I have research around and found that the Bank 1 sensor 3 O2 sensor is post Cat. But when I got under there I dont see anything that could resemble a sensor. Where is it. Please help. Marc

THE CAR NEEDS SERVICE, GET SOME?
no country of VIN told , no engine told. guess at V6 I will, 3.3L
that o2 heater is dead. or the 4wires to the oxy sensor are burned up on the exh. pipe. toss coin or better just look at it.
sensor #2 is behind the CAT. and is bad.
you posted no engine told at all, and is a 100% engine question.
S1 is always front of CAT, S2 is rear.
upstream or down stream, (S1 /S2 order)
B1 bank 1 means that bank that has cylinder #1 there.
on the 4 banger #1 is front of engine a belt drive end.
on a V6 , the #1 cylinder is most front (it's off set to bank2)
3.3L back bank row is B1. (drivers end)
so bank1 s2 is that exhaust manifold rear that goes to cat #1 and the rear of cat 1 is B1, sensor S2 rear of cat.
the heater circuit side never lies,it means the voltage or current flow to that heater is dead wrong. open/ or shorted heater or its wires to it.

the gen 3 car? 2014
engine line up is here.
Petrol:
2.0 L Theta II T-GDi I4
2.4 L Theta II MPi I4
2.4 L Theta II GDi I4
3.3 L Lambda II MPi V6
3.3 L Lambda II GDi V6
3.5 L Lambda II MPi V6
Diesel:
2.0 L R II CRDi I4
2.2 L R II CRDi I4

keep in mind my answers are USA only
and USA EPA. DICTATES CAT LAY OUT AND DESIGNS.
SOME CARS RUN Y PIPE TO A SINGLE CAT. V6
WHAT YOU HAVE THERE , IVE NOT 1 CLUE.

Notes in relation to bank numbers
•Bank 1 always refers to the group of cylinders which includes No.1 cylinder.
•The majority of small engine European vehicles will only have Bank 1 sensors fitted (i.e. 1 x Front & 1 x Rear).
•An inline 6-cylinder engine may have cylinders 1, 2 & 3 referred to as Bank 1 & cylinders 4,5 & 6 referred to as Bank 2.
•V configured engines would normally have Bank 1 & Bank 2 sensors, with again Bank 1 referring to the group of cylinders which contains No. 1 cylinder.
Notes in relation to sensor numbers
•Sensor 1 always refers to the front / regulating (Upstream / Pre-cat / Upper) sensor.
•Sensor 2 always refers to the rear / diagnostic (Downstream / Post-cat / Lower) sensor.
N.B. Some larger capacity vehicles may have 2 rear / diagnostic (Downstream / Post-cat / Lower) sensors on the same bank, which may be referred to by EOBD codes as sensor 2 & sensor 3. Sensor 2 would relate to the rear sensor closest to, or in the catalyst, with sensor 3 relating to the rear sensor furthest from the catalyst.
Notes in relation to position
In some catalogue entries, a sensor will be listed as fitting in left or right position. This is always taken from the point of view of sitting in the vehicle (i.e. For UK RHD vehicles Right = Offside, Left = Nearside).
Correct identification of the relevant oxygen sensor is more important than ever as many vehicles now may have devices such as temperature, pressure and NOx sensors fitted to the exhaust system in addition to the oxygen sensors.

What code is it giving ? p0172 system too rich bank 1 ? If it is staying around 0.8 millivolts try introducing a vacuum leak an see if it goes lean ! If it does it is working fine . You probably have leaking fuel injectors or a fault fuel pressure regulator ! You need a better scan tool to check fuel trims , short term am long term ,to see if the computer is adding or subtracting fuel ! Oxygen Sensor Test fixed rich signal Ford O2 Sensor Testing

have throttle body checked also the MDK throttle motor,
make sure that the dealer has followed service bulletin 12 07 99 for procedure to replace the MDK throttle motor
also have them check for codes,here are some codes,
it could be an expensive job,have them check everything,it is most likely the throttle motor,,throttle body is 600 if they switch it,dont let them do it without checking throttle motor,
codes,
MS41, MS42 and MS43 engine DME bmw codes...
• 1 Ignition coil Cyl 2
• 2 Ignition coil Cyl 4
• 3 Ignition coil Cyl 6
• 5 Fuel injector Cyl 2
• 6 Fuel injector Cyl 1
• 8 Air Flow Meter (HFM)
• 10 Coolant Temperature Sensor
• 11 Tank Pressure Sensor (EVAP System) or Radiator Outlet Temp (MS43)
• 12 TPS or Plausibility - Maximum Coolant Temp (MS43)
• 13 Plausibility- Radiator Outlet Temp
• 14 Intake Air Temperature Sensor
• 15 Plausibility - Cut Out Time
• 16 AirCon Compressor - PWM Signal or Plausibility Intake Air Temp (MS43)
• 17 Plausibility - Engine Coolant Temp
• 18 EWS Signal or Camshaft Sensor (MS43)
• 19 Activation VANOS Inlet Valve or Exhaust Valve (MS43)
• 20 "CHECK ENGINE" Light Failure
• 21 VANOS -Electrical Fault or Activation VANOS Inlet Valve (MS43)
• 22 Fuel Injector Cyl 3
• 23 Fuel Injector Cyl 6
• 24 Fuel Injector Cyl 4
• 25 Lambda Sensor Heater -Bank 1
• 27 Idle Control Valve - Malfunction
• 29 Ignition Coil Cyl 1
• 30 Ignition Coil Cyl 3
• 31 Ignition Coil Cyl 5
• 33 Fuel Injector Cyl 5
• 35 Aux. Air Injection System Relay
• 36 DME Main Relay
• 37 DME Main Relay : Delay
• 38 Clutch Switch -Plausibility
• 39 Brake Light Switch or Brake Light Test Switch
• 40 Brake Light Switch or Pedal Value Signal
• 42 Multi Function Steering Wheel- Plausibility
• 43 Multi Function Steering Wheel: Button
• 45 Multi Function Steering Wheel: Port
• 47 Temp Sensor -Downstream of Pre-Cat or Torque Limitation Level 1 (MS43)
• 48 DME Control Unit -Self Test 1
• 49 DME Control Unit or Torque Monitoring Level 2 (MS43)
• 50 EVAP Control Valve or Response Monitoring Level 2 (MS43)
• 51 Shut-off Valve -Charcoal Filter or Request Control Unit Reset (MS43)
• 52 Solenoid Valve -Exhaust Flap
• 53 Idle Speed Actuator
• 55 Lambda Sensor Heater -Bank 2
• 56 Ignition Current Feedback Resistor - Open Circuit
• 57 Knock sensor -Bank 1
• 58 DME Control Unit -Self Test 2
• 59 Knock Sensor -Bank 2
• 61 Lambda Sensor Heater -Bank 2 Post Cat
• 62 Aux. Air Injection System -Switching Valve
• 63 DME Control Unit or Ambient Temp Signal via CAN (MS43)
• 64 Plausibility - Ambient Temperature
• 65 Camshaft Position Sensor (Inlet MS43)
• 66 DME Control Unit
• 67 DME Control Unit
• 68 Tank Venting Valve
• 69 Fuel Pump Relay
• 70 DME Control Unit
• 71 DME Control Unit
• 72 DME Control Unit
• 74 AirCon Compressor Relay
• 75 Lambda Sensor Voltage -Bank 1
• 76 Lambda Sensor Voltage -Bank 2
• 77 Lambda Sensor Voltage -Bank 1 Post Cat
• 78 Lambda Sensor Voltage -Bank 2 Post Cat
• 79 Lambda Sensor Heater -Bank 1 Post Cat
• 80 ABS/ASC interface
• 81 MSR Signal -Active too Long
• 82 ABS/ASC Interface -Advance Adjustment
• 83 Crankshaft Sensor
• 90 Exhaust Temperature Pre Cat Conv - Bank 1
• 91 Exhaust Temperature Pre Cat Conv - Bank 2
• 92 Exhaust Temperature Post Cat Conv -Bank 1
• 93 Exhaust Temperature Post Cat Conv -Bank 2
• 94 Auxiliary Air -Air Mass Flow Sensor
• 95 Auxiliary Air Valve or Auxiliary Air Hose Blocked
• 96 Auxiliary Air Pump - Function
• 97 Auxiliary Air -Flow Rate too Low
• 98 Auxiliary Air -Flow Rate too High
• 99 Auxiliary Air Valve Jammed Open
• 100 DME Control Unit -Self-Test Failed
• 103 VANOS Error -Inlet Camshaft
• 104 VANOS Error -Exhaust Camshaft
• 105 VANOS Error -Position Inlet Camshaft
• 106 VANOS Error -Position Exhaust Camshaft
• 109 Throttle Valve Plausibility
• 110 Pedal Sensor Value Potentiometer 1
• 111 Pedal Sensor Value Potentiometer 2
• 112 TPS Potentiometer 1
• 113 TPS Potentiometer 2
• 114 Throttle Valve Final Stage
• 115 Reference Voltage -Voltage Regulator 1 or Throttle Pedal Adaptation (MS43)
• 116 Reference Voltage -Voltage Regulator 2
• 117 Plausibility -Pedal Position Sensor 1/2
• 118 Plausibility -TPS 1/2 or TPS1/Airflow Plausibility (MS43)
• 119 Throttle Valve Sensor -Mechanical Error or TPS2/Airflow Plausibility (MS43)
• 120 Plausibility Pedal Sensor or TPS
• 122 Engine Oil Temperature
• 123 Map Cooling Thermostat Control
• 124 Activation DISA Solenoid
• 125 Activation Electric Fan
• 126 Activation Tank Leak Pump Solenoid
• 127 Activation Pump Solenoid
• 128 DME/EWS Communication
• 129 CAN Signal SMG 1
• 130 CAN Signal ASC -Timeout
• 131 CAN Signal Instrument Cluster -Timeout
• 132 CAN Signal Instrument Cluster -Timeout
• 133 CAN Signal ASC -Timeout
• 134 SMG Intervention -Plausibility
• 135 Throttle Valve Re-Adaptation Required
• 136 Throttle Valve -Spring Test and Limp-home Position Failed
• 137 CAN Signal -Steering Angle Sensor
• 139 CAN Signal -Tank Level Sensor
• 140 Tank Leak Pump Solenoid - Reed Switch Open or Output Stage (MS43)
• 141 Tank Leak Pump Solenoid - Reed Switch Stuck Closed or Tank Level Sensor (MS43)
• 142 Tank Leak Pump Solenoid - Reed Switch Stuck Open or DMTL Module (MS43)
• 143 Tank Ventilation or Tank Leakage (MS43)
• 144 Fuel System - Large Leak Recognised
• 145 Fuel System - Small Leak Recognised
• 146 EVAP System Leak Detected (Small Leak) or Pedal Sensor Supply Voltage Pot 1 (MS43)
• 147 Pedal Position Sensor Potentiometer Supply Channel 1 (Pot 2 MS43)
• 149 Air Flow Sensor or Pedal Value Sensor Mismatch
• 150 Lambda Post Cat Bank 1 Max Limit
• 151 Lambda Post Cat Bank 2 Max Limit
• 152 Lambda Post Cat Bank 1 Min Limit
• 153 Lambda Pre Cat Bank 2 Max Limit
• 154 Lambda Pre Cat Bank 2 Min Limit
• 155 Lambda Pre Cat Bank 2 No Signal
• 156 Lambda Pre Cat Bank 1 No Signal
• 157 Lambda Post Cat Bank 1 Min Limit
• 159 Lambda Post Cat Bank 2 Max Limit
• 160 Lambda Post Cat Bank 2 (MS41) or Throttle Valve Stuck
• 161 Throttle Valve - Stuck
• 162 Throttle Valve -Control Deviation
• 168 Pedal Position Sensor Pot Supply 1 or MAP Cooling Thermostat Jammed (MS43)
• 169 Throttle Valve Output Stage Cut off after Fault
• 170 DME Control Unit -Self Test Failed
• 171 Plausibility - Throttle Valve
• 172 Pedal Sensor Potentiometer 1/2 Short Circuit
• 173 TPS Potentiometer 1/2 Short Circuit
• 174 Throttle Valve Potentiometer 1/2 Adaptation
• 175 Pedal Sensor 1 Adaptation
• 176 Pedal Sensor 2 Adaptation
• 186 Voltage Post Cat Bank 1
• 187 Voltage Post Cat Bank 2
• 188 Voltage Pre Cat Bank 1
• 189 Voltage Pre Cat Bank 2
• 190 EVAP -Reed Switch Open or Voltage Post Cat Bank 1 (MS43)
• 191 EVAP -Reed Switch Closed or Voltage Post Cat Bank 2 (MS43)
• 192 EVAP -Reed Switch Open
• 193 EVAP -Check Hoses
• 194 EVAP -Large Leak Detected
• 195 EVAP -Small Leak Detected
• 196 EVAP -Electrical Valve from LDP Pump or Barometric Pressure Sensor (MS43)
• 197 EVAP -Barometric Pressure Sensor
• 198 Cat Efficiency during Start -Bank 1
• 199 Cat Efficiency during Start -Bank 2
• 200 Lambda Regulation Bank 1 Pre Cat
• 201 Lambda Regulation Bank 2 Pre Cat
• 202 Lambda Regulation Bank 1 Post Cat
• 203 Lambda Regulation Bank 2 Post cat
• 204 Idle Control System -Idle speed not plausible
• 208 EWS -RPM Signal Error
• 209 EWS -Message Error
• 210 Ignition Feedback Resistor (ZSR)
• 211 Idle Speed Actuator -Mechanical
• 212 VANOS Bank 1 -Mechanical
• 214 Vehicle Speed Signal (VSS)
• 215 Lambda Sensor Bank 1 or ASC/MSR/EML -Interface not plausible
• 216 Lambda Sensor Bank 2 or EGS Position Signal
• 217 CAN bus error -EGS Signal not present
• 218 CAN module -Warning
• 219 CAN module -CAN Offline
• 220 Lambda Voltage Range Bank 1 Sensor 1
• 221 Lambda Voltage Range Bank 2 Sensor 1
• 222 Low Coolant Temperature or Lambda Sensor Control (MS43)
• 223 Lambda Sensor Switching Bank 1 Sensor 2
• 224 Lambda Sensor Switching Bank 2 Sensor 2
• 225 Cat Efficiency Bank 1
• 226 Cat Efficiency Bank 2
• 227 Mixture Deviation Bank 1
• 228 Mixture Deviation Bank 2
• 229 Lambda Sensor Switching Bank 1
• 230 Lambda Sensor Switching Bank 2
• 231 Lambda Sensor Switching Bank 1 Pre Cat
• 232 Lambda Sensor Switching Bank 2 Pre Cat
• 233 Catalytic Converter Overall Efficiency Bank 1
• 234 Catalytic Converter Overall Efficiency Bank 2
• 235 Lambda Heater Bank 1 Post Cat or Pre Cat Signal (MS43)
• 236 Lambda Heater Bank 2 Post Cat or Pre Cat Signal (MS43)
• 238 Misfire Cyl 1
• 239 Misfire Cyl 2
• 240 Misfire Cyl 3
• 241 Misfire Cyl 4
• 242 Misfire Cyl 5
• 243 Misfire Cyl 6
• 244 Crankshaft Interval Timing
• 245 Aux Air Injection System Bank 1
• 246 Aux Air Injection System Bank 2
• 247 Aux Air Injection System -Incorrect Flow Detected
• 248 Pre Cat Converter Efficiency -Bank 1
• 249 Pre Cat Converter Efficiency -Bank 2
• 250 Tank Venting Valve -Function
• 251 Tank Ventilation Diagnosis Error
• 252 Tank Ventilation System Vacuum
• 253 Activated Charcoal Filter Shut-off Valve Stuck Shut
• 254 Tank Ventilation System -Large Air Leak
• 255 Tank Ventilation System -Valve Stuck Open

If this is a V6 model, Bank 1 would indicate one side of the motor. Also, if it's the V6 model, I believe there are 3 O2 sensors, one per Bank, and one downstream(post-cat). If it is the V6 model, and it indeed is Bank 1, then just replace the O2 sensor, that's the most likely cause.

You will have a total of 4 O2 sensors, two on the left (driver's side-Bank 1) and two on the right (passenger side - Bank 2). Sensor 1 is before cat. convertor and sensor 2 after cat. converter. Hope this helps.

O2 sensors location is broken down like this. The bank refers to which side of the motor the sensor is on. Bank 1 is the same side of the engine as the number one cylinder. Bank 2 is the other side of the motor. Sensor refers to either before the cat or after the cat.

Bank 1 sensor 1 would be the O2 on the side of the motor that the number one cylinder is on and before the cat.

The number one cylinder is on the back side of the motor (the side closest to the firewall). The O2 sensor will be in the exhaust pipe near the exhaust manifold.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Sounds like you may have and O2 sensor problem . or a fuel delivery problem. Check your fuel presure to make sure it is with in specs I think some where around 55 - 60 lbs running at idle . You will need to have the engine control module scaned to see what codes you may have stored .doing this will lead you in the right direction for a repair , if you have O2 sensor codes start with seeing which ones are working or not . I think there are 4 on your truck 2 on each bank .. bank 1 sensor 1 bank 1 sensor 2 bank 2 sensor 1 bank 2 sensor two . bank one is the side with number 1 cylinder in the firing order the bank 1 sensor 1 or bank 2 sensor 1 are the sensor befor the cat converter and the other sensor are after the cat (pre cat and post cat )

bank 1 on a chevy is on the drivers side youll have 4 O2 sensors sensor 1 bank 1 is on the engine side of the cat converter sensor 2 bank 1 is after the cat converter (drivers side )
bank 2 is on the passenger side sensor 1 is pre cat sensor 2 is post cat converter

You need to check two things.

First, there is a heat shield on top of the front of the motor where the o2 sensor you already replaced is. It's got (4) 12mm bolts. Remove it and check the exhaust manifold (big thing you put the o2 sensor into) for cracks. If there are any cracks, you'll get o2 sensor codes. Replace the manifold.

Second, if the manifold is fine start the car and listen to the exhaust pipe under the engine for leaks. Or if you already know the exhaust is loud... that's gonna do it too. A leaking exhaust pipe on an Accent will set secondary o2 sensor codes.

SHOULD you try to change it yourself, you will need a 7/8" wrench, and a new o2 sensor, or flex pipe, or manifold depending on what's broke. Do NOT touch the tip of the new or old o2 sensor to ANYTHING when you take it out or install it or it will be ruined permanently. Run the car until it's at it's normal operating temperature. Raise the vehicle, and take the o2 sensor out of the car (see below to understand which one) using the 7/8" wrench or if you got fancy and bought an 02 sensor tool... use it.

Replace whatever parts are bad (manifold or pipe) or just replace the sensor you need. Then restart the car and take it to a parts store and have them clear the code. The code (and check engine light) won't return right away, it usually takes a few days. If you did everything right you should be good. If not, it's not a terribly expensive job to have done and by now you should be able to explain to them what's wrong based on what you found when you looked at the car. Just tell them everything you checked and what you saw.

Education: I'm not sure where that half baked answer about o2 sensor 1 being near cylinder one came from but here's the real deal.

Sensor 1 is always BEFORE the catalytic converter
Sensor 2 is always AFTER the catalytic converter

Bank 1 and Bank 2 refer to which SIDE of the exhaust on 6 and 8 cylinder engines. Bank 1 is usually left side, Bank 2 is usually right side.

So... O2 sensor B1S1 refers to the O2 sensor BEFORE the converter on the LEFT side of the motor.