DTC 56 Fuel Trim System Rich. Chevrolet Cavalier (CNG) 1998

For 1990-2009 cars only

Makes 🢒 Chevrolet 🢒 1998 🢒 Cavalier (CNG) 🢒 DTC 🢒 DTC 56 Fuel Trim System Rich

Handling Electrostatic Discharge Sensitive Parts Notice
Handling Electrostatic Discharge Sensitive Parts Notice
Engine Controls Components
Oxygen Sensors
OBDII Symbol Description Notice

Circuit Description

In order to provide the best possible combination of driveability, fuel economy, and emission control, the vehicle uses a Closed Loop fuel metering system. While in Closed Loop, the ECU monitors the oxygen sensor signal voltage. The ECU adjusts the fuel delivery based on this voltage. The long and short term fuel values indicate a change made to the fuel delivery. If an excessively rich condition is detected with fuel trim at the limit, the ECU set DTC 56.

This is a type B DTC.

Conditions for Setting the DTC

•	The vehicle is in CNG operation mode.

•	The ECT is greater than 85°C (185°F).

•	A rich condition exists.

Action Taken When the DTC Sets

The ECU will set a DTC 56 and turn ON the MIL after two consecutive driving cycles with the fault active.

Conditions for Clearing the MIL/DTC

The ECU turns the MIL off after three consecutive ignition cycles without a fault condition present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.

Diagnostic Aids

Always diagnose ANY PCM codes that are present before beginning this diagnostic procedure.

If present, diagnose and correct any ECU DTCs pertaining to the O2 sensor, the MAP sensor and the gas mass sensor (GMS) before attempting to diagnose DTC 56.

Any condition that results in rich fueling can cause DTC 56 to set.

Inspect for the following conditions:

•	MAP sensor DTCs and wiring problems.

•	O2 sensor DTCs and wiring problems.

•	Vacuum leaks.

•	Air leaks in the exhaust system.

•	A malfunctioning EVAP system.

•	Inspect the harness for the following conditions:

-	Backed out terminals

-	Improper mating

-	Broken locks

-	Improperly formed or damaged terminals

-	Poor terminal to wire connection

-	Damaged harness

Test Description

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

This step visually and physically inspects the items that can cause a rich condition.

This step inspects the MAP sensor and the O2 sensor faults by operating the vehicle on gasoline and interrogating the PCM for DTCs.

This step inspects for a short to power or a high resistance O2 signal circuit.

This step will verify the condition of the MAP sensor circuit. If the readings are different, the circuit is damaged or has high resistance.

This step will verify the condition of the ECT sensor circuit. If the readings are different, the circuit is damaged or has high resistance.

This step will verify the condition of the IAT sensor circuit. If the readings are different, the circuit is damaged or has high resistance.

Step

Action

Value(s)

Yes

Was the AF Powertrain OBD System Check performed?

Go to Step 2

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

Are any component-related DTCs set?

Go to component DTC tables

Go to Step 3

Visually and physically inspect the following items:

•	A collapsed or restricted air intake duct.

•	A plugged air filter.

•	The fuel pressure regulator for leakage. Refer to Fuel System Diagnosis in Engine Controls - 2.2L in the J Platform Service Manual.

•	The fuel injectors for leakage. Refer to Fuel System Diagnosis in Engine Controls - 2.2L in the J Platform Service Manual.

•	The ECU and the sensor grounds for being clean, tight, and in the proper location.

•	The PCM and the sensor grounds for being clean, tight, and in the proper location.

Was a repair necessary?

Go to Step 15

Go to Step 4

Perform the AF fuel system diagnosis. Refer to Fuel System Diagnosis .

Was a problem found and repaired?

Go to Step 15

Go to Step 5

Perform the EVAP Control System Diagnosis. Refer to EVAP Control System Diagnosis in Engine Controls - 2.2L in the J Platform Service Manual.
If the table isolates a problem, repair the problem as necessary.

Did the table isolate a problem requiring repair?

Go to Step 15

Go to Step 6

Remove the 20A CNG system fuse from the engine compartment fuse/relay center.
Start the vehicle.
Perform the AF OBD Trip. The fuel indicator will remain OFF due to the removal of the CNG system fuse.
Turn OFF the ignition. Wait 10 seconds.
Restart the vehicle.
Perform a second diagnostic drive trip procedure.
Inspect for PCM DTCs using the scan tool.
Replace the CNG system fuse.

Were any PCM DTCs set?

Go to the applicable PCM DTC table

Go to Step 7

Disconnect the O2S 1 connector.
Disconnect ECU connector C001.
Measure the O2S 1 signal circuit for resistance greater than the specified value using a DVOM.
Measure the O2S 1 signal circuit for continuity to ground using a DVOM.

Was a problem found?

1.0 ohms

Go to Step 13

Go to Step 8

Turn OFF the ignition.
Remove the O2S 1. Visually and physically inspect the O2S 1 for any silicon contamination. This will be indicated by a powdery white deposit on the portion of the sensor exposed to the exhaust stream.
If there is contamination present on the O2S 1 sensor, find the source of the contamination and make the necessary repairs.
Replace the O2S 1 sensor.

Was there silicon contamination present?

Go to Step 15

Go to Step 9

Connect the scan tool.
Turn ON the ignition. Leave the engine OFF.
Compare the BARO reading with a known good vehicle.

Is the MAP sensor voltage within the typical scan tool data, plus or minus the specified value BARO reading normal for your altitude?

0.4 V

Go to Step 10

Go to DTC 32 MAP Sensor Performance

Backprobe the MAP sensor signal circuit in ECU connector C001.
Measure the voltage of the MAP sensor circuit in the ECU connector using a DVOM. The voltage should be very close to the scan tool reading.

Are the readings close?

Go to Step 11

Go to Step 13

Backprobe the ECT sensor signal circuit in ECU connector C001.
Measure the voltage of the ECT sensor circuit in the ECU connector using a DVOM. The voltage should be very close to the scan tool reading.

Are the readings close?

Go to Step 12

Go to Step 13

Backprobe the IAT sensor signal circuit in ECU connector C001.
Measure the voltage of the IAT sensor circuit in the ECU connector using a DVOM. The voltage should be very close to the scan tool reading.

Are the readings close?

Go to Step 14

Go to Step 13

Repair the circuit as necessary. Refer to Wiring Repairs in Engine Controls - 2.2L in the J Platform Service Manual.

Is the repair complete?

Go to Step 15

Replace the GMS/MCV. Refer to Gas Mass Sensor/Mixture Control Valve Replacement .

Is the action complete?

Go to Step 15

Clear the DTC.
Inspect to see if the MIL light still illuminates after two driving cycles.

Did the MIL illuminate and the DTC set?

Go to Step 3

Go to Step 16

Are any ECU or PCM DTCs displayed that have not been diagnosed?

Go to the applicable DTC table

System OK