value at closed throttle to the maximum value at wide open throttle? | 0-100 % | ||||||
14 |
Did both values change to near the specified value? | 0 % | ||||||
15 |
Did the pressure test isolate a problem that requires a repair? | -- | ||||||
16 |
Did the system check isolate a problem requiring repair? | -- | ||||||
17 | Perform the Injector Balance Test. Refer to Fuel Injector Balance Test . Did the Injector Balance Test isolate a problem that requires a repair? | -- | ||||||
18 |
Did sensor require replacement? | -- | Go to Diagnostic Aids | |||||
19 | Replace the TP sensor. Refer to TP Sensor Replacement . Did you complete the replacement? | -- | -- | |||||
20 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Did you complete the replacement? | -- | -- | |||||
21 |
Does the scan tool indicate that DTC P0172 failed this ignition? | -- | System OK |
