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Emissions code P0420?

Tiny
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  • 2003 PONTIAC GRAND AM
  • 3.4L
  • V6
  • 2WD
  • AUTOMATIC
  • 215,000 MILES
The car listed above is a Gt model. I'm having a hard time finding the cause. It keeps reoccurring. Replaced cat, bank 1 and bank 2 02 sensors, emission purge solenoid, cleaned MAF sensor, repaired exhaust leak, new fuel filter, seafoam smoke treatment through intake, Gumout fuel system treatment. Please help.
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Friday, July 15th, 2022 AT 10:46 PM

19 Replies

Tiny
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  • MECHANIC
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Hi,

Is this the only code that shows up for you? If there are others, let me know what they are. Also, how long after the new converter did the code return?

The code indicates the converter is operating below the threshold. There are things that can damage a converter such as a rich fuel mixture, a misfire, a damaged exhaust system, contaminants in the fuel, and so on. Also, the downstream oxygen sensor should be checked.

Here is what I need. First, do you have a live data scan tool? If you do, reconfirm there are no additional codes. If there aren't, check the engine coolant temperature sensor when the engine is cold. The signal should be close to ambient temp. Next, check the short-term fuel trims and let me know what they are. Last, with the engine running, watch the downstream oxygen sensor. It should be moving between 0.1v and 0.9v. If there is variation, the sensor is bad or if it is outside the parameters I mentioned, it needs to be replaced.

Let me know if you are able to do this. Also, let me know if you can provide the info requested and if there are any other codes.

Take care,

Joe
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Saturday, July 16th, 2022 AT 6:02 PM
Tiny
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Sorry for the delay in my reply. Work has been keeping me busy. There had been several codes coming up dealing with emissions. I've been correcting as I go replacing parts. I can smell the fuel in my exhaust. So I believe it's running rich. I currently do not have a code reader for the live data. I'm off work July 19th and July 20th. I'm going to try the local auto parts stores and see if they have a rental for the scanner.
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Monday, July 18th, 2022 AT 7:10 AM
Tiny
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Hi,

Most parts stores will scan it for free if you can drive it to them. Otherwise, hopefully, they will have what you need for rent.

If you smell a rich fuel mixture from the exhaust, I suspect it would be fuel pressure related. You may want to test the fuel pressure to see if it is too high due to a faulty pressure regulator. Here is a link that explains in general how it's done:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

I attached the manufacturer's pressure specifications below for you. (Pic 1). Here is a quick and simple thing you can check as well.

The fuel rail is where the pressure regulator is located (see pic 2). With the engine off, remove the vacuum hose to it, and see if there is evidence of fuel in the hose. There shouldn't be.

If there isn't, start the engine and see if there is a vacuum being supplied to the hose. Don't run it too long like that because the regulator won't be working if the hose is disconnected.

Let me know.

Take care,

Joe

See pics below.

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Monday, July 18th, 2022 AT 1:05 PM
Tiny
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Okay, I got a Innova 3130 lat code scanner. Under Pontiac OEM Enhanced there are 4 dtcs listed 1. P0122 throttle position sensor 1 circuit low voltage.

2. P0433 EVAP purge solenoid valve control circuit.

3. P1122 throttle position sensor circuit intermittent low voltage.

4. P1122 throttle position sensor circuit intermittent low voltage.

I will run the other tests like you asked if needed still?
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Monday, July 18th, 2022 AT 8:35 PM
Tiny
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Hi,

Do me a favor. Locate the throttle position sensor and disconnect the connector. Inspect it for corrosion, damaged wires, and inspect the connectors to see if something is causing the low voltage.

If you look at pic 1, it shows the location.

Also, could you confirm that P0433 is correct and shouldn't be P0443?

Let me know what you find.

Joe
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Monday, July 18th, 2022 AT 9:50 PM
Tiny
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My bad. The code is p0443. And I'll look at the connector in a few minutes.
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Monday, July 18th, 2022 AT 10:09 PM
Tiny
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Sorry it's so late. I couldn't get the ambient temperature because live mode the car must be running. I pulled the connectors off the TPS and Ats. Cleaned with electrical contact cleaner. I'll give you the before and after I cleaned. Stft b1 1.6%. Ltft 10.9%
Ats 105.8
Tps.4

Stft 2.3%
O2s b1s1. 755
Stft b1s2 99.2%

O2s b1s1. 825
Stft b1s1 0.0
O2s b1s1. 490
Stft b1s1 99.2
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Tuesday, July 19th, 2022 AT 12:23 AM
Tiny
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Here's two pics.
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Tuesday, July 19th, 2022 AT 12:36 AM
Tiny
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Here's the o2s live data at idle.
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Tuesday, July 19th, 2022 AT 12:47 AM
Tiny
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Hi,

The sensors seem to be working. Why weren't you able to get the coolant temperature sensor info? The vehicle can be running. I suggest doing it when the engine is just started because the coolant will be close to ambient temperature.

Cleaning the connector sure made an improvement. If you clear the codes, do they come right back?

Joe
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Tuesday, July 19th, 2022 AT 7:01 PM
Tiny
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I replaced the TPS. Then cleared the codes. They haven't come back yet. But I haven't had a chance to drive it. Will tomorrow and I'll let you know how things go.
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Tuesday, July 19th, 2022 AT 9:52 PM
Tiny
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Hi,

Sounds like a plan. Let me know how things turn out for you. I'm interested in knowing.

Take care,

Joe
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Wednesday, July 20th, 2022 AT 7:56 PM
Tiny
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So, I drove it to the gas station and back, which is around four miles. I didn't want to push it. Just a short drive to see how it feels. On the way back I got into just enough to drop it down to second gear. Fast response on, no hesitation, and smooth quick acceleration. I'll drive it around eighty miles tomorrow to Attica, in and back. Back to work. So far so good. Will post tomorrow.
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Wednesday, July 20th, 2022 AT 8:55 PM
Tiny
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Okay. Yesterday I drove to and from work, which is around 75 miles. No codes and seemed okay. Today on my way to work after 35 miles the service engine light came on. I'm going to Napa on my lunch break to have them read that code. Will post when I know what it is.
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Friday, July 22nd, 2022 AT 8:01 AM
Tiny
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Okay, the code it threw is p0420 catalyst low efficiency. Also, I did the check on the fuel pressure regulator. No signs of fuel in vacuum line and it has plenty of suction.
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Friday, July 22nd, 2022 AT 12:34 PM
Tiny
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Could it be because the O2 sensor is back too far and isn't heating up enough? I could put the O2 in the cat. And put the plug in the O2 pipe that was added. That would move the O2 sensor forward maybe enough to stop throwing codes.
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Friday, July 22nd, 2022 AT 12:37 PM
Tiny
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Hi,

If the system is still the OEM design, the sensor position should be correct. I have a favor to ask that I asked earlier. I want you to drive the vehicle until it is at operating temperature. Then, check both the short-term and long-term fuel trims. Nothing other.

I read through the list you provided before and I'm struggling to make sense of a couple of things. In one section, it looks correct, and the other doesn't. I just want to know the fuel trim percentages.

Hang in there. We'll get it figured out. Also, since the P0420 returned, we have an issue with the converter. Did any of the other codes return?

Let me know.

Joe
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Friday, July 22nd, 2022 AT 7:40 PM
Tiny
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The exhaust system isn't the original except the header pipe and the pipe that zig zags over the axel. Oh, and the muffler. The rest was done by me and my cousin. The resonator pipe was eliminated. I need to know the distance from the header pipe back to where the O2 is supposed to go. No other codes besides the p0420. I'll have to get the code scanner again.
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Friday, July 22nd, 2022 AT 9:16 PM
Tiny
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Hi,

The only way I could tell the distance is if I had one in front of me. I attached a pic below showing it at the 3-way catalytic converter. Looking at the pic, I would estimate it is approximatly three inches after the converter. I wish I had a better answer for you.

Joe
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Saturday, July 23rd, 2022 AT 8:20 PM

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