For 1990-2009 cars

### System Description

In order to control exhaust emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it into nitrogen. The PCM has the capability to monitor this process using a Heated Oxygen Sensor (HO2S) located in the exhaust stream past the three - way catalytic converter. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the HO2S signal will be far less active than the signal produced by the front Oxygen Sensor (O2S 1). The PCM monitors the catalyst efficiency in two stages. When the catalyst efficiency falls below 75 %, the PCM will set a stage 1 fail flag but does not illuminate the Malfunction Indicator Lamp (MIL) at this point. When catalyst efficiency falls below 50 %, the PCM will set a stage 2 fail flag and the MIL will be illuminated.

### Conditions for Setting the DTC

• | DTC's P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0171, P0172, P0300-P0304, P0502, P0503, P0506, P0507, P0562, P0563, P0601, P1133, P1171, P1406 and P1441 not set.

• | Engine speed is between 1700 and 3700 RPM.

• | Engine Coolant Temperature (ECT) is above 76° C (168° F).

• | Intake Air Temperature (IAT) is above -29° C (-20° F).

• | Calculated Grams Per Second (GPS) is between 12 and 29 GPS.

• | Vehicle speed is between 35 and 80 mph (56 and 129 km/h).

• | HO2S 2 voltage varies greater than 282 mV.

• | Above conditions met continually for 180 seconds.

### Action Taken When the DTC Sets

• | The malfunction indicator lamp (MIL) illuminates.

• | The PCM records the operating conditions at the time when the diagnostic fails. This information stores in the Freeze Frame and Failure Records buffers.

• | A history DTC stores.

• | The coolant fan turns ON.

### Conditions for Clearing the MIL/DTC

• | The MIL will turn OFF after three consecutive ignition cycles in which the diagnostic runs without a fault.

• | A history DTC will clear after 40 consecutive warm-up cycles without a fault.

• | The MIL/DTCs can be cleared by using the scan tool.

### Diagnostic Aids

An Intermittent problem may be caused by a poor connection, rubbed through wire insulation, or a wire that is broken inside the insulation.

Any circuitry, that is suspected as causing the intermittent complaint, should be thoroughly checked for the following conditions:

• | Backed out terminals

• | Improper mating

• | Broken locks

• | Improperly formed or damaged terminals

• | Poor terminal to wiring connections or physical damage to the wiring harness

Refer to Intermittent Conditions .

### Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored on the scan tool for later reference.

2. If any component DTC's are set, diagnose those DTC's first. A fault in a component can cause the converter to appear degraded or may have caused it's failure.

3. This step includes checks for conditions that can cause the three-way catalytic converter to appear degraded. Repair any problems found before proceeding with this table.

4. If the three-way catalytic converter needs to be replaced, make sure that another condition is not present which would caused the converter to become damaged. These conditions may include: misfire; high engine oil or coolant consumption; retarded spark timing or weak spark. To avoid damaging the replacement converter, correct any possible causes of converter damage before replacing the catalytic converter.

#### DTC P0420-Three Way Catalyst (TWC) System Low Efficiency

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Were any component DTC's set? | -- | Go to Component DTC Tables first | Go to Step 3
3 | Visually/physically check the following items:

• | Exhaust system for leaks.

• | Rear Heated Oxygen Sensor (HO2S 2).

Was a problem found?

--

Go to Step 4

Go to Step 5

4 | Repair the exhaust system as necessary. Is the action complete? | -- | Go to Step 6 | --
5 | Replace the three way catalytic converter. Is the action complete? | -- | Go to Step 6 | --
6 |

1. Using the scan tool, clear the DTC's.
2. Start the engine and idle at normal operating temperature.
3. Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

Does the scan tool indicate that this diagnostic has ran and passed?

--

Go to Step 7

Go to Step 2

7 | Check if any additional DTC's are set. Are any DTC's displayed that have not been diagnosed? | -- | Go to applicable DTC table | System OK