Question about 2005 Chevrolet Cobalt

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My car feels like it might die when Im coming to a stop (very slow, at less than 5mph) I can see the headlights dimming. My car is reading code p0420 which could be a bad cat. Would a bad cat cause my car to do this?

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Do you have a 5 speed transmission ? I had this problem, my problem was so bad it would stall everytime I came to a stop sign. i thought i was gonna have to replace the throttle body. come to find out, it just needed cleaned and reset. The dodge garage did it in my town for $20

Posted on Apr 03, 2011

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Code reading p0420 on '99 chevy s10 blazer, low catalyst threshold. Is this a bad catalytic converter or burned valves?


there are a couple of things to consider here. That code indicates that the O2 sensor behind the cat is not functioning like the engine computer expects-either the cat itself is not functioning properly or you may have an exhaust leak, or the heater circuit in the O2 sensor is faulty. The cat should get up to 600 degrees and it burns unburned fuel before ti gets to the 2ed O2 sensor-if it cannot do this, the CEL comes on and you have a P0420. Before replacing the cat, clear the code and see if it comes back-if it does, you might try first replacing the O2 sensor. If that doesn't work, get a 'oil fouler' spark plug adapter and install the O2 sensor in it, then install both into the exhaust-this 'shields' the tip of the O2 sensor and slows it down, fooling the engine computer into seeing it switching slowly, as it is supposed to do. Your only other choice is to replace the cat, and an aftermarket cat is much less than an OEM unit.

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2 Answers

My car feels like it might die when Im coming to a stop (very slow, at less than 5mph) I can see the headlights dimming. My car is reading code p0420 which could be a bad cat. Would a bad cat cause my car...


No it may also have a bad or blocked cat though. It sounds like the alternator needs to be replaced or that the belt may be stretched or broken therefore not doing anything.

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2 Answers

I have p0420 on my 06 pont gp gt. it mostly comes on after I slow down after highway


Here is the trouble tree for that code it is most likely a bad cat but follow the trouble tree to see where it lead's you.
DTC P0420 or P0430

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. See: Testing and Inspection\Initial Inspection and Diagnostic Overview\Diagnostic Starting Point - Vehicle
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0420

* Catalyst System Low Efficiency Bank 1

Important: A new converter with less than 100 miles on it may set DTC P0420 or P0430 due to out-******* of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition. Hope this helps

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Check the alternator,it should read at the battery 13.65 to 14.25 volts with the headlights on,and ac on high blower ,it should not read less than this,if it does the alternator is bad.

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it sounds like your alternator is havin problems it might not be puttin out enough juice at idle to keep everything workin make sure you clean all the connections you can to

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