GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 1998 🢒 DeVille 🢒 Engine 🢒 Engine Controls - 4.0L and 4.6L 🢒 DTC P0420 Catalyst System Low Efficiency

Circuit Description

This diagnostic test monitors catalytic converter efficiency. Efficiency is the ability of the catalytic converter to perform its function in reducing exhaust emissions of hydrocarbons, carbon monoxide, and oxides of nitrogen. Catalytic converter efficiency is calculated using the oxygen storage capacity of the catalytic converter. The oxygen storage data is collected from the front bank, rear bank and post-converter oxygen sensors during idle conditions. The oxygen content of the exhaust is measured before (front bank and rear bank HO2S) and after (post HO2S) the catalytic converter.

When the Conditions for Running the DTC have been met the air fuel mixture is transitioned from rich to lean or lean to rich depending on the Post HO2S current rich or lean state. After the first transition is completed a second, opposite transition is performed. The amount of time it takes for the Front bank and Rear bank HO2 sensors to cross a calibratable stoichiometric threshold is recorded. The length of time it takes the Post converter HO2S to cross the same stoichiometric threshold is also recorded.

The longer it takes the Post HO2S to cross the stoichiometric threshold the better the oxygen storage capacity of the catalytic converter. If the Post HO2S crosses the stoichiometric threshold too close to the same time as the Front bank and Rear bank HO2 sensors then the catalytic converter efficiency would be considered low and DTC P0420 will set.

Conditions for Running the DTC
    •  DTC's P0101, P0102, P0103, P0105, P0106, P0107, P0108, P0116, P0117, P0118, P0120, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0139, P0140, P0141, P0143, P0144, P0146, P0147, P0151, P0152, P0153, P0154, P0155, P0171, P0172, P0174, P0175, P0300, P0322, P0371, P0372, P0440, P0442, P0446, P0502, P0503, P0506, P0507, P1108, P1114, P1115, P1133, P1134, P1139, P1140, P1153, P1154, P1320, P1350, P1370, P1441, P1508 or P1509 not set.
    •  DTC P0420 test has not run this ignition cycle.
    •  Catalytic converter at operating temperature.
    •  Engine is at idle.
    •  Engine is in closed loop fuel control.

Conditions for Setting the DTC

Catalytic converter efficiency is too low.

Action Taken When the DTC Sets

    •  The PCM will illuminate the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    •  The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records.

Conditions for Clearing the MIL/DTC

    •  The PCM will turn the MIL OFF after three consecutive drive trips that the diagnostic runs and does not fail.
    •  A Last Test Failed (current) DTC will clear when the diagnostic runs and does not fail.
    •  A History DTC will clear after forty consecutive warm-up cycles with no failures of any emission related diagnostic test.
    • Use a scan tool to clear DTCs.
    • Interrupting PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Clearing Diagnostic Trouble Codes in PCM Description and Operation.

Diagnostic Aids

Fuel with a high sulfur content can cause a marginal converter to look bad. Check fuel quality.

Only early 1998 model year production vehicles are equipped with a Pre-convertor Heated Oxygen Sensor (HO2S). If equipped, this HO2S is connected to the PCM but is no longer monitored or diagnosed. The remaining heated oxygen sensors retain the following naming convention:

    • Bank 1 Sensor 1 (Rear bank)
    • Bank 1 Sensor 3 (Post converter)
    • Bank 2 Sensor 1 (Front bank)

Test Description

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

  1. Checking for conditions that can cause the catalytic converter efficiency to appear low. Inspect and repair problems, if any, before proceeding.

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 appropriate DTC Tables

Go to Step 3

3
  1. Check exhaust system for leaks, damage, loose or missing hardware.
  2. Make repairs as needed.

Was a repair to the exhaust system made?

--

Go to Step 5

Go to Step 4

4
  1. Verify that the correct original equipment three-way catalytic converter is installed
  2. Check the converter for damage such as dents, severe discoloration, holes, etc.
  3. Ensure that oxygen sensors are properly installed and that oxygen sensors, wiring and connections are undamaged and properly retained.
  4. Make repairs as needed.

Was a repair made?

--

Go to Step 5

Go to Step 6

5
  1. Select DTC Status.
  2. Operate the vehicle under the running conditions needed for the DTC.
  3. Observe the scan tool display of DTC Status looking for this DTC to run and pass or fail.

Did this DTC fail?

--

Go to Step 6

Go to Powertrain On Board Diagnostic (OBD) System Check

6

Replace the catalytic converter.

Is the replacement complete?

--

System OK

--