MotoLogic P0171, P0172 User Manual

1/6/2021 DTC P0171 or P0172 • 2009 Chevrolet Cobalt • MotoLogic

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DTC P0171 or P0172

Diagnostic Instructions

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Review Strategy Based Diagnosis for an overview of the diagnostic approach.

Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0171

Fuel Trim System Lean

DTC P0172

Fuel Trim System Rich

Circuit/System Description

The engine control module (ECM) controls the air/fuel metering system in order to provide the best possible

combination of driveability, fuel economy, and emission control. Fuel delivery is controlled differently during Open

Loop and Closed Loop (CL). During Open Loop, the ECM determines fuel delivery based on sensor signals without

heated oxygen sensor (HO2S) input. During CL, the ECM adds HO2S inputs and level of purge to calculate the

short and long term fuel trim (FT) adjustments. If the HO2S indicates a lean condition, the FT values will be above

0 percent. If the HO2S indicates a rich condition, the FT values will be below 0 percent. The short term FT values

change rapidly in response to the HO2S voltage signals. The long term FT makes coarse adjustments in order to

maintain an air/fuel ratio of 14.7:1. A block of cells contain information arranged in combinations of engine RPM and

engine load for a full range of vehicle operating conditions. The long term FT diagnostic is based on an average of

cells currently being used. The ECM selects the cells based on the engine speed and load. The FT 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.

If the ECM detects an excessively lean condition, DTC P0171 sets. If the ECM detects an excessively rich

condition, DTC P0172 sets.

Conditions for Running the DTC

DTCs P0030, P0053, P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0116, P0117, P0118,

P0120, P0121, P0122, P0123, P0128, P0131, P0132, P0133, P0134, P0135, P0201–P0204, P0220,

P0222, P0223, P0300, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0496,

P0506, P0507, P060D, P1133, P1516, P2101, P2119, P2120, P2122, P2123, P2125, P2127, P2128,

P2135, P2138, P2176, P2A00 are not set.

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1/6/2021 DTC P0171 or P0172 • 2009 Chevrolet Cobalt • MotoLogic

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The engine is in CL status.

The engine coolant temperature (ECT) is between −7 and +120°C (+19.4 and +248°F).

The intake air temperature (IAT) is between −7 and +145°C (+19.4 and +293°F).

The manifold absolute pressure (MAP) is between 15–100 kPa.

The vehicle speed is less than 132 km/h (82 mph).

The engine speed is between 400–6,100 RPM.

The mass air flow (MAF) is between 1–512 g/s.

The barometric pressure (BARO) is more than 74 kPa.

The fuel level is more than 10 percent.

These DTCs run continuously when the above conditions have been met.

Conditions for Setting the DTC

The long term FT weighted average value is more or less than a calibrated value.

The above condition is present for approximately 3 minutes after the conditions for running the DTC have

been met.

Action Taken When the DTC Sets

DTCs P0171 and P0172 are Type B DTCs.

Conditions for Clearing the MIL/DTC

DTCs P0171 and P0172 are Type B DTCs.

Reference Information

Electrical Information Reference

Circuit Testing

Connector Repairs

Testing for Intermittent Conditions and Poor Connections

Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Important: Disregard any transmission symptoms, antilock brake system (ABS) indicators, and traction control

system (TCS) indicators until any fuel trim faults are repaired. A fuel trim fault may cause default actions such as

harsh shifts and illumination of the ABS/TCS indicators.

Verify that other DTCs are not set.

If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

Allow the engine to reach operating temperature. With the engine running, observe the HO2S parameter

with a scan tool. The HO2S value should vary from approximately 40 mV to approximately 900 mV, and

respond to fueling changes.

If the value does not vary from approximately 40 mV to approximately 900 mV, refer to DTC

P0131, P0132, P0137, or P0138 or DTC P0133, P0134, P013A, P013B, P013E, P013F, P0140,

P1133, P2270, P2271, or P2A00.

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