Engine light

  • 6 CYL
  • FWD
  • 190,124 MILES

Received a odb code po420. What is this and how do I fix.

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Thursday, December 23rd, 2010 AT 6:13 PM

2 Replies

  • 30,434 POSTS

Here is the trouble tree for that code.
To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides Of Nitrogen (NOx), a 3-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 these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx to nitrogen. The converter also has the ability to store excess oxygen and release the stored oxygen to promote these reactions. This Oxygen Storage Capacity (OSC) is a measurement of the catalysts ability to control emissions. The PCM monitors this process using a Heated Oxygen Sensor (HO2S) located in the exhaust stream past the three-way converter. When the catalyst is functioning properly, the HO2S 2 is slow to respond to a large change in the HO2S 1 signal. When the HO2S 2 responds quickly to a large change in the HO2S 1 signal, the OSC and efficiency of the catalyst is considered to be bad and the MIL will be illuminated if subsequent tests also indicate a failure.


1. Meet the conditions for engine warm up. Use the scan tool catalyst data list in order to verify the following:

o DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201, P0202, P0203, P0204, P0205, P0206, P0300, P0336, P0341, P0401, P0403, P0404, P0405, P0440, P0442, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1133, P1134, P1336, P1351, P1351, P1352, P1361, P1362, P1374, P1404, or P1441 are not set.
O The engine has been running for more than 10 minutes.
O The Engine Coolant Temperature (ECT) is between 70 C (158 F) and 124 C (255 F).
O The BARO is more than 75 kPa.
O The vehicle is in Closed Loop.
O The Intake Air Temperature (IAT) is between -20 C (-4 F) and 100 C (212 F).
O The battery voltage is more than 10.7 volts.

2. Warm up the catalyst:

o Fully open the hood.
O Place the transmission in Park (A/T) or in Neutral (MT).
O Set the park brake.
O Press and hold the service brake.
O Each time the engine is started the diagnostic can run up to 18 times. After the 10 minute run time, and before the diagnostic runs the first time, the engine must run an additional 5 minutes between 1500 and 2500 RPM. For any additional tests on the same key cycle, the engine must be between 1500 and 2500 RPM for 1 minute.
O In order to activate the diagnostic, return to idle and place the vehicle in Drive, depress the clutch pedal for manual.

3. Test the catalyst:

o Verify that the transaxle is in Drive (A/T) or is in Neutral with the clutch pedal depressed (MT).
O Within 60 seconds the air fuel ratio will go lean below 14.1 for up to 7 seconds, and may then may go rich above 15.3 for up to 7 seconds.
O Use the scan tool in order to determine if DTC P0420 has passed or failed this key cycle.

The PCM determines that the catalyst's oxygen storage capacity is below a threshold considered acceptable.


* 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.


* 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, it no failures are reported by this or any other emission related diagnostic.
* Use a scan tool in order to clear the MIL and the DTC.

Inspect for the following conditions:

* The PCM will not enable the catalyst test until following conditions are met:
* The engine speed is within 200 RPM of the desired idle.
* The throttle position is 1.5 percent or less.
* The Short Term FT 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 within 200 RPM of the desired idle.
* The throttle position is 1.5 percent or less.
* The short term FT is between -20 percent and 20 percent.
* The catalyst test may abort due to a change in the engine load. If this problem occurs use the scan tool in order to force the cooling fans on and return to Step 3 in the table.

IMPORTANT: When using the scan tool in order to force the cooling fans on, use the next list and previous list soft keys in order to enter the catalyst data list. If the catalyst data list is not entered this way the cooling fan control will be canceled.

* The number of attempted tests is limited to 18 per key cycle.
* More than 5 tests may have to be attempted in order to get 6 completed tests. An aborted test counts as an attempted test.
* If 18 tests have been attempted and a decision has not been made this key cycle, turn the key OFF for 30 seconds, start the vehicle, and perform the conditions for running the DTC, including the 10 minute run time.
* After returning to an idle the HO2S 1 signal may stay rich or lean for several seconds, causing the test to be delayed.

If the condition is determined to be intermittent, refer to Intermittent Conditions. See: Initial Inspection and Diagnostic Overview Diagnostic Strategies Intermittent Conditions


Steps 1-3

Steps 4-11

The numbers below refer to the step numbers on the diagnostic table:

2. 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.
3. Clearing the DTCs allows the catalyst test to be completed up to 6 times this ignition cycle. If the A/C is not turned off the diagnostic will not run. 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.
4. This step includes tests for conditions that can cause the three-way converter to appear degraded. Repair any problems found before proceeding with this table.

9. If the 3-way converter needs to be replaced, ensure that another condition which could damage the converter is not present. These conditions may include misfire, high engine oil or coolant consumption, retarded spark timing, weak spark, or a rich or lean fuel system. Correct any possible causes of converter damage before replacing the converter.

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Thursday, December 23rd, 2010 AT 6:24 PM
  • 16,270 POSTS

There has been a leak detected in the evap syatem, check the following.

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Thursday, December 23rd, 2010 AT 6:26 PM

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