Circuit Description
In order to maintain a reasonably low emissions of hydrocarbons (HC),
carbon monoxide (CO), and oxides of nitrogen (NOx), a three-way catalytic
converter (TWC) is used. The catalyst within the converter promotes
a chemical reaction which oxidizes the HC and CO present in the exhaust
gas. This reaction converts them into harmless water vapor and carbon
monoxide. The catalyst also reduces NOx, converting it to nitrogen.
The powertrain control module (PCM) monitors this process via the post
catalyst heated oxygen sensor (HO2S) signal. The post catalyst HO2S,
located in the exhaust stream after the TWC, produces an output
signal which indicates the oxygen storage capacity of the catalyst.
Oxygen storage capacity is one method of determining the catalyst
ability to convert exhaust emissions effectively. If the catalyst
is functioning correctly, the post catalyst HO2S signal will be far
less active than that produced by the pre-catalyst HO2S. If the PCM
detects excessive post catalyst signal activity, outside of an acceptable
range, for an extended period of time, the PCM will set this DTC.
This indicates that the TWC oxygen storage capacity is below a
threshold considered acceptable.
Conditions for Running the DTC
- Meet conditions for engine warm up. Use the scan tool catalyst
data list in order to verify the following.
| • | The following types of diagnostic trouble codes (DTCs) are not
set: |
| - | Injector or ignition control circuit |
| - | Evaporative emission (EVAP) system |
| - | Heated oxygen sensor (HO2S) |
| - | Vehicle speed sensor (VSS) |
| - | Idle air control (IAC) sensor |
| - | Exhaust gas recirculation (EGR) system |
| - | Throttle position (TP) sensor |
| - | Intake air temperature (IAT) sensor |
| - | Manifold absolute pressure (MAP) sensor |
| - | Engine coolant temperature (ECT) sensor |
| - | Mass air flow (MAF) sensor |
| - | Crankshaft position (CKP) sensor |
| • | Engine has been running longer than approximately 9 minutes. |
| • | IAT is between -18°C and +80°C (0°F and 176°F). |
| • | ECT is between 75°C and 120°C (167°F and 248°F). |
| • | BARO is above 75 kPa. |
- Warm up the catalyst.
| • | Transmission is in park (automatic) or neutral (manual). |
| • | Set the parking brake. |
| • | Engine speed is more than 1,500 RPM for 1 minute. |
- Test the catalyst.
| • | Transmission is in drive (automatic) or neutral (manual). |
| • | Within 5-20 seconds, HO2S 1 should toggle rich above
600 mv for up to 7 seconds, then it may toggle lean below
300 mv for up to 7 seconds. |
| • | Using the scan tool, check and see if DTC P0420 has passed or
failed this ignition cycle |
Conditions for Setting the DTC
The PCM determines that the catalyst oxygen storage capacity is below
a threshold considered acceptable.
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) when
the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic
fails. The PCM stores this information in the Freeze Frame and/or Failure
Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns the MIL OFF after 3 consecutive
drive trips during which the diagnostic runs and passes. |
| • | A last test failed, or the current DTC, clears when the diagnostic
runs and passes. |
| • | A History DTC clears after 40 consecutive warm-up cycles, if
no other emission related diagnostic failures are reported. |
| • | Use a scan tool in order to clear the MIL diagnostic trouble code. |
| • | Interrupting the PCM battery voltage may or may not clear DTCs.
This practice is not recommended. Refer to
Powertrain Control Module Description
, Clearing Diagnostic
Trouble Codes. |
Diagnostic Aids
Notice: Use the connector test adapter kit J 35616-A for any test that
requires probing the following items:
| • | The PCM harness connectors |
| • | The electrical center fuse/relay cavities |
| • | The component terminals |
| • | The component harness connector |
Check for the following conditions:
| • | The PCM will not enable the catalyst test until the following
conditions are met: |
| - | The engine speed is plus or minus 100 RPM from the desired
idle. |
| - | The throttle position (TP) is 0 percent. |
| - | The short term integrator is between -20 percent
and +20 percent. |
| • | The catalyst test will abort if the vehicle falls outside the
conditions listed below while the test is running: |
| - | The engine speed is plus or minus 125 RPM from the desired
idle. |
| - | The throttle position (TP) is 0 percent. |
| - | The short term integrator is between -20 percent
and +20 percent. |
| • | The catalyst test may abort due to a change in the engine load,
such as A/Cor the coolant fan. Repeat the test if a load change occurs during
the test. |
| • | More than 6 tests may have to be attempted to get 6 completed
tests. An aborted test counts as an attempted test. |
| • | The number of attempted tests is limited to 18 per ignition
cycle. |
| • | After returning to an idle, the HO2S 1 signal may stay
rich or lean for several seconds causing the test to be delayed. |
| • | Poor connections at the PCM -- Inspect the harness connectors
for backed out terminals, improper mating, broken locks, improperly formed
or damaged terminals, and poor terminal to wire connections. |
| • | Harness damage -- Inspect the wiring harness for damage.
If the harness appears to be OK, observe the display on the scan tool while
moving connectors and wiring harnesses related to the sensor.
A change in the display will indicate the location of the fault. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
If any component
DTCs are set, diagnose those DTCs first. A fault in a component
can cause the converter to appear degraded or may
have caused its failure.
-
Clearing the
DTCs allows the catalyst test to be completed up to 6 times
this ignition cycle. The diagnostic will abort if the A/C clutch cycles.
Turn the A/C OFF. The engine must be warmed-up. The converter
needs to be warmed-up by raising the engine speed above
idle for the specified time prior to each attempted
test. Check and see if DTC passed or failed this ignition
cycle. If the DTC does not pass or fail, look for a possible
reason that would cause the test to abort.
-
This step includes
checks for conditions that can cause the three-way converter
to appear degraded. Repair any problems found before
proceeding with this table.
-
If the three-way
converter needs to be replaced, make sure that another condition
is not present which could damage the converter. These
conditions may include: misfire, high engine oil or coolant consumption,
HO2S silicon contamination, retarded spark timing or
weak spark. Correct any possible causes of converter
damage before replacing the converter.
Step
| Action
| Value(s)
| 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 the applicable DTC table
|
Go to Step 3
|
3
|
- Clear the DTCs using the scan tool.
Important: If the A/C clutch cycles, the diagnostic will abort.
- Turn the A/C OFF.
- Start the engine and idle until normal operating temperature is
reached.
- Fully open the hood.
- Operate the vehicle within the conditions that are required for
this diagnostic to run. Refer to the Conditions for Running
the DTC.
Important: If more than 6 tests have been attempted and the DTC has not passed
or failed this ignition cycle the test may be aborting. Refer
to Diagnostic Aids .
- The diagnostic test may need to be completed up to 6 times in
order to pass or fail.
Does the scan tool indicate the DTC ran and passed this ignition cycle?
| --
| System Ok
|
Go to Step 4
|
4
|
- Visually and physically inspect the three-way catalytic converter
for damage.
- Check for the following:
| • | Dents, holes, or other physical damage |
| • | Severe discoloration caused by excessive temperatures |
| • | Internal rattle caused by damaged catalyst substrate |
- Also, ensure that the three-way catalytic converter is a proper
original equipment manufacturer part.
Was a problem found?
| --
|
Go to Step
7
|
Go to Step
5
|
5
|
- Visually and physically inspect the exhaust system for leaks.
- If a problem is found, repair as necessary. Refer to
Exhaust System Inspection
in Engine Exhaust.
Was a problem found and corrected?
| --
|
Go to Step 8
|
Go to Step 6
|
6
| Visually check for the following conditions:
| • | Damaged or contaminated heated oxygen sensors |
| • | HO2S wiring harness damage |
Was a problem found and corrected?
| --
|
Go to Step 8
|
Go to Step 7
|
7
|
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 three way catalytic converter. Refer to
Catalytic Converter Replacement
in Engine Exhaust.
Is the action complete?
| --
|
Go to Step 8
| --
|
8
|
- Clear the DTCs using the scan tool.
Important: If the A/C clutch cycles, the diagnostic will abort.
- Turn the A/C OFF.
- Start the engine and idle until normal operating temperature is
reached.
- Fully open the hood.
- Operate the vehicle within the conditions that are required for
this diagnostic to run. Refer to the Conditions for Running
the DTC.
Important: If more than 6 tests have been attempted and the DTC has not passed
or failed this ignition cycle, the test may be aborting. Refer
to Diagnostic Aids.
- The diagnostic test may need to be completed up to 6 times in
order to pass or fail.
Does the scan tool indicate the DTC ran and passed this ignition cycle?
| --
|
Go to Step 9
|
Go to Step 4
|
9
| Review Captured Info using the
scan tool.
Are there any DTCs that have not been diagnosed?
| --
| Go to
the applicable DTC table
| System OK
|
