4 Oxygen sensors ?

Hi, The post cat sensors are typically used by the ECM to monitor catalyst efficiency. The front sensors, judging by the voltage reading, are wide band sensors. Sometimes referred to as air/fuel sensors. The voltage reading is a little different than from o2 sensors. O2 sensors will often oscillate between 200 millivolts and 800 millivolts. Hi voltage is richer, low is leaner. Wide band sensors will tend to be somewhat steady at say 3.3 volts, 3.2 volts if rich, 3.4 volts if lean. The actual voltage reading varies by car manufacturers, but you get the idea. If you monitor the voltage as the engine warms up from cold start you’ll see how they respond.

When the fuel trims are at -10%-20% it means that the ECU is having to lean out the fuel mixture to compensate for a rich condition. The ECU uses the exhaust sensor to verify the correction.

I’ve read your previous posts on this problem. The reason for this site is to learn not to replace parts without cause but to learn to use deductive reasoning and our test equipment to establish why and where something has gone wrong. It has taken me some time to resist the urge to replace parts, but the satisfaction has been that when I do, the problem is resolved.

In order to fully understand your car’s problem/s it would be best to start with the exact codes you are getting and the STFT and LTFT for both banks of the engine.

From what I have read, your car is running rich for some reason. The STFT and LFTF from both banks will establish a basis for figuring out where things have gone wrong. As you have found out, replacing O2 sensors does not correct the problem.

Your car has four O2 sensors as most do. Two are located in the exhaust head pipe one each on either side of the engine and two down stream. One ahead of the cat and one after the cat. The two at the head exhaust pipe are the ones that tell the computer how to manage the fuel injectors to supply fuel to run your car. The computer receives data from other sensors to know how to control fuel.

Import is correct. The 2001 Jag uses Air Fuel Sensors (AF). They work differently from the narrow band 02 sensors.

You’re not going to get much data from using a DVOM as the AF sensors measure in mV changes(current). You will have a stoichiometric reading in voltage for your car. You need to have a scan tool which will give you an equivalent ratio reading. That will tell you what voltage reading is stoich. 14:7:1 for your car (each is different for makes and models). A reading of less than :1 is rich and greater than :1 is lean. That’s just the opposite of a 02 sensor.

Both upstream AF should be reading a steady voltage. You say you’re getting a rich condition code. That is the reason for the -trims. The computer is taking away fuel to compensate. Without knowing the stoich number I would have to reason that the 3.4v reading is below stoich.

Your engine is getting to much fuel by a bad pressure regulator or a leaking injector, bad EGR or EVAP problem. These are a few things that can cause too much fuel. Find the fuel delivery problem and I bet the trims, AF and CAT problems will go away.