DTC P0172 Fuel Trim System Rich. GMC Truck GMC K Pickup

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC K Pickup - 4WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 DTC P0172 Fuel Trim System Rich

Engine Controls Components
Engine Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The Heated Oxygen Sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 360°C (600°F) to produce voltage. The HO2S will produce more voltage when the exhaust is rich and less voltage when the exhaust is lean. The HO2S operating voltage range is around 0.10 volts (100 mV) to 1 volt (1000 mV).

The AF ECU utilizes a Closed Loop air/fuel metering system when the HO2S is at operating temperature. During normal Closed Loop fuel control operation the AF ECU will add fuel (enriches the mixture) when the HO2S indicates a lean exhaust content. When the HO2S indicates a rich exhaust content the AF ECU will subtract fuel (leans-out the mixture). This addition and subtraction to the air/fuel mixture may be indicated on a scan tool as the Long and Short Term Fuel Trim values. Short Term Fuel Trim values are adjustments to the fuel mixture that occur quickly. Long Term Fuel Trim values are adjustments to the fuel mixture that accumulate over longer periods of time. The Long and Short Term Fuel Trim values are displayed on a scan tool as both percent and counts. Positive percentage values indicate fuel is being added to the mixture. Negative percentage values indicate fuel is being subtracted from the mixture.

This DTC is used to indicate a fully lean fuel trim system that cannot compensate for a rich engine exhaust content.

Conditions for Running the DTC

•	The engine is operating on alternative fuel.

•	Engine coolant temperature is above 85°C (185°F).

•	The AF ECU is operating in Closed Loop fuel control.

Conditions for Setting the DTC

The AF ECU monitors that the Short Term Fuel Trim is at -50 percent OR Long Term Fuel Trim is at -45 percent for at least 5 seconds continuously.

Action Taken When the DTC Sets

•	A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.

•	The AF ECU will illuminate the MIL and store the DTC as History after TWO consecutive drive trips that the diagnostic runs and fails.

•	The AF ECU will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

•	The Last Test Failed status will clear when the DTC runs and passes.

•	The AF ECU will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.

•	The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.

•	All DTC Information can be cleared using a scan tool.

•	Interruption of Control Module (AF ECU / VCM / PCM or ECM) power or ground MAY clear DTC Information. Clearing DTC Information with this method is inconsistent and should not be performed.

Diagnostic Aids

Fault Not Present

Fault Not Present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to Intermittent Conditions Symptom diagnosis for additional information. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in the OBD System Check.

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

An HO2S contaminated by silicon will have a white, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual cause of silica contamination is the use of un-approved silicon RTV engine gasket material or the use of silicon based sprays or fluids within the engine. If the cause of this contamination is not corrected, the replacement HO2S will also get contaminated.

Perform the DTC P1441 diagnosis in the Engine Controls section of the gasoline service manual in order to determine if an EVAP purge condition is present.

When testing the circuits for continuity the resistance should be less than 5 ohms. When testing the circuits for shorts the resistance should measure infinite with all components disconnected from the circuit.

This step determines if a base engine concern is causing the rich condition or if the rich condition is only present when operating on CNG.

Step

Action

Value(s)

Yes

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

Go to Step 2

Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check

Capture the DTC information from the AF ECU into the scan tool.
Inspect BOTH the AF ECU and the gasoline control module (VCM/PCM or ECM) for other stored DTC's.

Are other DTC's stored?

Go to Step 3

Go to Step 4

Perform the other DTC diagnosis before attempting diagnosis of this DTC.

Is the other DTC diagnosis and/or repair complete?

Go to Step 14

This DTC may have been set by one of the following conditions:

•	A signal wire intermittently shorted to voltage

•	HO2S connector water intrusion

•	A silicon contaminated HO2S

•	A rich engine condition such as the following:

-	EVAP canister purge system malfunction

-	Gasoline contaminated engine oil

-	Incorrect CNG fuel pressure

-	A leaking gasoline fuel pressure regulator

-	An inaccurate MAF sensor

-	a restricted air fliter or intake air duct

•	Repair any of the above or similar engine conditions as necessary.

Was a condition found and repaired?

Go to Step 14

Go to Step 5

Reconnect all disconnected or removed components.
Using a scan tool clear all DTC information from BOTH the gasoline control module (VCM/PCM or ECM) and the AF ECU.
Using the scan tool communicate with the AF ECU and reset the Fuel Trim values to zero.
Turn the ignition OFF and wait 30 seconds.
Start the engine.
View the captured data Freeze Frame records for this DTC (that was stored in step 2). Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set.
Operate the vehicle within the Conditions for Running this DTC and the parameters observed when the DTC set.
Observe the Short Term Fuel Trim values.

Do the fuel trim values indicate a rich mixture condition is present or does a fuel trim DTC set?

Go to Step 7

Go to Step 6

The fault is not present. Refer to Diagnostic Aids.

Are any DTC's stored that have not been diagnosed?

Go to the Diagnostic Trouble Code (DTC) List

Go to Intermittent Conditions

Perform the AF Fuel System diagnosis and return to this step. Refer to Fuel System Diagnosis .

Was a CNG fuel system problem found and repaired?

Go to Step 14

Go to Step 8

Inspect the oxygen sensor Signal and Low circuit for continuity between the AF ECU connector terminals and the oxygen sensor connector terminals.
Inspect these circuits for shorts to other circuits or to ground.

Ensure the oxygen sensor, gasoline control module and the AF ECU connectors are disconnected before testing circuits.

Repair wiring as necessary.

Was a wire repair made?

Go to Step 14

Go to Step 9

Reconnect all disconnected or removed components.
Using a scan tool clear all DTC information from BOTH the gasoline control module (VCM/PCM or ECM) and the AF ECU.
Using the scan tool communicate with the gasoline control module and reset the Fuel Trim values to zero.
Turn the ignition OFF.
Remove the 20 amp CNG fuse in order to force a switch-over to gasoline operation.

The FIL will NOT illuminate due to the removal of the CNG 20 amp fuse.

Start the engine.
View the captured data Freeze Frame records for this DTC (that was stored in step 2). Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set.
Operate the vehicle within the Conditions for Running this DTC and the parameters observed when the DTC set.
Observe the Short Term Fuel Trim values.

Do the fuel trim values indicate a rich mixture condition is present or does a fuel trim DTC set?

Go to Step 10

Go to Step 13

Remove and visually inspect the oxygen sensor for silicon contamination.

Is the oxygen sensor contaminated?

Go to Step 12

Go to Step 11

Repair the possible causes of rich engine operation. Refer to the items in step 4.

Is the repair complete?

Go to Step 14

Important: Before replacing a contaminated HO2S determine and repair the cause of the contamination.

Replace the HO2S. Refer to Heated Oxygen Sensor Replacement in the Engine Controls section of the gasoline service manual.

Is the replacement complete?

Go to Step 14

Replace the GMS/MCV. Refer to Gas Mass Sensor/Mixture Control Valve and Mounting Bracket Replacement in the C/K Supplement.

Is the replacement complete?

Go to Step 14

Reconnect all disconnected or removed components, connectors, fuses, etc.
Using a scan tool clear all DTC information from BOTH the gasoline control module (VCM/PCM or ECM) and the AF ECU.
Using the scan tool communicate with the BOTH the gasoline control module and the AF ECU in order to reset the Fuel Trim values to zero.
Start the engine.
View the captured data Freeze Frame records for this DTC (that was stored in step 2). Observe the data parameters that give an indication of how the vehicle was being operated when this DTC set.
Operate the vehicle within the Conditions for Running this DTC and the parameters observed when the DTC set until the scan tool indicates the diagnostic ran.

Does the scan tool indicate the DTC passed?

System OK

Go to Step 2