Code P0420 after code P0303 repair?

Tiny
DOC1WHO
  • MEMBER
  • 2010 CHEVROLET AVEO
  • 1.6L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 165,000 MILES
Chevy 2010 Aveo5 LT, 1.6l Engine, Auto Transmission, Hatchback.
165,000 Miles

After misfire P0303, replaced coil pack, spark plugs OK, Vehicle reported P0420.
No instance of the code before Misfire, verified by scanner.

Live data from scanner shows:
STFT: Bouncing between -6.0 to 3.0
LTFT: 6.0
o2 sensor 1: showing correctly
o2 sensor 2: cold - bouncing, at full temp (highway speeds) flat voltage between.5-.7 volts

Both Cat convertors changed less then a years ago with OEM.
Brand new o2 sensors replaced at the same time with OEM.
Full exhaust replaced at the time of Cat replacement.

Vehicle shows no power reduction, no acceleration issues, no starting issues.
Noticed recent poor fuel consumption.

Possible resolutions:
Leaking fuel injector(s), not overly noisy
Bad or fuel soaked EVA canister (no codes shown)
Bad #2 o2 sensor
Manifold Catalytic Converter

Ideas?

Thanks for your help
Tuesday, November 19th, 2024 AT 9:26 AM

11 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
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Did the P0303 code go away after the repair, and did you clear the codes once the repairs were completed? The P0420 code (Catalyst System Efficiency Below Threshold) often appears after dealing with a P0303 (Cylinder 3 Misfire) because a misfire can cause unburned fuel to enter the catalytic converter. So, I would clear the codes again, also can I ask how the engine is running?
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Tuesday, November 19th, 2024 AT 10:40 AM
Tiny
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Cleared codes after repair. About 2 days after repair, P0420 code appeared.
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Tuesday, November 19th, 2024 AT 10:43 AM
Tiny
KEN L
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I would clear it again for a retest.
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Tuesday, November 19th, 2024 AT 10:55 AM
Tiny
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I did, code showed up again after about 2 hours of driving. Combination of city and highway driving. I noticed that at highway speeds the B1S2 sensor displaying as normal (flat response, .5-.7 voltage reading). It is almost like the Cat is not maintaining operating temp. Maybe a flaky B1S2 sensor.
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Tuesday, November 19th, 2024 AT 11:10 AM
Tiny
KEN L
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Is the engine still misfiring? How does the engine seem to run?
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Thursday, November 21st, 2024 AT 9:57 AM
Tiny
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Replacement of the coil pack repaired the misfire. Cleared all codes.
P0420 showed up about 2 hours of City/highway driving.
Engine shows no power reduction, no acceleration issues, no starting issues, stable idling.

Thinking to change out o2 sensor #2 to test for flaky sensor.
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Thursday, November 21st, 2024 AT 10:51 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,509 POSTS
Sorry to say it but that is common. The P0420 code is because the unburnt fuel running into the converter actually causes them to run hot and glaze over the metals that make it work. This can be even worse if the engine uses any oil as the fuel and oil combine to create a glass like sealer that really wipes out the converter quick. 99% of the time replacement of the converter is the only option. The P0420 code actually tells you that the sensor is operating. If you want to try it, you could use some Cataclean and see if it brings it back enough to turn out the light. If the misfire hasn't been going on long and oil use is minimal I've seen it work.
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Friday, November 22nd, 2024 AT 1:49 AM
Tiny
DOC1WHO
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Misfire lasted about 10 miles. From interstate to work. Replaced Coil pack same day. Reset codes and drove home after work. No codes for about 2 days then threw P0420. I am thinking that the downstream o2 might have an issue. The downstream o2 shows random spikes when at highway speeds, mostly stable between.5-.7 with random spikes to.100. Going to change it for a test. I am thinking there might be intrusion of something causing the random spikes. At low RPM the downstream o2 looks like upstream o2 sensor.
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Friday, November 22nd, 2024 AT 11:21 AM
Tiny
STEVE W.
  • MECHANIC
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If the downstream sensor tracks the upstream at idle that would point to a bad cat, not a bad sensor. The random spikes are likely the test method the system uses to verify the converter is working. What the EDU does is to test the sensors. It will momentarily make the mix very rich, then drop back to normal. Then it will monitor the up and down stream sensors to see if they respond. When the system goes rich, the upstream sensor should see it and change rapidly in response, the downstream sensor however should stay at a low level all the time, if it is showing tracking and the spikes the converter is not doing its job.
It's likely that the converter was borderline operational, and the extra fuel just finished it off.
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Friday, November 22nd, 2024 AT 5:31 PM
Tiny
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Attached is pic of Highway speed downstream o2. Watching downstream "cold" o2 sensor. It tracks with upstream until the converter starts to warm up. Unless there is something external is added in the upstream side the downstream o2 sensor should be a flat line with no spikes (at temperature). The only change should be in speeding up or slowing down, either way it should quickly return to a flat line. Spiking in the downstream o2 sensor could indicate faulty o2 sensor or something entering the flow upstream.
I did notice that when the ECM completes the "CAT" test then it triggers the fault. Until that test is completed there is not fault CEL set. I also disconnected the battery for the weekend to fully rest the ECM. So far CEL has not set. Thanks for the help!
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Monday, November 25th, 2024 AT 7:21 AM
Tiny
STEVE W.
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The spikes are the convertor test running. The test is simple, the engine goes into closed loop. Then the once it has triggered the PCM will dump extra fuel into the engine. The upstream sensor sees it and should reacts within a set time limit if it does, the upstream tests good and the PCM can trust it, that same fuel dump hits the converter and then the downstream sensor. If the cat is operating properly the sensor should only see a minor change as the cat burns the extra fuel. The spike you are seeing is not the sensor it's that the convertor isn't burning off the extra fuel and the PCM sees that and you get the P0420 code because of it. The programming in that car has a very tight tolerance between a "good" cat and a marginal one. And yours is just hitting the marginal area. That is why I suggested the cleaner. It may remove the soot and glaze and eliminate the spike that is setting the code.
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Monday, November 25th, 2024 AT 1:39 PM

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