Circuit Description
This diagnostic test is designed in order to measure the efficiency
of the three-way catalytic converter (TWC) system. Catalytic convertor efficiency
is a measure of its ability to store oxygen after converting the levels of
hydrocarbon (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) to less
harmful gases. The control module is able to evaluate the catalyst efficiency
once the vehicle has met the enable criteria and the vehicle is at idle instead
of the steady cruise speeds used in the past.
Once the conditions for running this diagnostic trouble code (DTC) are
met, the control module commands either a lean or rich air/fuel ratio depending
on the current state of the exhaust oxygen level. The control module issues
a rich command if the exhaust is currently lean, or a lean command if the
exhaust is currently rich. After completion of the first command, a second
and opposite command is issued.
For example, if the control module were to command a rich mixture, the
upstream heated oxygen sensor (HO2S) voltage would increase immediately. The
rich mixture is delayed in reaching the downstream HO2S due to the conversion
process occurring within the converter. The higher the efficiency, the
more the delay before the rich or lean mixture is detected by the downstream
oxygen (O2) sensor. As a result of the lower conversion efficiency
within a damaged or poisoned catalyst, the delay in the rich or lean mixture
reaching the downstream O2 sensor is significantly shorter. This DTC monitors
the amount of time required for both the upstream and downstream HO2S voltages
to cross a calibrated voltage threshold in response to the rich or lean
command.
Conditions for Running the DTC
| • | No active secondary AIR DTCs |
| • | No active CMP sensor DTCs |
| • | No active ECT sensor DTCs |
| • | No active fuel trim DTCs |
| • | No active IAT sensor DTCs |
| • | No active MAF sensor DTCs |
| • | No active MAP sensor DTCs |
| • | No active O2 sensor DTCs |
| • | No active transmission DTCs |
| • | No active EVAP system DTCs |
| • | No active TP sensor DTCs |
| • | No active VS sensor DTCs |
| • | The engine speed is 900 RPM or more for more than 35 seconds
since last idle period. |
| • | The engine has been running for at least 346 seconds and
the long term fuel trim is stable. |
| • | The predicted catalyst temperature is at least 475°C(887°F). |
| • | The system is in closed loop. |
| • | The BARO is 73 kPa or more. |
| • | The IAT is between -7°C-75°C(20°F-167°F). |
| • | The ECT is between 75°C-117°C(167°F-243°F). |
| • | The engine has been idling for less than 60 seconds. |
| • | The actual engine speed is within 100 RPM of the desired
idle speed. |
Conditions for Setting the DTC
The VCM determines that the oxygen storage capacity of the catalyst
has degraded below a calibrated threshold.
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp
(MIL) the first time the diagnostic runs and fails. |
| • | The control module will set the DTC and records the operating
conditions at the time the diagnostic fails. The control module stores the
failure information in the scan tools Freeze Frame/Failure Records. |
Conditions for Clearing the MIL or DTC
| • | The control module turns OFF the MIL after 3 consecutive
drive trips when the test has run and passed. |
| • | A history DTC will clear if no fault conditions have been detected
for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature
has risen 22°C (40°F) from the startup coolant temperature
and the engine coolant reaches a temperature that is more than 70°C (158°F)
during the same ignition cycle. |
| • | Use a scan tool in order to clear the DTCs. |
Diagnostic Aids
The use of fuel with a sulfur or lead content can degrade a marginal
converter's performance. Be sure to check fuel quality.
An intermittent may be caused by any of the following conditions:
| • | 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.
-
This table checks for conditions that can cause the three-way catalytic
converter efficiency to appear degraded. Inspect and repair the exhaust
system as necessary. Refer to
Exhaust System Inspection
in Engine Exhaust.
-
If the three-way catalytic converter is replaced, make
sure that the following conditions are not present to cause the catalyst to
be damaged:
| • | High engine oil consumption or coolant consumption |
| • | Retarded spark timing or weak spark |
Step
| Action
| Value(s)
| Yes
| No
|
1
|
Important: Before clearing the DTCs, use the scan tool in Capture Info to save
the Freeze Frame and Failure Records for reference. The control module's data
is deleted once the Clear Info function is used.
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?
| --
|
Go to Step 2
| Go to the
Powertrain On Board Diagnostic (OBD) System Check
|
2
| Are any other
DTCs set?
| --
| Go to the applicable DTC table
|
Go to Step
3
|
3
|
- Check exhaust system for the following conditions:
| • | Loose or missing hardware |
- Repair as necessary. Refer to
Exhaust System Inspection
in
Engine Exhaust.
Did you find a problem?
| --
|
Go to Step
5
|
Go to Step 4
|
4
|
- Verify that the correct original equipment three-way catalytic
converter is installed.
- Check the converter for the following:
- Ensure that oxygen sensors are properly installed and that wiring
connections are undamaged and properly retained.
- Repair as necessary. Refer to
Exhaust System Inspection
in Engine Exhaust.
Did you find a problem?
| --
|
Go to Step
5
|
Go to Step 6
|
5
|
- Using the scan tool, clear the DTCs.
- Start the engine.
- Allow the engine to idle until the engine reaches normal operating
temperature.
- Select DTC and the Specific DTC function.
- Enter the DTC number which was set.
- 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?
| --
| System OK
|
Go to Step 6
|
6
|
Notice: In order to avoid damaging the replacement three-way catalytic converter,
correct the engine misfire or mechanical fault before replacing the three-way
catalytic converter.
Important: Refer to the test description before replacing the catalytic converter.
Replace the catalytic converter. Refer to
Catalytic Converter Replacement
in Engine Exhaust.
Is the action complete?
| --
| System OK
| --
|
