Have a 2001 Jaguar XJR 4.0 supercharged and stuck in limp mode computer won't read O2 sensors we think it has aftermarket O2 sensors that are not wired correctly can anyone offer any assistance

 

You can test your sensors. Give the wires a visual inspection to ensure there is no damage and that no friction points have caused shorts. If there is, you may want to replace the sensor.

If everything looks good, you want to start the car and Allow it to run until it reaches at least 200 degrees Fahrenheit. This step ensures that the reading will be accurate. Turn off the engine temporarily while you set up the connectors. Then take your voltmeter and back probe and measure the oxygen sensor at a few different points to see if the measurements are faulty. You can connect the back probe to the signal wire and connect the voltmeter’s positive lead to the back probe. Next, you will connect the voltmeter’s negative lead to a ground on the chassis. Set the voltmeter to the 1-volt scale. Start the vehicle and check the voltmeter’s reading. It should change rather quickly. If it is only reading 0.5 volts, the vehicle might not be warmed enough. If it isn’t changing after the engine is warmed, there is something wrong with the sensor.

 

Upstream O2 sensors are 4 wire wideband sensors. Downstream are typical 4 wire narrowband sensors. 4 wire wideband sensors can not been measured the same way than the narrowband. Same goes with the obd testers. Tester will only show you the fuel mixture from the upstream sensors. From the downstream sensors tester will give you the signal voltage.

If you know what you are doing, you can backprobe upstream sensor signali with a oscilloscope.