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Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 4410
Experience:  ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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Chevrolet Malibu: I have a 2009 Chevy Malibu. I just replaced

Customer Question

I have a 2009 Chevy Malibu. I just replaced the front cat and downstream 02 sensor because of a p0420 code. I read the mode 6 data with my android phone and shows that the cat efficiency monitor failed.
Min 0.35 Max 0.99 Current 0.321
Submitted: 6 months ago.
Category: Chevy
Expert:  Jerry Newton replied 6 months ago.
Don't get hung on oxygen sensor voltage. You can't read it or decipher it as fast as the computer can, the computer is looking for very specific trends as the voltage varies. Where did you get the converter that you installed? What's the vin on your car?
Customer: replied 6 months ago.
It threw a light for the same code. I can't get the vin until this evening. I bought from a guy that drove it from Texas with a misfire. I replaced the motor with a 2011 Malibu 2.4l. It had a permanent code of p0420 and no matter what I do it won't go away. I had a reputable exhaust shop replace the front cat. And I replaced the bank 1 sensor 2 with a bosh universal
Expert:  Jerry Newton replied 6 months ago.
If this is a California emissions car (not necessarily a car FROM California), the converter is different from Federal emissions cars, and they're a lot more money. If your exhaust shop welded in a replacement, that's suspect. I also don't put any faith in universal Bosch sensors, they are terrible. I'd definitely put an AC Delco sensor in this, and if that's not an OE style converter (if they cut the pipe and welded in a new one), then that needs to go away, too. Weld in converters do NOT meet the stringent standards required for the OBD system.
Customer: replied 6 months ago.
I paid the shop $270 for the work. They looked up the correct cat for it. What I don't understand is why it's reading lower than the minimum. Is it burning cleaner than it should?
Customer: replied 6 months ago.
If so, will just replacing the 02 sensor with a ac delco do it? I also have to drive it 2hrs straight for it to set the cat readiness monitor. I only have 1 week left on my 1month dmv permit
Expert:  Jerry Newton replied 6 months ago.
The post cat O2 sensor should read low. The converter oxygenates the exhaust, adding oxygen, and this drives the oxygen sensor voltage low (high oxygen = low voltage). This is what the computer is looking for: is the post cat O2 sensor voltage consistently lower than the pre cat sensor? This is something we can't determine by watching the values on a scan tool or even a scope, the computer has a calibration that varies depending on any number of operating conditions, and the thresholds are programmed into the software. We don't know what the thresholds are, and it wouldn't matter if we did, because we can't monitor and measure it fast enough with our naked eye anyway. We rely on the engine computer to do this math for us, and it does it very reliably. If the shop only charged you $270 to replace the converter, then it's not the correct converter. Half of that money was likely labor, which means the converter was about $135, and they made money on that converter, which means they probably paid $50 for the converter itself. $50 converters don't meet the emissions standard, and will turn the light on and you'll have a P0420. This is the reality of it. The correct converter for this car will cost much more, the part along will be $400-600, plus labor. If you're not paying that much for a converter, you're not getting the converter you need. You can't go cheap on this, the computer will not be satisfied, and you'll never get P0420 to go away. A "new" converter is not a "good" converter. Don't get hung up on the fact that it's new, that doesn't mean anything. Other makes and models (VW, for example) require a converter that's about $1000. There are companies that make cheaper replacements, but if you don't put the expensive (that is, correct) converter in, P0420 will plague you until you do. This converter is just wrong.

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