1996 olds achieva check engine light remains on. Already replaced both oxygen censors and catalytic converter but the light remains on. Scan tool was placed on the o2 censors but it show "no activity".

P0420 means that the catalytic converter is operating below efficiency threshold. Normally, you only get this code if both O2 sensors ARE OPERATING PROPERLY. If you change the O2 sensors and still get the same code, it's an indication your catalytic converter needs to be replaced. But before you replace it, ensure that everything else in the engine is operating properly, so that whatever may have damaged your current catalytic converter doesn't damage a new one.

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.

Hi there:Likelly needs a down strem o2 sensor...

Circuit Description
The PCM has the ability to monitor this process by using a rear Heated Oxygen Sensor (HO2S 2). The rear HO2S 2, located in the exhaust stream past the catalytic converter, produces an output signal relative to the oxygen storage capacity of the catalytic converter. The PCM then calculates the ability of the catalytic converter to convert the exhaust emissions effectively. Whenever the catalytic converter is functioning properly, the rear HO2S 2 signal will be far less active than the signal produced by the front Oxygen Sensor 1 (O2S 1).


Conditions for Setting the DTC

• DTCs P0106, P0107, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0301, P0302, P0303, P0304, P0404, P0405, P0508, P0507, P1404, P1441 not set.
• Engine run time is greater than 25 seconds.
• Engine Coolant Temperature (ECT) is greater than 40 degrees C (104 degrees F).
• Air/fuel ratio is between 14.5:1 and 14.7:1.
• Throttle Position (TP) angle is between 5% and 50%.
• Engine has met the above conditions for 140 seconds.
• O2S 1 voltage is greater than 1042 mV for 50 to 75 seconds.

Action Taken When the DTC Sets

• The Malfunction Indicator Lamp (MIL) will illuminate.
• The PCM will record operating conditions at the time the diagnostic fails. The Freeze Frame and Failure Records buffers will store this information.
• A history DTC is stored.

Conditions for Clearing the MIL/DTC

• The MIL will turn off after 3 consecutive ignition cycles in which the diagnostic runs without a fault.
• A history DTC will clear after 40 consecutive warm up cycles without a fault.
• A scan tool can clear the DTCs.

Diagnostic Aids
Whenever the oxygen sensor pigtail wiring, connector or terminal are damaged, the entire oxygen sensor assembly must be replaced. Do not attempt to repair the wiring, connector or terminals. In order for the sensor to function properly, the O2S must have a clean air reference. This clean air reference is obtained by way of the oxygen sensor wires. Any attempt to repair the wires, connectors or terminals could result in the obstruction of the air reference and degrade the performance of the oxygen sensor. Check for the following conditions:

• A rich exhaust - An overly rich exhaust may load the catalytic converter causing high rear HO2S 2 signal voltages.
• A silicone contamination - A silicone contamination of the rear HO2S 2 may cause a false rich condition. A powdery white deposit on the rear HO2S 2 will indicate this condition.
• A faulty HO2S 2 - Whenever the rear HO2S 2 is internally shorted, the rear HO2S 2 voltage displayed on a scan tool will be over 1.0 volt. Disconnect the rear HO2S 2. Jumper the rear HO2S low circuit to the engine ground. Whenever the rear HO2S 2 voltage goes from 100 mV to around 450 mV, replace the rear HO2S 2.
• An intermittent test - Observe the rear HO2S 2 on a scan tool while moving the related connectors and wiring harness with the key ON. Whenever the fault is induced, the rear HO2S 2 display will change. This may help isolate the location of the malfunction.
• A damaged harness - Inspect the PCM harness for a short to voltage in the rear HO2S 2 low circuit.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
2. This step determines if DTC P0138 is the result of a hard failure or an intermittent condition.

1. Disconnecting HO2S 2 and jumpering CKTs 1668 and 1669 to ground should cause the scan tool to display HO2S 2 voltage below 100 mV (0.1 V). If the signal voltage is still high, the PCM is malfunctioning.

1. Replacement PCMs must be reprogrammed. Refer to the latest Techline information for programming procedures.

Hope this helps.

You probably have a large vacuum leak. Catalytic converters can only glow hot if they are getting too much fuel and too much oxygen (or air). A vacuum leak will put way too much oxygen in the exhaust and will cause the oxygen sensors to read lean. Then the computer will start adding a bunch of fuel to try and compensate for the lean oxygen sensors....and Voila! A raging inferno ensues at the catalytic converter that is hot enough to melt steel!

You need to scan it , to find what the code is.
Usually check engine light doesn't have to do anything with oil.

did you already run an OBD 2 scan?(free at most parts shops)

Facing the engine, look at the back of engine and down. It will be mounted on the manifold.

P0422 - Catalyst below threshold.

Catalytic converter is clogged or out of medium.

Replace it.


ACTUAL DIAGNOSTIC:

Fault Code Definition
Code P0422 indicates that the Bank 1 Main Catalytic Converter's emissions efficiency has fallen below the minimum allowable limit. This threshold is tracked by a Catalyst Monitoring Oxygen Sensor located on or near the outlet of the Catalytic Converter.

Symptoms

* Check Engine Light will illuminate
* In many cases, no abnormal symptoms may be noticed

Common Problems That Trigger the P0422 Code

* Defective Catalytic Converter
* Engine misfires have damaged the Catalytic Converter
* Internal engine damage resulting in high oil consumption and/or a leaking Head gasket has damaged the Catalytic Converter

Common Misdiagnoses

* Oxygen Sensors are replaced when the real problem is a damaged Catalyst
* Catalyst is replaced when the real problem is internal engine damage, which is producing elevated emissions levels
* Catalyst is replaced when a misfiring engine is the real problem



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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

P0422 - Catalyst below threshold.

Catalytic converter is clogged or out of medium.

Replace it.


ACTUAL DIAGNOSTIC:

Fault Code Definition
Code P0422 indicates that the Bank 1 Main Catalytic Converter's emissions efficiency has fallen below the minimum allowable limit. This threshold is tracked by a Catalyst Monitoring Oxygen Sensor located on or near the outlet of the Catalytic Converter.

Symptoms

* Check Engine Light will illuminate
* In many cases, no abnormal symptoms may be noticed

Common Problems That Trigger the P0422 Code

* Defective Catalytic Converter
* Engine misfires have damaged the Catalytic Converter
* Internal engine damage resulting in high oil consumption and/or a leaking Head gasket has damaged the Catalytic Converter

Common Misdiagnoses

* Oxygen Sensors are replaced when the real problem is a damaged Catalyst
* Catalyst is replaced when the real problem is internal engine damage, which is producing elevated emissions levels
* Catalyst is replaced when a misfiring engine is the real problem



Thanks for using FixYa - a 4 THUMBS rating is appreciated for answering your FREE question.#$%^&