Need to pass emissions test, dianostic test - Catalyst efficiency blow threshold (bank 1) tried to order part - now I need to know if it pre or post, left or right Oxygen sensor, can it be determined by the code which sensor is needed

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

P0420 Catalyst System Efficiency Below Threshold Bank 1 Probably the cat is bad .You need to have it tested How To Diagnose Check Engine Light Code P0420 Catalyst Bank 1

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

P0420 = catalyst efficiency below threshold (bank 1)

I think it is a coincidence that the check engine light came on with this code at the same time as you installed the new alarm.

the power train control module or computer checks the output of the oxygen sensors (pre-catalyst and post-catalyst) and compares them. When the post-catalyst sensor output starts to resemble the pre-catalyst sensor output with little difference, the computer sets the catalyst efficiency code. Check the output of the oxygen sensors. A worn out or lazy oxygen sensor might be at fault. If the sensors check okay, then the catalytic converter may need to be replaced. This is unusual for a 2005 unless the engine has been having misfires. Raw unburned fuel will quickly destroy a catalytic converter.

if the code is p0420
it means
catalyst system efficiency below threshold
(cylinder bank no.1)

Catalyst System Efficiency Below Threshold Bank 1,
Is telling you that the drivers side converter is not efficient in emission control,It's probably starting to get plugged up.

p1423 Secondary Air Injection Bank 1 flow too low

P0431 Warm Up Catalyst, Bank 2 Efficiency Below Threshold/Limit

P1411 Secondary air injection fault, this is related to the P1423.

If the readings were close to passing, use 3oz of acetone per 10 gals of fuel and it should pass. Then you have a whole year to figure out what's wrong with it! Main cat is the large one. all others are pre-cats.