O2 sensor the mechanic said the front and rear o2 sensor is bad, what is the location and what they look like???????

The 1st sensor is located closest to the engine and the last is located at the rear of the exhaust system. Generally, if we are talking about O2 sensors: Sensor 1 = Before Catalytic converter Front (Upstream O2 sensor) Sensor 2 = After Catalytic Converter Rear (Downstream O2 sensor)
https://mechanicbase.com/troubleshooting/bank1-vs-bank2/

some have 4 sensors
what engine.
what country and if in calif , that too, is a country. By EPA regs.
the H25 engine is here.
got a 2.5L? v6

there are 4 sensor, 2 front , 2 rear of cat.
or can be 3, if the rear merge to 1 cat. but not this one.

the front sensors are hard to change
and the connectors , lock up backwards from all other connectors
on the car.
H25
http://www.fixkick.com/sensors/O2%20sensor/25L/25L-O2-H3LL.html


if you post country and engine,
anwers get better 100x.
we cant guess that.

this page covers front no rear, rear is child easy to fix.
rears only change if you get P0420s. (lazy cat errors)
the fronts control fuel mix. hot, closed loop.

o2 sensors are at the rear of the cat converter-- HO2s sensors are at the front of the cat converter . Use the special tool to remove /replace them as if you don't then the wires will be damaged.

the p0421 code on this vehicle is either caused by a bad catalytic converter or an oxygen sensor. your mechanic has a scan tool that he can verify if the converter is faulty by comparing the rear o2 sensor to the front sensor. If the rear o2 sensor's voltage is switching from rich to lean (01-1.0v) similar to the front's, then, it's a bad converter. The rear o2 sensor should keep a steady voltage reading. Have him do this test while at cruising speeds. good luck

There are four sensors on the Continental cat converter. Because it is front wheel drive, the engine is rotated 90 degrees compared to a rear wheel drive. Next up is location. Front O2 sensor should be near the radiator.It will have a green connector where it is plugged into wire harness. It is called left sensor on the frt wheel drive cars. The right hand O2 sensor is downstream on the inlet pipe right above number 2 cat, the one that looks like a catchers mit, ahead of the flex pipe.Sensor 1 reads air/fuel mixture and is commonly called O2 sensor 1. remember green connector. Sensor 2 monitors the function of your catalytic converter and have a blue connector plug. Front wheel drive cars are a bit tougher because everything is 90 degrees off of diagnosis, front,rear, number 1,number 2. BTW, the sensors with blue connectors are called CMS sensors for catalytic monitor system and rarely go bad. Auto Zone scans for free and I would have it scanned before popping 40 bucks for new.

Hello: Code 420 normally indicates a catalytic converter is operating below 60% efficiency. You need to take your car to another shop and request a COMPLETE diagnosis not a dart board guess.

On almost every car, there is an O2 sensor located in the front, on the exhaust pipe, or manifold, and there is an O2 sensor located after the Catalytic converter. The front O2 sensor is the most important because it monitors the air/fuel ratio. The rear O2 sensor checks your Cat converter for proper operation and filtering.

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

These are both air fuel ratio sensors, These sensors are located in the exhaust manifolds, the front one (bank 2) is easy to get to, the one in the rear is more difficult, they require a special socket to remove and install. The O2 sensors are located behind the catalytic converter and are not the same as the A/F ratio sensors.Replacing the sensors is not what the codes are telling you.You could have a plugged air filter giving you diminished air intake. That's not a bad sensor. A mass air flow sensor could be reading bad and giving you a bad mix of fuel so that your O2 sensor reads a problem. Get it? Or the O2 sensors could be bad after all, probably not both though.

your vehicle is equiped with one 02 sensor only and has disc brakes front and rear

The Crankshaft Position Sensor is located: Under hood, driver side, front engine area, mounted rear of crankshaft pulley
The Knock Sensor is located: Under hood, center, front engine area, below intake manifold, mounted on block