3l, 6 cyl (California emissions). Where is the 4th O2 or (A/F) sensor?

Hello again :>D

Question edited to add maker and complete the year.
Question moved to model category.

If you have California emissions, then you will have 3 sensors. If you are getting a P0420 code, you can try changing the upstream sensor first. If you still get a P0420 code, then you will have to replace the manifold/catalytic convertor.

https://www.google.com/search?q=2008+Ford+Fusion+2.3.+Mine+has+3+x+o2+sensors

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Sounds like the 3rd and last sensor on your exhaust is bad, or wiring to it is bad.

bank one is left side top of engine standing looking at it , O2 sensor is located on exhaust manifold there is one on each manifold (right-left)

You did not say what your car was ,however most newer cars use 2 oxygen sensors per bank, if it is a 4 cyl replace downstream o2 sensor,if 6 or 8 cyl bank 1 sensor 2 is downstream sensor on bank that has cyl 1 on it.upstream sensor is in front of catalytic converter downstream is after the cat

Check Engine Light Codes and Meanings
Long blinks Short blinks Problem
1 1 Camshaft position sensor
1 2 Mass air flow sensor
1 3 Engine coolant temperature sensor
1 4 Speed sensor
1 11 EVAP control system (flow)
1 14 Fuel injection system right bank (too rich)
1 15 Fuel injection system right bank (too lean)
2 1 Ignition circuit
2 3 Closed throttle position switch
2 5 Idle speed control
2 9 Fuel Injection system left bank (too rich)
2 10 Fuel Injection system left bank (too lean)
3 1 ECU
3 2 Exhaust gas recirculation system flow problem
3 3 Front left O2 sensor
3 4 Knock sensor
3 5 Exhaust gas recirculation temperature sensor
3 6 EGRC-BPT valve
3 7 Closed loop control
3 8 Closed loop control
4 1 Air intake temperature sensor
4 2 Fuel tank temperature sensor
4 3 Throttle position sensor
4 7 Crankshaft position sensor
5 3 Front right O2 sensor
5 4 Automatic transmission control unit
6 3 Cylinder #6 misfire
6 4 Cylinder #5 misfire
6 5 Cylinder #4 misfire
6 6 Cylinder #3 misfire
6 7 Cylinder #2 misfire
6 8 Cylinder #1 misfire
7 1 Multiple cylinder misfire
7 2 Three way catalytic converter (right side)
7 3 Three way catalytic converter (left side)
7 4 Evaporative emission control system pressure sensor
7 5 Evaporative emission control system
7 7 Rear O2 sensor
8 1 Vacuum cut valve bypass valve
8 2 Crank position sensor
8 3 Absolute pressure sensor
8 4 Automatic transmission control unit
8 6 Evaporative emission canister purge control solenoid valve
8 7 Evaporative emission Canister purge control solenoid valve circuit
9 1 Front Right O2 sensor
9 3 Evaporative emission Canister vent control valve
9 5 Crankshaft position sensor
9 8 Engine coolant temperature sensor
10 1 Front left O2 sensor
10 3 Park and Neutral position switch
10 5 EGRC solenoid valve
10 8 Evaporative emission canister purge volume control valve
11 1 Inhibitor switch
11 2 Revolution sensor
11 3 Improper shifting to first gear (Automatic transmission)
11 4 Improper shifting to second gear (Automatic transmission)
11 5 Improper shifting to third gear (Automatic transmission)
11 6 Improper shifting to fourth gear (Automatic transmission)
11 7 Solenoid valve lock up function
12 1 Shift solenoid B (Automatic transmission)
12 3 Overrun clutch solenoid valve (Automatic transmission)
12 4 Torque converter clutch solenoid valve (Automatic transmission)
12 5 Line pressure solenoid valve (Automatic transmission)
12 6 Throttle position sensor (Automatic transmission)
12 7 Engine speed signal
12 8 Fluid temperature sensor
13 2 MAP/BARO Switch Solenoid Valve
13 5 Fuel pump control module
13 8 Cooling fan

There are two type of emission classes.

Federal and California emissions.

Look under the hood for the emission class sticker.
Smog check station's machine in CA are programmed to handle both types.

You do not need to change anything before the inspection.

==== Side notes: You should check the service record for the last Catalytic converter,O2 sensors and EGR valve change.
Service interval for
Catalytic ~180,000 miles O2 ~150,000 miles EGR ~160,000 miles


If you can locate the last smog test (even from out of state) Check for the
CO NOx

HiI!!
6 cyl. and 8 cyl. engines are divided into 2 Banks, standing in front of the Engine, the Bank 1 is the side where the #1 spark plug is (on V6 Ford Engines is on the PASSENGER SIDE). Bank 2 is on the DRIVERS SIDE.
Check the following diagram for HO2 sensor locations (This diagram applies to Ranger, Explorer and Mountaineer models) :



2 would be Bank 1 and 2A would be Bank 2.
Good Luck!! A HELPFUL - 4 THUMBS - rating for this solution would be greatly appreciated. Thank you for using FixYa.

Bank 1 usaually is the front if this is a 4 cyl, if a 6 cyl, it would be bank 1 which is cyl 1 side, sensor 1 is befor cat converter, sensor 2 is after cat

Probably not. The California car is all together different. Usre the Cal, O2 Sensor.

Well, if you have the 6 cylinder, there are 3 02 sensors in that car.
One on front manifold, one on back manifold, and one in the center pipe.You changed one, good idea to change the other 2. That is if you have the 6 cyl.