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Dan, Technician
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Experience:  Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
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2002 Malibu: v..6, has P0420 code, upstream O2 sensor jumps

Resolved Question:

2002 Malibu 3.1 v-6, has P0420 code, upstream O2 sensor jumps from .995 to .065, downstream O2 sensor tracks the same but not as bad. Catalytic converter was replaced six months ago. All other parameters are within range and pass the emissions inspection. could this be the upstream O2 sensor going bad and are there any service bulletins on this?
Submitted: 5 years ago.
Category: Chevy
Expert:  Dan replied 5 years ago.

Welcome to JustAnswer, my name is XXXXX XXXXX I'm here to help get your problem resolved. Ask follow up questions, thanks.


This code almost always means the converter has failed and if you are burning coolant, oil, or too much fuel then it can cause converter to fail early. A OEM converter should also be used to prevent converter failing right away. The oxygen sensor could be the cause but not common and there is no TSB. Let me know if you have questions, thanks.




P0420 Catalyst System Bank 1 Low Efficiency - With Automatic Transmission

Possible CausesSetting Conditions
  • Base engine problems (i.e., high engine oil or coolant usage)
  • Catalytic converter has failed (check for physical damage and discoloration)
  • Check for air leaks in the Air Induction system (e.g., in the intake manifold)
  • Continuous engine misfire conditions, or weak or low coil output
  • HO2S has failed (it may be contaminated with fuel, silicone or water)
  • Rear HO2S is loose in the mounting hole (check it for a leak)
  • Front HO2S older (aged) than the rear HO2S (HO2S-12 is lazy)
DTC P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0206, P0300, P0325, P0327, P0336, P0341, P0401, P0403, P0404, P0405, P0410, P0412, P0418, P0440, P0442, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1133, P1134, P1336, P1351, P1352, P1361, P1374 and P1441 not set; engine runtime over 10 minutes, engine running in Closed Loop, system voltage more than 10.7v, BARO signal over 75 kPa, ECT signal from 158-255ºF, IAT signal from -4ºF to +212ºF, then under the following conditions:
Warm The Catalyst
Fully open the hood; and then place the Transmission in Park and set the parking brake. Each time the engine is started, the diagnostic can run up to 18 times. After completion of the 10-minute runtime and before the diagnostic runs the first time, the engine must run an additional 5 minutes at between 1,500-2,500 rpm. For any additional tests on the same key cycle, the engine speed must be between 1,500-2,500 RPM for one minute. In order to activate the diagnostic, return the engine to idle with the gear selector in Drive.
Test The Catalyst
With the gear selector in Drive and the service brake depressed, within 60 seconds, the A/F ratio will go rich (to less than 14.1) for up to 8 seconds, then the A/F ratio may go lean (more than 15.3) for up to 10 seconds. Then the PCM determined that the catalyst oxygen storage capacity was below an acceptable threshold. Use a Scan Tool in order to determine if DTC P0420 passed or failed this ignition cycle.

This code sets in 1 Trip(s).
This code will turn on the MIL (Malfunction Indicator Lamp)

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