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Cannot resolve code P0420

Tiny
HVDES
  • MEMBER
  • 2005 PONTIAC GRAND PRIX
  • 3.8L
  • V6
  • 2WD
  • AUTOMATIC
  • 184,000 MILES
Hi, I have been battling this P0420 code on my car (GT model) for a while now and was hoping someone would have some more insight on the subject.
- I have replaced both oxygen sensors as well as the catalytic converter.
- Replaced the PCV valve.
- I have put in new plugs and wires.
- I have cleaned the MAF sensor and air filter.

When driving, the car has a hard time accelerating. Especially up-hills and when accelerating to highway speeds, there is a ticking noise that sounds similar to a card in the spokes of a bicycle but is not heard when idling. Also, when I start the car it fires up right away then will cut immediately off if I do not tap the gas pedal.
Could it be a bad MAF sensor? I tried to test it with a multimeter but it is nearly impossible with the harness that it has.
Anyway, thanks.
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Thursday, July 26th, 2018 AT 7:29 PM

10 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good morning.

What brand catalytic converter did you use for the replacement?

The noise could be a valve tap from a valve issue with the motor. You need to have that checked as it may be causing a misfire and that will overheat the catalytic converter and cause the code as well.

Roy

P0420
Descriptor
Engine Controls
Probable Causes
Catalytic Converter
Catalytic Converter Bank 1 Blocked
Catalytic Converter Bank 1 Damaged
Diagnose Other DTCs First
Engine Misfire Detected
Engine Oil or Coolant Consumption Excessive
Exhaust Leak
Exhaust System Damaged
Exhaust System Hardware Loose or Missing
Fuel Mixture Lean
Fuel Mixture Rich
Heated Oxygen Sensor (HO2S) Bank 1 Sensor 2
Heated Oxygen Sensor (HO2S) Bank 1 Sensor 2 Damaged
Heated Oxygen Sensor (HO2S) Bank 1 Sensor 2 Loose
Heated Oxygen Sensor (HO2S) Bank 1 Sensor 2 Pigtail or Wiring Harness Contacting Exhaust or Ground
Oxygen Sensor (O2S) or Wiring Harness Damaged
Three Way Catalytic Converter (TWC) Does Not Meet Manufacturer Specifications
Weak or Poor Spark
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Friday, July 27th, 2018 AT 5:45 AM
Tiny
HVDES
  • MEMBER
  • 4 POSTS
I do not think it is valve tap as I only hear it when I am above about 2,500-3,000 rpm's and it is not very loud but, the converter I put on is from eBay and I think it is this exact one.

https://www.ebay.com/itm/2004-2005-2006-Pontiac-Grand-Prix-3-8L-Direct-Fit-Catalytic-Converter-/253764966466?hash=item3b15921842
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Friday, July 27th, 2018 AT 3:14 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Okay.

It looks like this one does not have enough precious metals to keep the light out. That is the issues with the cheap converters.

Walker and Bosal are the only ones we use due to come backs from the 420 returning.

Roy
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Friday, July 27th, 2018 AT 3:18 PM
Tiny
HVDES
  • MEMBER
  • 4 POSTS
But the lack of quality of the converter should not be causing the car to accelerate slow and strangely would it? The car also seems to show all the symptoms listed here for a bad MAF sensor.

https://www.carsdirect.com/car-repair/how-to-tell-if-you-have-a-faulty-mass-airflow-sensor?src=20&lnk=eyzsD2QGsYg-jBWJdbuvzn%2Fj4mDqmhlHtw

Yesterday I unplugged the senor and drove it down the street to see if there was a difference in how it accelerates and it seemed to be running the same.

Thanks
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Friday, July 27th, 2018 AT 3:27 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Okay, the original question was your 420 code. We through that the sluggish.

I would start by doing a fuel pressure test to be sure you have enough fuel for the injectors to work correctly.

also, I would do a compression test to see if there is a issue with the timing chain.

Roy

https://www.2carpros.com/articles/how-to-test-engine-compression

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
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Friday, July 27th, 2018 AT 4:33 PM
Tiny
HVDES
  • MEMBER
  • 4 POSTS
I have tested the car with a scanner and it says all sensors and systems passed except the EVAP system, but I do not have any codes related to it. Do you think a bad solenoid or vapor canister could be causing these problems?
Thanks again.
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Saturday, July 28th, 2018 AT 11:05 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
They are only monitors. That does not mean there is no problems with those systems.

You need to do the two tests I suggested in my previous answer.

Roy
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Sunday, July 29th, 2018 AT 5:51 AM
Tiny
GJGOGOL
  • MEMBER
  • 3 POSTS
ASEMASTER6371, how does one know if a cat has enough precious metals?
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Monday, August 21st, 2023 AT 2:34 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,363 POSTS
I believe all catalytic converters have precious metals, here is more information:

Catalytic converters are devices used in the exhaust systems of vehicles to reduce harmful emissions by catalyzing various chemical reactions.

Platinum (Pt):

Palladium (Pd):

Rhodium (Rh):

These precious metals are often supported on a ceramic or metal substrate
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Tuesday, August 22nd, 2023 AT 10:42 AM
Tiny
GJGOGOL
  • MEMBER
  • 3 POSTS
KEN L - Thank you for your reply. ASEMASTER6371 mentioned that the cat may not have enough precious metals to keep the light out. I am looking for a new cat and I don't see anywhere where the seller states "how much" precious metals are used. I am "assuming" that if it is either EPA or CARB compliant, they have sufficient metals in them. I could be wrong, thus my question.
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Tuesday, August 22nd, 2023 AT 11:28 AM

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