Failed emissions test (CO at idle)


my name is Alexander and I am new to this forum and I would appreciate your help. My car failed the emissions test one week ago, and I was told that the fuel was too old (March 2020) and that the car needs some fast kilometres on the Autobahn. So at first I burned the old fuel till minimum, then put E10 and a gasoline cleaner in the car and went on the Autobahn again.

Today the CO was too high at idle (0,97), maximum is 0,30 in germany. At 3000 rpm it was okay. He tested left and right exhaust, also he closed the exhaust with old cloth.

The guy said go to Jaguar and check for codes. I talked to a Jaguar dealer/mechanic and he said that the car is too old and that he can’t help me. :frowning:

Then I called Bosch and they said that they might can help me/read codes. :frowning: :cry:

I have an ELM Blutooth Scanner but I am not sure if this is helping much. When I bought the car it had CO of 0,00. Now, about 2500 kilometres later its all messed up?

There was water under the tank lid but not above the gas-cap. The water drainage was clogged.

I have no check-engine light and the car states 10-12 liter/100 km which is quite okay. It runs smooth, has good power.

Thanks for reading and any help is appreciated. :slight_smile:



Hi Alex,

To me it sounds as if the catalytic converters have reached the end of their life. Because the ECU does not measure things after the cats there would be no error codes.

I had this same (I think the past 1991/1992 Euro1/Euro2 emissions are the same in all EU countries) on another brand of car from 1995 as it had had the cats deleted. CO was way above the allowed, more than twice so at idle, everything else was ok, but that’s enough to fail an MOT / TÜV.

I had a wifi OBD2 reader on our ex X308 (mf. 1997) and a couple of apps on the iPad that could read the generic stuff and the P-codes.

I might be worth checking what new cats would cost (probably a lot) and what options there are.


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It could be that the O2 sensors are failing. Does your code reader give live data? If so look at the live voltage from the sensors. This should be changing all the time. If it isn’t then I would start by changing them. Also look at your long and short term fuel trims, as if they are way above 0 then than that will tell us something.

Oh, and what model of car do you have?

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Hi Pekka,
hi Peter,

thank you very much for your input. :slight_smile: The mechanic also suggested cat or O2 sensor(s) and I googled till early in the morning because I don’t have the money for a brand new cat. :frowning: What I read is that my X300 4.0 1996 only has a OBD1 connection. My ELM Bluetooth is a ELM2. According to that its not possible to get anything out of this or am I wrong? Is it possible to check each O2 on its own while still in the car? Or do they get removed and then tested? I found some O2 sensors in the UK: Jaguar xj6 XJR x300 Lambda Lambdasonde lhe1682aa | eBay. Are these the right ones please? Before buying I would prefer testing my O2’s. Maybe they aren’t that old and were changed by the pre-owner.


Hi Alex,

The lambda values can be measured also with a multimeter. I don’t know the part numbers but they might be generic Bosch sensors, ca. €30-€50,- each.

On my ”other brand” car, an F116 aka 456GT the original cats were available. Cheapest I could find was Eurospares in the UK. €4500,- each! Plus VAT and shipping! I nearly fainted!

After a lot of googling (in German and Italian additionally to English) I found 3rd party high flow cats from BM Cats in England. €950,- the pair including shipping and VAT. What a relief. I wouldn’t have been able to register the car at all without them.

You could also have leaking injectors as your 1996 4.0L X300 only has too high CO at idle, but I think if the lambdas and cats work as they should, that should not be as severe as you now have.

You said ”old fuel” and then you topped up with ”E10”. If I had similar symptoms I would drain the fuel and use fresh 98E5 or V-power. Less ethanol, less combustion issues IMO as it nornally would inject a bit more fuel the more you have ethanol. Our ex XJ8 3.2 MY1998 ran fine with 95E10, never an issue at emissions MOT / TÜV.



Hi Pekka,

I called Bosch and they said that they might be able to help me. Multimeter, so pull them out and test them? Maybe I need to tell Bosch what to do. :wink:

Wow, what a nice car the 456! My dream is the red Testarossa. :heart_eyes:

I also added an industrial strength gasoline cleaner (Liqui Moly 5153) to the E10. E10 has more Oxygen than E5 in it so many people stated it helps with the CO.



Yes, the TR has the same ECU’s as the 456, 2 x Bosch Motronic 2.7. Same lambdas etc although completely different engine.

True about the O2, it’s just that at least in the X308 it would inject a bit more E10 than E5 at idle.

Your cheapest replacable parts are the lambdas but I’m just afraid that if the cats are finished / kaputt, it will not help. In my case the only way to get CO down from ca 0.62-0.78 below 0.30 was to buy and install two new cats (plus seals and new lambdas.) YMMV.


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the TR is my dream since Out Run and Miami Vice. One day…

I got you. Some even advised to put “spiritus” pure Ethanol in the car for the test.But I want to find the fault, not just pass the test. The original lambdas are expensive (for me), because the car needs special ones from what I read. So not 75.- € each but 400.- € OEM from Jaguar. I haven’t found a good source for aftermarket O2 sensors yet.


Hi Alex, I am not sure the O2 sensors are standard ones.
You can get them in the UK for around £45

Your 96 should be ODB2. So plug your reader into it and give it a go, SO much easier then getting a multimeter onto the wiring…….

This is a ODB unit highly recommended by user
[Ship from UK] Autel MaxiAP AP200 Bluetooth OBD2 Code Reader with Full Systems Diagnoses
(If in Germany (auto barn ref) then it will ship from mainland Europe warehouse to avoid Brexit hassle)

If you do swap them, it is VERY important that you don’t swap the wires around, so carefully mark which sensor goes were. You will know if you get it wrong because the short term fuel trim reading will be off the scale. If you find a Jaguar independent that has old IDS software, this has an option to correct a swap but it’s easier (and cheaper) to get it right in the first place.

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Hi Peter!

Wow, this is so cool! :smiley: Thank you so much for you kind help! I will order the Autel AP200 directly at Autel in Germany. Its only 52.- € incl. shipping and thats a steal if I can readout the X300 entirely. I also checked the reviews at Youtube and this thing seems really neat. Only thing thats missing seems the programming (except for the keys).

The O2 sensors seem right. I also googled and most people stated that I the car needs titanium dioxide sensors. Not sure if these are those and I don’t know the OEM part number. Is this the OEM number, please? Are the O2 sensors heated? Maybe my fuse is just blown and they remain cold?

Yes, I read about the swapping issue. I will put some paint on them (front/back).

btw, I love your car. The colour is so beautiful. A dark cherry :slight_smile: :heart_eyes:

Thanks again, you saved me! :heart_eyes:

Kind regards


Thank you, I love my car too!
The part number for the sensors is LHE1682AA and are the ones I linked to at £45. They are available from Jaguar but cost £450 so I know which ones I would use lol

In fact the last time I was at a Jaguar dealer, picking up parts, they received a package from britishparts and told me that if they don’t have stock they will get them from there!

The sensors are heated, but I don’t think there is a fuse involved as it all goes through the engine control module. If they are the original ones then they are 25 years old and will be slowly failing………

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Hey Peter,

mine is british racing green with the oatmeal interior. thanks again for providing valuable information. :smiley: haha, so they sell the £45 part for £450? nice business plan. :smiley:

I hope that the Autel OBD will give some hints whats wrong. Right now I only have the ELM327 and the Car Scanner Android App.

Have a nice day, Peter!



I received the Autel AP200 and checked for codes and got the famous P1621 and some other stuff which I erased. I started the engine and checked again for codes after some minutes. No codes. The problem is that the Autel Scanner couldn’t read the O2-sensors. :frowning:

I made some pdf-files about the scans. Please have a look at them and help me. I have no clue what to do next. :frowning:

Any help is very much appreciated!


Alex2021-09-19_17-33-35.pdf (41,1 KB)
2021-09-19_18-56-06.pdf (40,1 KB)

The scanner is working properly as the modules don’t have O2 info, that comes from the live date you are reading!

There are 3 values you should be monitoring for the 2 sensors

These are
long term trim
Short term trim

The voltage should be constantly changing, this is telling the ECU what’s happening now in the exhaust, if it’s too rich, too lean or just right.

The long term trim is telling YOU what HAS happened. In an idle world this should be 0. If it plus of minus 2 or 3 then that’s not too bad, but if it’s a lot more than that then you will have a leak somewhere.

The short term trim is telling YOU what IS happening at that moment.

There is a better explanation here Understanding Short Term and Long Term Fuel Trims

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Hi Peter,

I see. :slight_smile: If I understand it correctly my short and long term trims seem to be allright; 0% and 4,7% and well under the stated normal range of 5-8%. Maybe the O2-connectors are mixed up? The car got a “mini-kat” 2019 after the passed emissions test which was CO: “0.00” and maybe they swapped them accidently. The idle is a bit lumpy but I am not sure if my trim readouts would be that normal if they swapped them.

Thanks! :slight_smile:

Yes and no lol
The short term should be changing as you drive……it doesn’t seem to be

You need to check the voltage to make sure it is changing.
If it isn’t then that may be why the short term is steady, and means you should change the sensors……

hehe, I wasn’t driving, i tested in idle :smiley: I’ll drive and check again. Maybe the ECU needs to adapt to the E10 super? Maybe resetting via battery cutoff would help? I’m afraid that I break the sensors off if they are from 1996

The ECU timing advance is set to use quite poor fuel quality so will not be affected by the E10. This is why the ‘Andy bracket’ can advance the timing by 5 degrees with out Ill effect.

The sensors are not reversed, if they were then your readings would be all over the place.

This is a screenshot of what you should see by monitoring the short term trim and voltage of both banks. This is using a different ODB setup but yours will be similar :slight_smile:


You came here for help. We are all trying to tell you what is wrong and you seem to want to debate the issue.

ECU doesn’t adapt to E10 super, or any fuel in any way that would cause an issue.

Resetting is not feasible, nor helpful. Resetting would simply reset the data, not change the problem.

Running the tests at idle or while driving makes no difference to the way your oxygen sensors respond. I don’t know why you want to resist this.

Change the oxygen sensors and hope that’s the solution. If not, you’ll be shopping for a new ECU.

You can continue to try to change the inevitable, but until you replace the oxygen sensors, we cannot help you any further. If you’re afraid to replace it yourself, take it to a shop. There is nothing unusual about these sensors, so no need for any kind of specialist. (The sensors themselves are indeed atypical, but replacing them is no different than any other sensor)

Hi Peter,

ah I see. :smiley: Whats the name of the OBD program you used, please? I stumbled over another possible culprit for my problem: the temperature sensor on top of the thermostat housing (part # LHE1600AA). when its faulty the car stays in open loop and then the o2 sensors are not showing any sign of life. what do you think about that? the gauge in the instrument cluster rises normal (I know there is another sensor for the gauge) and my scan shows a OBD coolant temp from 2 to 72 degrees. This should be okay?! Maybe I should order this part first anyway? Can I just change it or is the coolant water spraying out while changing? :smiley:

Hi Veekay,

I am sorry that you see it that way.

I don’t see myself debating, more like that I want to understand my car completely and not just throwing parts at it. I read about some guys who had the same problem like me and they changed the O2’s, even the cats for instance and nothing happened to solve the initial problem. So I guess a discussion is wise, before buying parts. Also I have good reasons to minimise the contact to others like the workshop or working at my car which I don’t want to discuss here.