Hi,
I have read all the MOT/emission failure threads I can find and cannot seem to find the information/circumstances I need.
1999 XJ8 3.2 (AJ27, 4 oxygen sensor model); 154,000 miles, original O2 sensors, new plugs last autumn (NGK upgrade pregapped).
I gave the car a good run before the test but the tester aborted the test after the lambda reading was so high (1.14). He said he suspected an exhaust leak but revved the engine higher, to compensate, and the value only got worse.
The car has been down on power recently, when cold, and also hesitates (almost dies) when accelerating from idle even when warm.
No warning lights but I do get an intermittent p1646 (the yellow light v occasionally flickers; that I can only see using my OBD reader).
I went for a run this morning with the OBD reader monitoring engine values:
Both downstream O2 sensors showed varying values based on acceleration; both upstreams were solid on 0.35mA irrespective of acceleration. Both Long Term and Short Term fuel trim were solid on -0%.
Other sensors were changing as expected; including the MAF.
I am tempted to replace the upstream O2 sensors but that is an expesive job (if it does not solve the problem).
Any help appreciated.
Aidan

 

I don't know if this will help. Mine is the AJ26 4.0 normally aspirated engine. I had the same issue with a high lambda reading at the MOT, both O2 sensors were changed, problem solved.

For about 4 months before the MOT I had been grappling with a sudden increase in fuel consumption from the long term 23.5 down to less than 19, I had changed the air filter and plugs, cleaned the MAF and throttle body, but it was all down to the O2 sensors. I'm now getting 27+ on a motorway run.

 

In the absence of any other codes or an obvious leak, it's all pointing to the HO2S:



(click on the image to enlarge it)

Harness faults are possible but unusual unless the sensors have been disturbed.

Graham

 

Thanks for your thoughts NigelMay and Graham.
I feel better about getting new O2 sensors now.
Graham, all 4 O2 sensors were disconnected/reconnected several times last year when I had to drop the exhaust system to replace the gearbox (twice because the torque converter was not mated up correctly the first time).
Cheers, Aidan

 

Aidan,

Are you absolutely certain all the sensor connections were put back in the original positions? The reason I ask is I recall reading the sensors are fitted and then setup to the ECM using the dealer diagnostic system.

There is a warning in JTIS about sensor removal:

Under no circumstances must the sensor wiring be cut to facilitate removal, should this prove necessary.
NOTE: Early production vehicles have wire color codes that are different from that shown. Use connector pin numbers for wire identification.

Other members may be able to verify this and add further details.

Graham

 

Hi Graham,
All 4 sensors were simply unplugged at the start of the job and plugged back in at the end.
(Repeated when the replacement gearbox had to be dropped out and put back again).
No ECM reset was undertaken. Did we screw up?
Could this lead to constant zero (ok, minus zero) long and short term fuel trim on both banks?
The constant 0.35mA too??
Aidan

 

What he means is, are you SURE you have the sensors plugged in corrctly, left to right?

And Graham, it was the X-300 that had the infamous "sensor orintation" procedure. AFAIK, the X-308 has assigned channels.

 

Update: I have ordered two Bosch 15627 from AutoZone | Auto Parts & Accessories | Repair Guides & More in the US.
After discounts, total cost including delivery to the US was £90 (I was already
booked for a long weekend in NY).
I just hope the problem is not the ECU.
I did check the connections and they looked ok. I probably should have disconnected one and run the car to get a new error code.
Cheers, Aidan

 

Mine failed its MOT recently on emissions. Readings showed both upstream lambda sensors, which were replaced and now the car drives like it has a turbo charger.

For months I had been thinking the lack of performance, until it was put into sport mode, was due to it being a gentleman's carriage built for comfort. I also noticed some hesitation and juddering on acceleration. I have only owned the car 1 year and did not know what to expect and suspect the sensors were duff when I got it and the previous MOT was fiddled. My stainless steel exhaust was always dirty, now I barely have to clean it week after week.

Not a cheap job, but worth every penny. My Jaguar trained, personal friend and mechanic did say be careful with cheap sensors not "designed" for Jags, as they can be a false economy.

From my understanding of them, they contain precious metals that deteriorate over time as they do their work so eventually will need replacing. I think 154k and 14 years is probably pretty good going.

 

Looking forward to a follow up.
LTFT solid at 0 does not sound right. Is it even running CL (closed loop)? Sounds like it isn't and in that case the trims are likely junk.

 

Thanks migrosmarket, JagV8, all,
No more progress on the Jag as I have been away for a week.
The car is in for a reconditioned suspension arm today (my specialist did not want to do a lower ball joint on the car).
Once I get it back I will fit two Bosch Wideband A/F Sensors (model: 15627). An exact fit for the car, if not OEM.
JagV8, when you say 'trims are likely junk' do you mean the readings are invalid or that some other component will need to be replaced? I understand the concept of close loop in this case, but would not have the slightest idea how to verify it, other than see a relationship between OBD2 readings.
Oh, I had my wife get an independant emissions test while I was away. Only the Lambda failed at 1.09 and 1.11 in two tests.
Thanks all, hopefully I will have better news next week.
Aidan

 

Hi All,
Just a note to say that both upstream O2 sensors have now been replaced (not an easy
job, although no external heating required).
My OBD2 now shows varying sensor current, and varying short and long term fuel trim on
both banks. The car is also much more responsive when driving.
The sensors, although branded Bosch are identical to the (Denso??) ones that were removed.
Same pattern of holes in the sensor and identical shaft outline.
Hopefully it will pass the MOT retest.
Thanks for all advice.
Aidan

 

Great news. MOT passed today with no related notes. Lambda reading of 1.0
Looking forward to better performance and (hopefully) economy with my new O2 sensors.
Aidan