P0420 ENGINE CODE

  • Tiny
  • caddiac
  • Chevrolet Suburban

My '98 Suburban, 5.7L, 4sp auto, 4wd and 176,000 miles is giving the subject code indicating " Catalyst Efficiency Below Normal (Bank 1)". It started following an episode of misfiring that was identified as code 0300 " Random/Multiple engine misfire detected". I chose to drive a couple of hundred miles with an misfiring engine rather than walk a couple of hundred miles. After replacing the distributer cap and clearing the code, the engine seemed to run normally. After a short time, the 0420 appeared. If I clear the code it will return after the driving less that 10 miles. Any idea what my problem might be?

Tuesday, October 9th, 2007 AT 6:53 PM

7 Answers

  • Tiny
  • Service Writer
  • Expert
  • 9,255 posts

Most likely the cat is bad. Unless the is an air leak in front of the cat in the exhaust; it is very likely the misfires ended up damaging the cat. Ouch.

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Tuesday, October 9th, 2007 AT 7:15 PM
  • Tiny
  • caddiac
  • Member

What is the harm in living with the light on? Aside from the environmental issues - which I am concerned about - is there additional harm to be done if I don't repair immediately?

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Wednesday, October 10th, 2007 AT 6:13 PM
  • Tiny
  • Service Writer
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  • 9,255 posts

Poorer gas mileage, potential burned exhaust valves come to mind. Depending what happens. May give a lean mixture that can cause piston damage. But straight forward this is only my specuation.

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Wednesday, October 10th, 2007 AT 8:04 PM
  • Tiny
  • caddiac
  • Member

So should I replace one or both catalytic converters? Since there is one in each header coming together at the muffler I am not sure which is bank 1.

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Thursday, October 11th, 2007 AT 7:47 PM
  • Tiny
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  • 9,255 posts

Usually it is the left side, but let me check tommorow.

MAke sure the o2 sensors are not damaged or loose, that here are no leaks between the converter and rear converter flange.

You can check the grounds for the VCM, other sensor grounds, make sure there are no other codes right now.

I would still expect the cat to be bad.

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Thursday, October 11th, 2007 AT 8:06 PM
  • Tiny
  • caddiac
  • Member

I purchase a set of catalytic converters from a internet supply house I found through this site. A " shade tree" attempt to install the converters resulted in a split 15 mm socket.

So I payed a local muffler shop to do the job. They had to cut them off, but my internet part fiit perfect and the problem was solved.

Parts and labor, around $500 including a donation to get some professional advice on this site. I consider that a deal!

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Thursday, November 1st, 2007 AT 7:26 PM
  • Tiny
  • Service Writer
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  • 9,255 posts

Good Idea replacing both, it's better off in the long run. Glad it worked out for you. And thanks for the donation!

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Friday, November 2nd, 2007 AT 4:55 AM

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