I have an '01 Galant LS. I got an engine code "O2 Sensor Heater Circuit (Bank 2 Sensor 1)". I have replaced O2 sensors before, but I don't know where this sensor is on this engine. Can anyone give me a hint?

there are about a dozen O2 sensor codes. Tell us which one you have

If they are codes P0135 and P0155 they refer to both O2 sensors nearest the engine on the exhaust. The ones before the catalytic converter on each exhaust pipe.
P0135 = O2 Sensor Heater Circuit Bank 1 Sensor 1
P0155 = O2 Sensor Heater Circuit Bank 2 Sensor 1

If the sensors were replaced an DTC'S come back has to be a wiring problem or PCM ,
The ECM detects that the HO2S heater low control circuit voltage is between 2.6-4.6 volts when the heater is commanded OFF, indicating an open HO2S heater circuit.
Is the fuse open?
All four codes refer to the heating element circuit ! there is a relay in the under hood fuse box that supplies B+ voltage to the O2 sensor heating elements . I would change that or test the relay , videos on you tube on testing relays .
MAIN Relay
--
LO SPEED Relay, HI SPEED Relay, S/P FAN Relay, PRE O2 Fuse, IGN-1 Fuse, ECM Fuse, POST O2 Fuse, and INJ Fuse
Also check
POST O2 Fuse
10 A
HO2S Bank 1 Sensor 2 and HO2S Bank 2 Sensor 2
PRE O2 Fuse
10 A
Heated Oxygen Sensor (HO2S) Bank 1 Sensor 1 and HO2S Bank 2 Sensor 1

P0052 DTC trouble code may be caused by one or more of the following:
A short in the heater circuit in the sensor.
A failed O2 sensor heater.
Wiring/connectors broken/frayed leading to sensor and/or relay.
Failed PCM/ECM.

P0302 TOYOTA - Cylinder 2 Misfire Detectedhttp://engine-codes.com/p0302_toyota.htmlForum CodePossible causes- Faulty spark plug 2
- Faulty ignition coil 2
- Ignition coil 2 harness is open or shorted
- Ignition coil 2 poor electrical connection
- Clogged or faulty fuel injector 2
- Injector 2 harness is open or shorted
- Injector 2 poor electrical connection
- Improper spark plug 2
- Insufficient cylinder 2 compression
- Incorrect fuel pressure
- Intake air leak
P0135 - O2 Sensor Heater Circuit Malfunction Bank 1 Sensor 1http://engine-codes.com/p0135.htmlForum CodePossible causes- Faulty Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit
- Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit open shorted to ground
- Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit poor electrical connection
- Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit fuse
- Faulty Engine Control Module (ECM)
Tech notes The code means that there is a problem with the heater element circuit of the heated oxygen sensor. The Engine Control Module (ECM) monitors how long it take the sensor to warm up and start sending an adequate signal. The code means the the sensor is taking too long to warm up. Replacing the O2 Sensor 1 usually takes care of the problem
P1705 TOYOTA - Direct Clutch Speed Sensor Circuit Fault.
This is related to the sensor that gives info about the slippage rate o fthe transmisssion to the Transmission Control Module..

P0155 is o2 sensor heater circuit malfunction bank 2 sensor 1 it is the o2 sensor circuit in front of the cat converter the o2 sensors are located mounted to the exhaust pipe in front and behind the catylic converter the problem is in the circuit [wiring or sensor itself] check wiring and repair if necessary if the short or open circuit is the sensor need to replace sensor +++ P0161 is 02 sensor heater circuit malfunction BANK 2 SENSOR 2 ++++++ this is the sensor behind the cat converter

All cars sold in the US after 1995 use the same on board diagnotics called OBD-II. The codes are the same. Search OBM-II error codes and you'll get a listing.

P1150 Air/Fuel Sensor Circuit Range/Performance. (cylinder bank 1 Sensor 2)

P1150 may mean that one or more of the following has happened:
water or corrosion in the O2 sensor connector, loose terminals in the connector, wiring burnt on exhaust components, open or short in the wiring due to rubbing on engine components, holes in exhaust allowing unmetered oxygen into exhaust system, unmetered vacuum leak at the engine, bad O2 sensor or bad PCM.

P1155 Air/Fuel Sensor Heater Circuit. (cylinder bank 1 Sensor 2)
P1155 may mean that one or more of the following has happened: O2 Heater element resistance is high, Internal short or open in the heater element or O2 heater circuit wiring high resistance

Solution :Replace oxygen sensor and see if the problem goes away.

P0157 Oxygen Sensor Circuit Low Voltage (Bank 2 Sensor 2)

I would try replacing the Oxygen sensor, then clear the code.

The O2 sensor would be the after the catalytic converter ( sensor 2) on the exhaust pipe of the number 2 cylinder uses ( bank 2).

P0161 02 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2)
check the wiring to the bank 2 sensor, that is the one behind the catalytic converter on the opposite side from the bank where #1 cylinder is located, see firing order diagram below.

there is a fuse for the o2 heaters if one of the sensors blew the fuse they will all be out