Check engine light on 2005 saturn vue

1996-newer (OBD-II) vehicles use oxygen sensors and a catalytic converter to reduce emissions. There is one O2 (oxygen) sensor in front of the catalytic converter (called upstream), and one behind it (downstream). When the engine is warmed up and running normally the upstream oxygen sensor reading fluctuates and the downstream one is fairly steady. In the case where both readings are fairly steady, a P0420 check engine light code is set.
Possible Causes: Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter
Diagnostic Help:
To diagnose a P0420 trouble code we begin by checking for exhaust leaks under the vehicle. Then use an oscilliscope to check the waveforms of both upstream and downstream O2 sensors. The front reading should vary, the rear should be steady. Replace as necessary. Check the rear Heated O2 Sensor. Check the catalytic converter, replace if necessary. If necessary, check for wiring and component concerns. See our article "Automotive Circuit Testing 101", if you need more assistance with this. Remember to refer to an model-specific repair manual for proper instruction.

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.

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


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 (remember to rate and comment this answer).

It is pointing to a bad catilatic converter, as well as possibly, 02 sensor. Hope this help you, thanks. Mike from fixya

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.

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.

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.

Check ti test it and keep us updated.

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 SolutionsSome 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

Code p0303 means one cyliner is misfireing. Check spark plugs and wiring cables. If one cylinder isn't working good that could be the possible couse for the first code P0420 and all other engine problems.

A p0420 code indicates a Catalytic System efficiency failure, Bank 1.
Basically, the computer is seeing reduced efficiency of the catalytic converter or one of it's components (such as an 02 sensor)

There may be a few root causes, such as a faulty oxygen sensor, faulty coolant temperature sensor, faulty converter, Large exhaust leak, or retarded (slow) timing.

P0420 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 Need a catalytic converter? Visit our catalytic converter shopping area for the lowest price on your replacement converter!
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.

The codes are Catalyst System Efficiency Below Threshold (Bank 1)&(Bank 2).

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.

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

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.

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

There is one 02 sensor in front of the catalytic converter (called upstream), and one behind it (downstream). When the engine is warmed up and running normally the upstream oxygen sensor reading fluctuates and the downstream one is fairly steady. In the case where both readings are fairly steady, a P0420 check engine light code is set.

Possible Causes:
Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter