GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1998 🢒 Cavalier (CNG) 🢒 DTC 🢒 DTC 55 Fuel Trim System Lean

Object Number: 384484 Size: MF
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 lean condition is detected with fuel trim at the limit, the ECU set DTC 55.

This is a type B DTC.

Conditions for Setting the DTC
    • The vehicle is in CNG operation mode.
    • The CNG fuel pressure sensor (FPS) voltage is greater than 1.95 volts (with the fuel level above 1/8 tank).
    • The ECT is greater than 85°C (185°F).
    • A lean condition exists.

Action Taken When the DTC Sets

The ECU will set a DTC 55 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 55.

Any condition that results in lean fueling can cause DTC 55 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.

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

    2.

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

    3.

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

    4.

  4. This step inspects for O2 low circuit wiring problems. If the O2 low circuit is open, the ECU will read the O2 signal voltage improperly.

    5.

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

    6.

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

    7.

  7. 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

No

1

Was the AF Powertrain OBD System Check performed?

--

Go to Step 2

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

2

Are any component-related DTCs set?

--

Go to component DTC tables

Go to Step 3

3

Visually and physically inspect the following items:

    • The oxygen sensor must be installed securely. The pigtail harness must not contact the exhaust manifold or the ignition wires.
    • The vacuum hoses for splits, kinks, and the proper connections.
    • The throttle body, the intake manifold, the induction hoses and the EGR valve for vacuum leaks.
    • The crankcase ventilation valve, the spring, and the grommet for proper installation.
    • 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.
    • The exhaust system for corrosion in close proximity of the O2S 1 location. Also inspect for leaks and loose or missing hardware.

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

--

Go to Step 15

Go to Step 4

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

5

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

Were any PCM DTCs set?

--

Go to the applicable PCM DTC table

Go to Step 6

6

  1. Disconnect the O2S 1 connector.
  2. Disconnect ECU connector C001.
  3. Measure the O2S 1 signal circuit for resistance greater than the specified value using a DVOM.
  4. 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 7

7

  1. Disconnect PCM connector C2.
  2. Measure the O2S 1 low circuit between PCM connector C2 and ECU connector C001 using a DVOM for resistance greater than the specified value.

Was a problem found?

0.5 ohms

Go to Step 13

Go to Step 8

8

Inspect for the following conditions:

    • Loose or missing clamps on the GMS/MCV inlet and outlet hoses.
    • The condition of the GMS/MCV inlet and outlet hoses.
    • Verify that the GDI is properly installed on the throttle body.

Was a problem found and repaired?

--

Go to Step 15

Go to Step 9

9
  1. Connect the scan tool.
  2. Turn ON the ignition. Leave the engine OFF.
  3. 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

10

  1. Backprobe the MAP sensor signal circuit in ECU connector C001.
  2. 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

11

  1. Backprobe the ECT sensor signal circuit in ECU connector C001.
  2. 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

12

  1. Backprobe the IAT sensor signal circuit in ECU connector C001.
  2. 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

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

--

14

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

Is the action complete?

--

Go to Step 15

--

15
  1. Clear the DTC.
  2. 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

16

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

--

Go to the applicable DTC table

System OK