GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 Sonoma Pickup - 2WD 🢒 DTC 🢒 DTC P0172 Fuel Trim System Rich

System Description

In order to provide the best possible combination of driveability, fuel economy, and emission control, a Closed Loop air and fuel metering system is used. While in a Closed Loop, the PCM monitors the oxygen sensor signal voltage. The PCM adjusts fuel delivery based on signal voltage. The long and short term fuel trim values will indicate a change made to the fuel delivery. The ideal fuel trim values are around 128 (0 percent). Whenever the oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 128 (0 to 100 percent). Whenever a rich condition is detected, the fuel trim values will read below 128 (0 to 100 percent), indicating that the PCM is reducing the amount of fuel delivered. Whenever the exhaust emissions reach an excessive level due to a rich condition, a fuel trim DTC will set.

Conditions for Running the DTC
    • No active MAP DTC's
    • No active IAT DTC's
    • No active ECT DTC's
    • No active TP DTC's
    • No active Oxygen Sensor DTC's
    • No active Injector Control DTC's
    • No active Misfire DTC's
    • No active KS DTC's
    • No active CKP DTC's
    • No active CMP DTC's
    • No active EVAP DTC's
    • No active VSS DTC's
    • No active IAC DTC's
    • No active PCM Memory DTC's
    • Intake Air Temperature (IAT) is between -25°C (-13°F) and 115°C (175°F).
    • Engine Coolant temperature is between 60°C (140°F) and 115°C (239°F).
    • Engine speed is between 850 RPM and 3400 RPM.
    • Vehicle speed is below 82 mph (132 km/h).
    • MAP is more than 26 kPa.
    • BARO is more than 72 kPa.
    • The fuel level is more than 10 percent.

Conditions for Setting the DTC
    • Fuel Trim Index is below 75 (-46 percent).
    • Above conditions present for over 16 seconds.

Action Taken When the DTC Sets
    • The Malfunction Indicator Lamp (MIL) illuminates if the fault is active within the same conditions for 2 out of 80 ignition cycles.
        OR
    • The MIL will illuminates after two consecutive ignition cycles in which the diagnostic runs with the fault active.
    • The PCM records the operating conditions at the time the diagnostic fails. The Freeze Frame and Failure Records buffers stores this information.
    • A history DTC is stored.

Conditions for Clearing the MIL/DTC
    • The MIL will turn off after 3 consecutive ignition cycles in which the diagnostic runs without a fault within the freeze frame conditions that the DTC failed.
    • A history DTC will clear after 40 consecutive warm up cycles without a fault.
    • A scan tool can clear the DTCs.

Diagnostic Aids
    • Check for a poor connection at the PCM. Inspect harness connectors for the following conditions:
       - Backed out terminals
       - Improper mating
       - Broken locks
       - Improperly formed or damaged terminals
       - Poor terminal to wire connection
    • Check for a damaged harness. If the harness appears to be OK, observe the O2S 1 display on the scan tool while moving the connectors and wiring harness related to the engine harness. A change in the display will indicate the location of the fault.

An intermittent may be caused by any of the following conditions:

    • A poor connection
    • Rubbed through wire insulation
    • A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table.

  1. The Powertrain OBD System Check prompts you to complete some of the basic checks and to store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data captured when the malfunction occurred. The scan tool stores this data for later reference.

    2.

  2. A more negative value indicates the PCM is leaning the fuel mixture (Rich condition) and a more positive value indicates the PCM is richening the fuel mixture (Lean condition).

    3.

  3. This step checks for a fuel delivery concern.

Step

Action

Value(s)

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

Important: : If any DTCs are set other than P0172, refer to those DTCs before proceeding with this diagnostic.

  1. Install the scan tool.
  2. Idle the engine in closed loop at normal operating temperature.
  3. Monitor the Long Term FT using the scan tool.

Is the Long Term FT value closer to 0% than the specified value?

--14 %

Go to Step 3

Go to Step 4

3
  1. Turn ON the ignition leaving the engine OFF.
  2. Review the Freeze Frame and Failure Records for this DTC and observe the parameters.
  3. Turn OFF the ignition for 15 seconds.
  4. Start the engine.
  5. Operate the vehicle within the conditions for this diagnostic to run, and as close to the conditions recorded in Freeze Frame or Failure Records as possible.
  6. Select DTC and the Specific DTC function.
  7. Enter the DTC number which was set.

Does the scan tool indicate that this diagnostic failed this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

Visually and physically check the following items:

    • The air filter for being restricted.
    • The air intake duct for being collapsed or restricted
    • The MAF sensor for proper installation and any screen blockage.
    • IAC: If a low or unsteady idle is being experienced.
    • Check for throttle body coking or objects blocking the IAC passages.
    • O2S 1 electrical connector for contact with the exhaust system.
    • Check the fuel pressure regulator for proper operation or a slow diaphragm leak. Refer to Fuel System Diagnosis
    • Excessive fuel in the crankcase. Change oil if necessary.

Did any of the above checks isolate a condition requiring repair?

--

Go to Step 9

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Turn ON the ignition leaving the engine OFF.
  3. The fuel pump should run for about 2 seconds.
  4. It may be necessary to cycle the ignition ON more than once in order to obtain maximum pressure.
  5. Monitor the fuel pressure with the fuel pump running. The fuel pressure should be within the first specified value range.
  6. When the fuel pump stops, the fuel pressure may vary slightly, then it should hold steady within the second value range.

Is fuel pressure within the specified value ranges and does it hold?

248-325

kPa (41-47 psi)

Go to Step 6

Go to Step 12

6

Perform the Fuel Injector Balance Test .

Was an injector problem found?

--

Go to Step 10

Go to Step 7

7

Perform the Evaporative Emission Control System Diagnosis .

Was a problem found?

--

Go to Step 8

Go to Diagnostic Aids

8
  1. Repair the EVAP System.
  2. Use the Fuel Trim Reset function on the scan tool.
  3. Operate the vehicle in Closed Loop while monitoring the Long Term FT display.
  4. The Long Term FT should increase above the specified value.

Is the Long Term FT value closer to 0% than the specified value?

-14 %

Go to Step 11

Go to Step 4

9
  1. Repair the items found in Step 4.
  2. Use the Fuel Trim Reset function on the scan tool.
  3. Operate the vehicle in Closed Loop while monitoring the Long Term FT display.
  4. The Long Term FT value should increase above the specified value.

Is the Long Term FT value closer to 0% than the specified value?

-14 %

Go to Step 11

Go to Step 5

10
  1. Replace the fuel injector. Refer to Fuel Injector Replacement .
  2. Use the Fuel Trim Reset function on the scan tool.
  3. Operate the vehicle in Closed Loop while monitoring the Long Term FT display.

The Long Term FT value should increase above the specified value.

Is the Long Term FT value closer to 0% than the specified value?

-14 %

Go to Step 11

Go to Step 5

11

Rich condition is not present. Does a driveability symptom still exist?

 

Go to Symtoms

Go to Step 13

12

Refer to Fuel System Diagnosis .

Is the action complete?

--

Go to Step 13

--

13
  1. Using the scan tool, clear the DTCs.
  2. Start the engine.
  3. Select DTC and the Specific DTC function.
  4. Enter the DTC number which was set.
  5. Operate the vehicle, with the conditions for Setting this DTC, until the scan tool indicates the diagnostic Ran.

Does the scan tool indicate the diagnostic passed?

--

Go to Step 14

Go to Step 2

14

Does the scan tool display any additional undiagnosed DTCs?

--

Go to Applicable DTC Table

System OK