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Category: Chevy
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Experience:  ASE Certified and over 25 years Experience in the Automotive Field
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Chevy colorado: p0420..the o2 sensors are within spec and the spark

Customer Question

my 04 chevy colorado is setting of a p0420 code. both of the o2 sensors are within spec and the spark is advancing, what is the most likely problem in this situation?
Submitted: 9 years ago.
Category: Chevy
Expert:  GM Expert replied 9 years ago.


Thanks for using JA,

I have pulled the check list for the p0420 code and found it to do the same thing that you are doing and check the O-2 sensors and ensure they are reading within specs. The next thing it says to check is the Cat converter and I am sending you the check list and hope it helps. Please if this info helps click on accept.

thanks msgtperrin

Did you perform the Diagnostic System Check-Engine?

Go to Step 2

Go to Diagnostic System Check - Engine Controls


  1. Allow the engine to reach operating temperature.
  2. Ensure Closed Loop is achieved.
  3. Increase the engine speed to 1,500 RPM for 1 minute.
  4. Return the engine to stabilized idle.
  5. Monitor the HO2S 1 and the HO2S 2.

Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter?

Go to Step 4

Go to Step 3


Important: The test may need to be completed up to 6 times in order to pass or fail.

If more than 6 tests have been attempted and the DTC has not passed or failed this ignition cycle, the test may be aborting.



  1. Observe the Freeze/Frame Records for this DTC.
  2. Clear the DTCs with a scan tool.
  3. Turn OFF all accessories.
  4. Operate the vehicle within the Conditions for Running this DTC.

Does the scan tool indicate DTC P0420 ran and passed this ignition?

Go to Diagnostic Aids

Go to Step 4


Important: Verify that the three-way catalyst (TWC) is a high quality part that meets the original equipment manufacturer (OEM) specifications.

Visually and physically inspect the catalytic converter for the following conditions:

A severe discoloration caused by excessive temperatures
Road damage
An internal rattle caused by damaged catalyst substrate
Restrictions--Refer to Restricted Exhaust in Engine Exhaust.

Did you find a condition?

Go to Step 8

Go to Step 5


Visually inspect the exhaust system for the following conditions:

Leaks--Refer to Exhaust Leakage in Engine Exhaust.
Physical damage
Loose or missing hardware
The heated oxygen sensor (HO2S) 2 for proper torque

Did you find and correct the condition?

Go to Step 9

Go to Step 6


Visually inspect the HO2S 2 for the following conditions:

The pigtail and wiring harness contacting the exhaust or a ground
Physical damage

Did you find a condition?

Go to Step 7

Go to Step 8


Replace the HO2S 2 sensor. Refer to Heated Oxygen Sensor 2 Replacement .

Did you complete the replacement?

Go to Step 9



Notice: Refer to Three-Way Catalytic Converter Damage Notice in the Preface section.

Replace the catalytic converter and the exhaust manifold. Refer to Catalytic Converter Replacement and Exhaust Manifold Replacement in Engine Exhaust.

Did you complete the replacements?

Go to Step 9



  1. Clear the DTCs with a scan tool
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions For Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 10


Observe the Capture Info with a scan tool.

Are there any DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK