Check eng light with code po431 replased ox/sensors

The diagnostic trouble code (DTC) P0420 "Catalyst System Efficiency Below Threshold" is one possible outcome following the Catalytic Converter Monitor (test). This monitor is one of the last tests to be performed by the powertrain control module (PCM) because other failures in the system could compromise this test's result. Catalyst efficiency is tested by observing the oxygen sensor signals both before and after the catalyst. Before the catalyst, the signal should appear as a sine wave as it switches back and forth between rich (0.9 volts) and lean (0.1 volts). After the catalyst, the signal should be flat-lined on the rich side (about 0.7 volts). This is because a healthy catalyst will store oxygen and therefore remove the "lean" portion of the signal/waveform. If the signal after the catalyst still appears as a sine wave, the PCM concludes that the catalyst can no longer store oxygen and should therefore be replaced. In your specific scenario where a downstream (post-cat) O2 sensor was just replaced, I would theorize that this sensor had failed and was flat-lined. Then when it was replaced two things happened: 1.) the oxygen sensor began to work properly which allowed the oxygen sensor monitor (test) to complete, and 2.) the catalyst monitor then ran and used the new (accurate) sensor data to determine that the catalyst efficiency was below threshold. You can read more here: https://www.yourmechanic.com/article/p0420-obd-ii-trouble-code-catalyst-system-efficiency-below-threshold-bank-1-by-jamahl-walker

Have the catalytic converter and your O2 sensor checked.

2003 saturn vue, we dont have the engine size, the codes should be and are the same, po 128 means; cooling temp is out of spec, make sure the coolant is full, then replace the coolant emp sensor and make sure the connection is good, i would also replace the thermostant, this is a common code on these and most of the time its the coolant temp sensor or thermostant, ive learned to replace then both to avoid future engine lights with code po128. next we will check the code # po421, this code is that the computer sees that the cat converters are taking to long to warm up, code po 431 is also the same as po421, this code is related to the thermostant and sensor, the sensor give the computer the wrong temp readings and then the compuetr thinks the engine is hotter than it is and thinks the converters should be also hot like the motor but the coolant temp is giving a false reading, if you replace the coolant temp sensor and the thermostant and fill coolant , then reset the light and you will solve the problem, i hope this is helpful, also the sensor is located to the right of the engine if you are standing in front of the motor, heres a picture of the sensor, remember to check connector heres a pic of the connector, good luck.

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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If your check engine light is on use DB2 monitor to check code, replace sensor indicated. Check fuel filter if more than 30,000 miles, Could be ignition problem or weak fuel pump

curious to know if you got this solved. Guessing downstream o2 sensor. did your vue idle high? act like it was getting too much gas? Let me know please.

if your check eng light is on have it checked out lot of times it will not be a major repair just a faulty solenoid witch is a lot less to replace than a overhaul good luck

what eng size

Here is the definition of the code in GM manual:
P0128 - Coolant Thermostat Below Thermostat Regulating Temperature, this indicates the engine is running cold, u need a thermostat

could be the knock sen-cor, map sen-cor. hope this helps.