2000 Chevy Prizm 130000 Miles Bought vehicle used. Check engine light stays on showing code p0420 (I think) or catalyic converter reading lean air/fuel mixture and bank 1. High knocking noise at 2500 RPM and knocking noise at idle. Engine oil constantly checked and filled. *May have been driven while out of oil once without knowledge*

I doubt they did a 'tune up' at all. you only replace a fuel pump when it has failed. And fortunately for them, you can't see if it has been changed because it is inside the fuel tank. As my old dad used to say "they saw you coming", meaning you got fleeced because you are female. Put this on your car, leave it there. It talks to your smartphone and you can even reset codes. The Catalytic converter one is not critical, but the overheating is.

https://www.fixd.com/

P1296 is a coolant overheating code.

P1296 VOLKSWAGEN code possible causes - Engine-Codes ...

https://www.engine-codes.com > p1296_volkswagen
'P1296 VOLKSWAGEN code possible causes · Low engine coolant level · Faulty Engine Coolant Temperature (ECT) sensor · Faulty engine coolant thermostat · Engine ...

and your P0420 code is the Catalytic Converter code you originally went for.

P0420 VOLKSWAGEN Code - Catalyst System Efficiency ...

https://www.engine-codes.com > p0420_volkswagen
As the code description implies the P0421 code means that the vehicle's control module has detected that the three-way catalytic converter is not working ...

0 2 sensor near the catalytic converter or

replace the catalytic converter

The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar.

Read more at: https://www.obd-codes.com/p0420
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https://www.obd-codes.com/p0420

Your converter may be failing again, and this can be caused by coolant leaking into combution chamber (bad head gasket), worn engine parts such as rings, or using certain additives in the fuel.

DTC P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

You will likely not notice any drivability problems, although there may be symptoms.

A code P0420 may mean that one or more of the following has happened:
Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
******** spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

Some suggested steps for troubleshooting a P0420 error code include:
- Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
- Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
- Inspect the downstream heated oxygen sensor (HO2), replace if necessary
- Replace the catalytic converter

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

Hope this helps; also keep in mind that your feedback is important and I`ll appreciate your time and consideration if you leave some testimonial comment about this answer.

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http://www.obd-codes.com/p0420

TSB Number: 3316 NHTSA Number: 10017210 TSB Date: n/a Date Added to File: October 27, 2005 Failing Component:Electrical System Summary:Diagnostic trouble code P0420 may set on low mileage vehicles, or shortly after catalytic converter replacement
TSB Number: 3316 NHTSA Number: 10017210 TSB Date: n/a Date Added to File: October 27, 2005 Failing Component:Electrical System Summary:Diagnostic trouble code P0420 may set on low mileage vehicles, or shortly after catalytic converter replacement
there is a service bulletin for this.

http://www-odi.nhtsa.dot.gov/cars/problems/tsb/results.cfm?start=1&SearchType=DrillDown&type=VEHICLE&year=2005&make=CHEVROLET&model=COBALT&vehtype=PC&typenum=1&component_id=0&summary=true

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Technical Description

Catalyst System Efficiency Below Threshold (Bank 1)
What does that mean?

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.
Symptoms

You will likely not notice any drivability problems, although there may be symptoms.
Causes

A code P0420 may mean that one or more of the following has happened:

* Leaded fuel was used where unleaded was called for
* An oxygen sensor is not reading (functioning) properly
* The engine coolant temperature sensor is not working properly
* Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
* Retarded spark timing
* The oxygen sensors in front and behind the converter are reporting too similar of readings

Possible Solutions

Some suggested steps for troubleshooting a P0420 error code include:

* Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
* Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
* Inspect the downstream heated oxygen sensor (HO2), replace if necessary
* Replace the catalytic converter

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

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what is the code that turned the light back on?????? if its a P0420, the cat is not good enough. you may need one good enough for California or a better quality converter.

from what i am finding the code means o2 sensors. they recommend replacing them and if this does not work than replacing the cat converter

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

causes

A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• An oxygen sensor is not reading (functioning) properly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings

possible soloutions

Some suggested steps for troubleshooting a P0420 code include:

• Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
• Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
• Inspect the downstream heated oxygen sensor (HO2), replace if necessary
• Replace the catalytic converter

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.


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A p0420 code is set when the computer determines that the catalytic converter isn't doing it's job. In almost all instances that code can't set as a result of a bad o2 sensor so they probably didn't have to be replaced. The catalytic converter will need to be replaced, prefferably by an oem part. This can be pretty expensive and should be considered before purchasing the vehicle.

I have replaced a few bad converters on late model subarus in the past few years.