1996 MY XJS, AJ16 running lean, B1S1 O2 flatlines

I have a stubborn “LEAN MIS-FIRE” problem with my 1996 MY 4.0L AJ16.

Even after replacing too many parts:
All O2 sensors and harness connectors (O2 sensor orientation test completed with IDS/WDS)
Throttle Position Switch (TPS reset using IDS/WDS)
MAF (I have 2 new aftermarket and 2 used)
coils, and harness connectors with shrink wrap over all failing insulationPlugs (Champion RC12YC)
Fuel filter, and fuel pressure regulator (pressure is good), Air filter
Rebuilt fuel injectors (that looked different, so I rebuilt the original injectors, flushed them and reinstalled the originals)
Exhaust manifolds (they were cracked), and front catalytic converter, EGR valve
(I’m sure there were more)

I still have not been able to fix this, and could really use some expert help.

Symptom 1: repeated MIL codes
P0171, ADAPTIVE FUEL, Combustion too LEAN, Bank 1
P1138, HO2 sense circuit lack of “swing”, Bank 1, Sensor 2 Sense circuit indicates lean combustion (No HO2S response)
P1315, Persistent misfire (one cylinder identified and injector switched off)
P1314, Misfire Rate catalyst damage, Bank 2
P0300, Random misfire detected

Symptom 2: Champion Spark plugs look VERY LEAN (bright white)

Symptom 3: Low voltage on O2 sensor “C” (B1S1)
Here are the O2 sensor voltage traces (using IDS 118.5/WDS with a genuine VCM).
Sensor A= Bank 1, Sensor 2 (downstream)
Sensor B= Bank 2, Sensor 2 (downstream)
Sensor C= Bank 1, Sensor 1 (upstream)
Sensor D= Bank 2, Sensor 1 (upstream)

I have changed all 4 O2 sensors, and the Bank 1 Sensor 1 O2 sensor (sensor “C”) twice, and still get the same results.

I have checked ALL the O2 sensor wiring and it is OK.

When I turn the engine OFF, key ON, I can watch the voltage rise back to 5v.

I believe this O2 sensor problem is causing my AMFR and FMFR to drift out of spec.

I found a way to “hack” reset the AMFR using IDS/WDS: Vehicle Configuration - Set-up and configuration - Engine and transmission - Fuel trim set up. I don’t actually have a smog machine, but if I lie and say the “measured” carbon monoxide level is 05.% or 1% or 1.5%, WDS updates the ECU at least a little (engine runs rough during change)…keep repeating the process until you get the PASSED screen and the level is right. I was able to change my AMFR from -12.5kg/hr all the way to 531 g/hr. Spec according to Bulletin Number 18-53 and 05-1-31 is AMFR -4.2 to +2.8 kg/hr. The problem is, even after I get the AMFR adjusted to spec, it drifts right out again.

Is the O2 sensor telling the truth? Is that really is the O2 level? what is going on here?

Symptom 4

I also wanted to re-set the FMFR per Bulletin Number 18-53 and 05-1-31 (apparently, fixing the initial AMFR problem WONT be enough; you must also re-establish the proper AMFR and FMFR for the ECU…it can’t do it without your invention)

After setting up the 4 traces, per the bulletins, the signal for the Calculated engine Load Value “CLV” reading keeps dropping out (error messages that I can’t look up). The transmission had started shifting very badly, so I replaced the VSS. Shifts are better, but the CLV signal is still dropping out. When I check ONLY the CLV, it seems fine.

My smog is past due by 1 1/2 years, and the great state of California could tow my car away to ‘save the world’ if they catch me driving it (so I don’t drive it).

Have you replaced the inline fuel filter? AJ16’s are known to overheat
from weak mixture if the fuel filter is blocked. It can cause the
exhaust manifolds to crack.


Yes I have. Contrary to the service manual, it’s not near the rest axle, it’s behind a cover in the left front wheel well.

My God. From your post it seems I should pardon all V12 owners - no more reliability piss out of 5.3.

Oh, by the way, AJ16 suffered from timing chain stretch, which led to symptoms like those in the subject of your topic.

Before taking the mortgage for the spare parts set in your Jag - have you checked the timing?

Deleted initial post because I didn’t read through this properly

I haven’t looked at the timing chain; maybe later.

I only have a problem with bank 1.

I switched the upstream O2 sensors, and the problem stayed on bank 1.

I switched the O2 wiring to see if the problem would switch to bank 2, but it stayed on bank 1.

Then I switched the coils and plugs from bank 1 to 2 (1 to 4, 2 to 5, 3 to 6), and the problem stayed on bank 1.

What do you think about the knock sensor for bank 1? WHY IN THE WORLD are the knock sensors not included in the PIDs or in the MIL light codes?

Hello Scott - just asking for some clarification of nomenclature - I think you stated that you have the 1996 4.0L (six cylinder) engine - is this an in-line straight block with dual overhead cams? I have been under the impression that the “bank” terms generally referred to the opposing sides of an engine block, such as used in a V6, V8, V10 or V12 - with the first three cylinders (1,2,3), of an in-line six, using the exhaust manifold routing to a designated cat (may have upstream and downstream O2 sensors), muffler and piping, and the next three cylinders (4,5,6) using a different exhaust manifold routing to it’s designated cat (also may have upstream and downstream O2 sensors), mufflers and piping, does this mean that the first three cylinders are being called “bank 1” and the next three cylinders being called “bank 2” - this seems to be what I am reading from your posts - this would seem an oddity to me, but I noticed that you stated a reading for a problem with the “bank 2” O2 sensors - this is where I am looking for some clarification - why would you have any “bank 2” conditions - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 10/17/2020 2255hrs. EDT USA.

Jaguar divides the L6 into two banks, cylinders 1-3 are bank one.

It could be a convention that is true for all engines; for example, cylinder #1 is ALWAYS the cylinder farthest away from the flywheel.

Hello Scott - okay on the “bank 1” and “bank 2” clarification (thought this might be what you were stating but not sure) - did not know that Jaguar used this designation for separation on the 4.0L - when did the cylinder number sequence change cylinder number 1 as being designated as the farthest away from the flywheel, which is opposite to how the older engines were numbered, such as in the E-Type and XK - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 10/17/2020 2336hrs. EDT USA.

Just to chime in, texterry is correct. The first three cylinders are considered Bank 1, and have their own exhaust manifold, oxygen sensors and even catalytic converter! The latter three are Bank 2.

sromkee, this was very important information, regarding how to switched coils and oxygen sensors around. This will cutt out a lot of the BS theories.

This may be an ECU failure. I had this happen to me once. I was lucky enough to score a substitute ECU and that resolved my issue.

Have you checked the pins on the ECU. They tend to get corroded over time. That would be the best of solutions.

I would also download the electrical diagrams from jagrepair.com and do an ohm test for not only the oxygen sensor wiring, including grounds, but also from the ECU to the oxygen sensor harness connections at the firewall.

If all of that checks well, I’d really start looking at the ECU.

I’m happy to see you figured out how to set the AMFR and FMFR, the PDU does not have that option. I always wondered how to set it with my WDS. I even bought a product called “Gunsons Gastester” hoping it would help, but I ended up scoring a PDU instead, so I abandoned that idea and re-sold the gastester on ebay.

One thing not mentioned anywhere, maybe worth trying (apart from timing chain). If you are unsure if the ECU values are correct (not confirmed with exhaust gas analyzer) - replace capacitor on your alternator… Usually £5.
Also - you shouldn’t have instant errors after clearing those, usually it takes 30-40 cycles to log one. Instant error log sounds like bad ECU.
Another thing is that most of the measured ECU values are read/calculated from ‘virtual’ TDC in sequence - hence suggestion of the timing chain…

After checking, repairing and rechecking the wire harness, every connection, every plug and every sensor and the ECU, I have determined that all of those are working now, but I still have unburned O2 going into bank 1. It is most apparent at idle; if I raise the idle enough (foot on gas), or introduce extra air (vacuum leak) the upstream O2 sensor chart (Sensor 1 Bank 1) shows the normal pattern but then the downstream O2 sensor pattern goes flat showing unburned O2.

What’s the probability that this is a slight coolant leak? My coolant consumption is very very low. I use 25% coolant and a bottle of HyperKuhl PH Balance.

my compression is good at 145-152 PSI across the six cylinders

You have an issue with one bank and not the other, so the intake side, or everything from the throttle body out to the front of the car is fine.

Have you replaced the intake manifold gasket?

You have done everything else there is to check. If it isn’t the intake manifold gasket, then it will be the ECU, or a wire that isn’t making good contact between the ECU and the coils, sensors, injectors, etc.

When you have the manifolds off, you might be able to see something interesting.

By the way, if you’re going to replace the intake manifold, you should absolutely replace the bastard hose. Part NBC2273AB