I have a 1996 XJS with the 4.0 AJ16 on 120k miles.
From what I can tell, it's not entering closed loop mode, which is causing raised CO, HC and CO2 levels, which has caused the failure of one of the CATs in the Y pipe of the exhaust (recent exhaust gas test showed good emissions from one tail pipe, and bad from the other).
The lambda readings are both around 1.270v - I understand they should switch high/low, and read upto 3.5 volts?
From the data, coolant temperature is around 90C and air temp (today with ambient around 21C) was around 38C
There are no stored fault codes.
Work so far:

• Replaced both O2 sensors
• Replaced exhaust Y piece with CATs
• Replaced air temp sensor (was seeing upto 60C air temp previously - although ambient air temp was higher).

No improvement so far. Data from other sensors looks as expected.
Any suggestions what to look at next?

 

There is one fuse X that heat the sensors up to the proper temp to read correctly to get your regulation correct

Right engine bay fuse box fuse X

10 , 12 , 14 , or 16 .........................looking

 

See page 60
jagxj1996.pdf (jagrepair.com)

For your XJS that fuse would be Right fuse panel fuse # 20 and Right fuse panel fuse # 7

See page 64 for your XJS

jvs1995en.pdf (jagrepair.com)

EMS power relay must click closed ( swapping options and fuse # 20 and # 7 , # 7 to control the relay ? )

There is an external diode involved on the relay control circuit ....................

Fuse # 7 goes through the inertia switch for some reason , you can jumper between the 2 white wires on the inertia switch connector on your XJS per XJS wiring guide

 

Thanks, however the XJS doesn't have a right side engine compartment fusebox, only a left one and no fuse for the O2 sensors (according to the electrical guide I have, anyway).
Regardless, I have checked every fuse in the engine bay, all are good.
I can open a connector and check if the heater voltage is present.

 

OK, some confusion here because my car is a UK right-hand drive model - and all positions are reversed.
Therefore the relays and fuse box contents that the electrical guide shows on the right side of the car (page 10) are on the left side.
The fuses there are all good, and I can feel the ECM relay operate. If I remove it, the engine cranks but doesn't start, so the relay must be working,

 

I can open a connector and check if the heater voltage is present.

Yes , researching

On the car side of the O2 sensor connectors it will be the White / Brown wire

EMS " power " relay clicking I see you have 2 EMS relays

 

From the diagram, there's a EMS Power Relay (white socket), and an ECM controlled relay (blue socket).
I removed the power relay - and it wouldn't start.

 

The " power " relay as the White / Brown wire going on down also powers the coils as power " sitting " on them , 6 timed ground path wires from the ECM produce the spark

 

I've done some quick testing.
I measure battery voltage on one of the pins of the O2 sensor connector. I also connected one of the old sensors and could feel it heating.
So we can eliminate O2 sensor heating as a potential cause.

Reading up on events that can trigger the check engine light (in the Engine management SELF-STUDY TRAINING COURSE 801S document, page 28), one of them is: Engine control module fails to enter closed-loop operation within a specified time period.
The check-engine light is working - it comes on before cranking, but doesn't again, and there are no stored codes. So the ECU doesn't consider there to be anything wrong.
Is a faulty ECU a possibility?

 

You can read resistance through the sensor pigtail at pin 3 and 4 for the heating element in the installed sensors

Connectors clean ?

Then we get back around to the ECU connectors

O2 sensor return signal wires A and B

Sensor return wires shield ground terminals clean to protect that signal

The pin / sockets positions will be the same between the X300 and XJS just different wire colors ( both O2 sensor and ECU connectors )

The sockets can be missing 1 of 2 pinching tabs for best connection and corrosion on the wire as it enters the socket wire crimp

You can have fresh sockets in the unused socket positions as there are many






You should only have downstream sensors ( S2 ) for England

ECU connector ( red ) for return signal :

Red 18 aft bank cylinders

Red 16 fwd bank cylinders

Black 4 both O2 sensor heater grounds

And we get to ECU grounds including heavy external ground wire attached to ECU case ( sometimes missing )

ECU pins / wire ground terminals ( about 2 or 4 )



working

O2 sensor connector for heater :

pin 3 White / Pink wire on the car side

To pin 4 White / Brown wire on the car side

The ECM power relay can read battery volts but not carry the current to properly run the devices ( burnt contacts inside the relay , relay swapping option ? )

You can feel some heat but is it enough as exhaust gasses run past the sensor

The ECU is a first-generation design working out the real-world bugs as they came up with codes so best first try from engineering