GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2002 🢒 DeVille 🢒 DTC 🢒 DTC P0420

Circuit Description

The powertrain control module (PCM) tests the ability of the catalytic converter to lower exhaust emissions. The PCM determines the performance of the catalytic converter by calculating the oxygen storage capacity. The PCM calculates this data by comparing the oxygen content of the exhaust entering the catalyst, read from the front and rear heated oxygen sensor (HO2S) to that leaving the catalyst, read from the post HO2S.

At the appropriate time, the PCM transitions the air fuel mixture from rich to lean or lean to rich depending on the current state of HO2S bank 1 sensor 2, rich or lean state. Then, the PCM initiates a second, and opposite transition. The PCM compares the amount of time for the fuel control sensors to cross a calibrated threshold to the amount of time for HO2S bank 1 sensor 2 to cross the same threshold. The longer the HO2S bank 1 sensor 2 takes to cross the threshold the higher the oxygen storage capacity. An HO2S bank 1 sensor 2 that crosses the threshold close to the same time as the front and rear HO2S indicates a catalyst with low oxygen storage capacity, and DTC P0420 will set.

Conditions for Running the DTC
    • DTCs P0030, P0036, P0101-P0103, P0106-P0108, P0112, P0113, P0116-P0118, P0121-P0123, P0130-P0135, P0137, P0138, P0140, P0141, P0150-P0155, P0171, P0172, P0174, P0175, P0201-P0208, P0300, P0335, P0336, P0340, P0341, P0351-P0358, P0385, P0386, P0404, P0405, P0410, P0412, P0418, P0440, P0442, P0443, P0446, P0449, P0452, P0453, P0502, P0503, P0506, P0507, P1031, P1032, P1133, P1134, P1336, P1359, P1360, P1372, P1415-P1416, P1441, P1153, or P1154 are not set.
    • The engine speed since the end of the last idle period is more than 800 RPM for a maximum of 46 seconds.
    • The idle period is less than 60 seconds (1 minute).
    • The actual engine speed is within 100 RPM of desired engine speed (Non-California).
    • The actual engine speed is within 125 RPM of desired engine speed (California).
    • The engine run time is more than 10 minutes.
    • The predicted catalyst temperature is between 420-650°C (788-1202°F).
    • The barometric pressure (BARO) is more than 75 kPa.
    • The engine coolant temperature (ECT) is between 71-120°C (160-248°F).
    • The intake air temperature (IAT) is between -6 to +200°C (21-329°F).
    • The engine is in Closed Loop fuel control.
    • Tests attempted this ignition cycle is less than 6.
    • Test attempted during idle period is 1.

Conditions for Setting the DTC

The catalytic converter efficiency is too low.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

Fuel with a high sulfur content can cause inaccurate converter test results. Check the fuel quality.

Test Description

The number below refers to the step number 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

Yes

No

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important: If any other DTCs are set, diagnose the other DTCs first.

Are any other DTCs set?

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3
  1. Inspect the exhaust system for leaks, damage, loose, or missing hardware. Refer to Exhaust Leakage in Engine Exhaust.
  2. If the condition is found, repair as necessary.

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4
  1. Inspect for the following conditions:
    2. • The correct original equipment catalytic converter is installed.
    • The converter for damage such as dents, severe discoloration, holes, etc.
    • Properly installed oxygen sensors
    • Damaged wiring, or improper connections
  2. If a condition is found, repair as necessary.

Did you find and correct the condition?

Go to Step 6

Go to Step 5

5

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.

Did you complete the repair?

Go to Step 6

--

6
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

Go to Step 7

Go to Step 2

7

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK