Codes po401,po421,po431 what do these codes mean?

Those are your only codes? you really shouldnt have drilled out your cats, its hard to diag an 02 sensor online. a scanner could show if they are functioning properly. It might be an issue with your cats though and require replacement.

Replace all spark plugs.

EXHAUST RELATED ERROR CODES

There are 711 possible PO generic DTC error codes. Only 7 of the 711 codes are exhaust related.

PO401: Insufficient EGR Flow

PO402: Exhaust Gas Recirculation (EGR) Excessive Flow Detected

PO410: Secondary Air Injection System Malfunction

PO420: Catalyst System Efficiency Below Threshold (Bank 1)

PO421: Warm-Up Catalyst Efficiency Below Threshold (Bank 1)

PO430: Catalyst System Efficiency Below Threshold (Bank 2)

PO431: Warm-Up Catalyst Efficiency Below Threshold (Bank 2)

THINGS TO ASK YOURSELF OR YOUR CUSTOMER

Have there been any driveability or performance issues with the vehicle, have there been any recent repairs or tune-up work performed, if so was the check engine light reset and were the codes recorded on previous repair orders.

Do not rely on scan tool data alone. In most applications, the customer will have the check engine light reset prior to repair. If the original cause of converter failure is intermittent, only the catalyst failure code may show up (masking the original reason for converter failure).

EXHAUST RELATED ERROR CODES

There are 711 possible PO generic DTC error codes. Only 7 of the 711 codes are exhaust related.

PO401: Insufficient EGR Flow

PO402: Exhaust Gas Recirculation (EGR) Excessive Flow Detected

PO410: Secondary Air Injection System Malfunction

PO420: Catalyst System Efficiency Below Threshold (Bank 1)

PO421: Warm-Up Catalyst Efficiency Below Threshold (Bank 1)

PO430: Catalyst System Efficiency Below Threshold (Bank 2)

PO431: Warm-Up Catalyst Efficiency Below Threshold (Bank 2)

THINGS TO ASK YOURSELF OR YOUR CUSTOMER

Have there been any driveability or performance issues with the vehicle, have there been any recent repairs or tune-up work performed, if so was the check engine light reset and were the codes recorded on previous repair orders.

Do not rely on scan tool data alone. In most applications, the customer will have the check engine light reset prior to repair. If the original cause of converter failure is intermittent, only the catalyst failure code may show up

A code P0401 most likely means one or more of the following has happened:

• The DPFE (differential pressure feedback EGR) sensor is faulty and needs to be replaced
• There is a blockage in the EGR (tube), most likely carbon buildup
• The EGR valve is faulty
• The EGR valve may not be opening due to a lack of vaccuum

Possible SolutionsIn fixing this code, it is quite common for people to just replace the EGR valve only to have the OBD code return. The EGR valve is not always the culprit.

• Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's & DPFE voltage. There should be a noticable difference in RPM's with the EGR open
• Clean out the EGR valve and/or tubing to remove deposits
• Check the voltage at the DPFE, compare to specified values (refer to a repair manual for your specific model)
• Replace the DPFE sensor (with a good quality / OEM one)
• Replace the EGR valve

Hi

Please check the link to get a detailed idea about the code:-

http://www.obd-codes.com/p0421

Please get back to us if you have further query else please accept the suggestion.
Thank you for contacting fixya.com

when u get a code it can mean a # of things-never assume and just replace the part--computer systems today are integrated in circuits which means its complicated to diagnose--it may be something as simple as a tuneup or vacuum leak--get a manual on code troublshootn-in library or online

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.

IF IT RUNS OK DONT DO ANY THING

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