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 secondary AIR DTCs are active. |
| • | No CMP sensor DTCs are active. |
| • | No ECT sensor DTCs are active. |
| • | No EGR DTCs are active. |
| • | No fuel trim DTCs are active. |
| • | No IAC DTCs are active. |
| • | No IAT sensor DTCs are active. |
| • | No MAF sensor DTCs are active. |
| • | No MAP sensor DTCs are active. |
| • | No O2 sensor DTCs are active. |
| • | No transmission DTCs are active. |
| • | No EVAP system DTCs are active. |
| • | No TP sensor DTCs are active. |
| • | No VS sensor DTCs are active. |
| • | No misfire DTCs are active. |
| • | The engine speed is 900 RPM or more for more than 35 seconds
since the last idle period. |
| • | The engine has been running for more than 346 seconds and
the long-term fuel trim is stable. |
| • | The predicted catalyst temperature is more than 475°C (887°F). |
| • | The system is in closed loop. |
| • | The BARO is 73 kPa or more. |
| • | The IAT is between -7-117°C (20-167°F). |
| • | The ECT is between 75-117°C (167-243°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 determined 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) if a failure is detected during 2 consecutive key cycles. |
| • | The control module sets the DTC and records the operating conditions
at the time the diagnostic failed. The failure information is stored in the
scan tool 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 high sulfur or lead content can degrade the performance
of a marginal convertor. Be sure to check the fuel quality.
An intermittent may be caused by any of the following conditions:
| • | Rubbed through wire insulation |
| • | A broken wire inside the insulation |
Thoroughly inspect any circuitry that is suspected of causing the intermittent
complaint. Refer to
Testing for Intermittent Conditions and Poor Connections
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 in 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.
-
Before you replace
the three-way catalytic converter, make sure that the following conditions
are not present:
| • | High engine oil consumption or high coolant consumption |
| • | Retarded spark timing or weak spark |
Step
| Action
| Values
| Yes
| No
|
1
| Did you perform
the Powertrain On-Board Diagnostic (OBD) System Check?
| --
|
Go to Step 2
| Go to
Powertrain On Board Diagnostic (OBD) System Check
|
2
| Are any other DTCs set?
| --
| Go to applicable DTC
|
Go to Step 3
|
3
|
- Check the exhaust system for:
| • | Loose or missing hardware |
- Repair the exhaust system as necessary. Refer to
Exhaust System Inspection
and
Exhaust Leakage
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:
- Ensure that the oxygen sensors are properly installed and that
the wiring connections properly retained and are not damaged.
- Repair any conditions 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, within the Conditions for Setting this DTC,
until the scan tool indicates that the diagnostic Ran.
Does the scan tool indicate that this 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.
Replace the catalytic converter. Refer to
Catalytic Converter Replacement
in Engine Exhaust.
Is the action complete?
| --
| System OK
| --
|
