Warm up catalyst efficiency below threshold bank1 on galant 02 v6: someone pls tell what is this ? and where is bank1 located on this car ?

This is not the sensor this is bad catalyst converter you have to replace if you put chip one code return very quickly.

Visit the on-line store, know as reliable-store, they have the repair manuals. I'm sure you will get the solution of your problem.
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this code marks a bad cat.
The ecu can get this wrong.
it means the rear 02 is reading oddly, Aka. the cat monitor (OBD2)
it can be a , bad 02 oxy sensor rear, bad fueling or spark.
Some smarter cars, can shut down the 421 monitor when, it detects other failures. I have such a car. but not all cars work the same here. it's best to take look to see if all things are good first.

if the engine has full power, it's not a broken up cat.
the cat back pressure can bet tested easy.
make sure there are not other signs, of trouble.
Is the 02 front holding closed loop at hot idle, and light cruise.?
if not check for exhaust leaks near either 02 sensor.
the front sensor sets AFR trim.
then check just that, look at LTFT, make sure its centered. nearer zero. not -40 or +40% out of wack. Long term fuel trim.
most cars run about -4% they naturally remove this much fuel from the rich side. It may be an air leak near the rear 02.
so look there for any cracks. near that area.

P421=Warm up catalyst system efficiency below threshold, bank 1
P431=Warm up catalyst system efficiency below threshold, bank 2
Sounds like you may have clogged or damaged catalyst converters or bad 02 sensors.
Those codes I think are taking from after the catalyst by the sensors in the exhaust pipes. Be careful taking it to a mechanic for further checking since the catalyst converters are not cheap through them. If you need to replace the converters, shop around and get prices on aftermarket parts. Don't be ripped off by a hungry mechanic or especially a dealer.

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

hi, the first issue is most likely the heated O2 sensor they are pretty common failures. the second cat efficiency can be caused from the first problem. If not then the convert-or is bad and needing replaced. Under Federal law the converters are covered by the manufacture for 8 years so you may want to have this checked soon. Good luck

The code means Warm-up catalyst efficiency below threshold.....

P0421 : Warm Up Catalyst Efficiency Below Threshold

420 is catalist below threshold bank one.
432 is catalist below threshold bank two.

What codes come on? P0420

The converter's only purpose is to reduce emissions from the engine, in rare cases the converter can disintegrate which could result in a loss of power but this is not very common,

This means that you're Cat needs repair or replacement, You're car will not pass legally smoged without fixing this. :

Good luck