How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask The Audi Doctor Your Own Question
The Audi Doctor
The Audi Doctor, Technician
Category: Car
Satisfied Customers: 2428
Experience:  Over 30 years experience, 17 years with Audi, Factory trained
Type Your Car Question Here...
The Audi Doctor is online now
A new question is answered every 9 seconds

Failed MOT due to emissions, the engine is mechanically fine

This answer was rated:

For anyone who has experience with emissions, here's a scenario (and its more academic than practical): A customer booked his car into the workshop for a test after a MOT failure in relation to emissions. I can chose any model but i think i will stick to a VW Passat 2.0, a 2002 model. It's a GASOLINE engine, compliant with the Euro4 emission standards, ignition and injections systems are functioning correctly, its consuming the recommended fuel and the engine is MECHANICALLY fine. The CO readings are high, 0.9% and the HCs are quite high as well at 550ppm. In my example, the catalytic converter has been partially melted as the cat worked too hard to keep up with the high emissions. Could someone outline a series of reason for the high levels of CO and HCs that led to the destruction of the cat? Upstream and downstream O2 sensors not working properly, coolant temperature sensor, etc...

I also pointed out that i had high levels of O2 due to a leak in an exhaust pipe before the upstream oxygen sensor. I'm looking for other components (mainly sensors) as the engine has to be mechanically fine and the ignition and injection system cannot be the problem. 

Details about possible causes would be appreciated along with P codes that might come up as a result.



Thank you for choosing Just Answer or with your question.

In the scenario given the readings do not appear to be high, and are definitely not high enough to cause a catalyst failure. If there were no catalyst in the system the readings listed would be quite acceptable for this year vehicle. These reading could be verified by removing a pre catalyst Oxygen Sensor out of a vehicle and measuring an exhaust sample through the hole before the catalyst. The MOST LIKELY cause for a catalyst that has been partially melted would be due to a misfire. Given the year model vehicle I would suspect an ignition coil failed and the vehicle was driven for an extended period, possibly repeatedly. This would be a misfire severe enough to cause the MIL (Malfunction Indicator light) to flash.

In your example of the upstream exhaust leak if the leak were to effect the exhaust sample with a high rate of Oxygen I imagine it would take some time to cause catalyst damage. If this were the case it could be confirmed by monitoring the fuel trims in Measuring Value Block #32. If the trims were well into the positive, (I imagine above +20%) in the bank with the failed catalyst then it could be the cause, however it would be very unlikely.

Both examples would be accompanied with an illuminated MIL and likely flashing in the case of the misfire. One would be "ignition" related, and one would be "Injection" related but in either case the catalyst was damaged due to unburned fuel (HC) going through the catalyst for an extended period of time. Given the facts, It IS NOT possible for the "engine to be mechanically fine and the ignition and injection system cannot be the problem"

I hope that I have answered your questions and addressed your concerns. If you have any more questions for me please don’t hesitate to ask. I want you to be 100% satisfied with my answer.

Bonuses are always APPRECIATED. Please rate my answer as OK, GOOD, or EXCELLENT. With a lower rating I will not be compensated. Please contact me with your concerns if you cannot rate my answer OK, GOOD or EXCELLENT.

Thank You!
Jake “The Aud/VW Doctor”
Customer: replied 4 years ago.

That is all relevant information in regard to emissions, but i have to stick to a few points. First of all, my example is just fictional: Of course it has to make sense. The engine has to be Mechanically fine, without any modifications. The ignition and injection systems are perfect as well.
The cause of the problems has to be incorrect data coming from different sensors (like the oxygen sensors, upstream and downstream) or the coolant temperature sensor... or anything else that would lead to an incorrect fuel metering and would cause the engine to run rich.

The hole in the exhaust system affected to O2 value, rising the O2 value in the exhaust at 2.3% (therefore, the engine management ECU suspects that the engine is running lean and it is injecting more fuel in) - that lead to the catalytic converter failure and it damaged other sensors. I'm looking for these "other sensors" that could be damaged or that could affect the fuel metering, along with some P codes that might be generated.

Obviously any emission related problem would illuminate the MIL lamp and that's not a problem in my example. (there are actually some people who take their car to a MOT test with the MIL lamp on).

So I have to stick to a few points, where the engine is mechanically fine, and the fuel injection and ignition systems are ok... A flawed fuel metering has to be the problem caused by some sensors that could be responsible for that... This is what i'm basically looking for.



Customer: replied 4 years ago.

Relist: Incomplete answer. The answer is relevant to emissions, but it's not exactly what i'm looking for. This is only a hypothetical vehicle that i'm using in a project that i'm working on. All I need is a quick word of advice, and some faulty components that could affect the CAT (O2 sensors for example) and whats the most likely scenario in which the faulty O2 sensors would affect the fuel trim, therefore leading the CAT to work too hard. The fault doesn't have to appear in most cars out there, if it's possible to appear in one single car in the whole world that is meeting my guidelines (engine, injectors and ignition system in good working conditions) i'll be happy. Just something to make sense.



Hello Daniel,

There is a major contradiction in your question that limits my answer.

" the ignition and injection system cannot be the problem" & "The ignition and injection systems are perfect "

I call this the "Engine Management System" as they are both connected and overlap and sometimes difficult to separate. What ever you call it, if they "can not be the problem" and are "perfect". That eliminates the possibility that any of the "sesnsors" are failed as they are trickle parts of the "system". An Oxygen Sensor, Mass Air Flow sensor, Coolant Temperature Sensor, etc. are all part of the "engine management system" and the failure of any one these sensors would constitute that the engine management system was NOT "perfect" or "not the problem".

If you want to eliminate sensors from the system and not include them, yes, there are a number of possibilities that could cause the system to run rich and possibly damage the catalyst provided the operator ignored the illuminated MIL and continued to operate the vehicle even though the On Board Diagnostics illuminated the MIL indicating there was a "problem in the system". I hope you can see where I am having a problem with the question as it has been posed. C.O. will not damage the catalyst. HC (unburned fuel) is the leading cause of catalyst damage (overheated).

Problems that could cause high levels of HC;

  • Misfire - Due to ignition coil, spark plug, dirty/leaking/failed injector, vacuum leak to the point of misfire, wiring, failed ECM
  • Upstream Oxygen sensor reporting VERY lean
  • MAF sensor grossly over reporting
  • CTS (Coolant Temperature Sensor G62) reporting much colder than actual temperature
  • Very high fuel pressure - likely failed regulator or restricted return
  • Restricted Intake system
  • Restricted exhaust system
  • Air leak into the exhaust upstream of the upstream Oxygen sensor
  • A fuel saturated carbon canister
  • A improperly functioning purge system
  • Incorrect, poor quality, or contaminated fuel in the fuel tank
Nearly all of these possibilities would illuminate the MIL and likely even cause the MIL to flash before irreparable catalyst damage would result. On board diagnostics is an important part of the engine management system and ignoring the illuminated MIL would be a key factor in the failure of the catalyst.

I hope this answers your question. If not possibly you could word it in such a way that I could give you an academically accurate and correct answer.

Please rate my answer as OK, GOOD, or EXCELLENT. With a lower rating I will not be compensated. Please contact me with your concerns if you cannot rate my answer OK, GOOD or EXCELLENT.

Thank You!
Customer: replied 4 years ago.

That's exactly the answer that I wanted!! Thanks for that Jake! I do appreciate a lot where you're coming from and I have a lot of respect for your vast experience and knowledge. The thing is that i'm working on a project and im approaching the deadline very quickly. I didnt read the task properly in the recent days and I had mechanical engine problems, I had to delete everything and look for an alternative. I kinda know where i'm going but i felt like i needed some guidance. The task is very clear in stating that: the engine is mechanically good and that there are no problems with the injection and the ignition system (my understanding is that that is ruling out dripping/leaking injectors, any misfires as they would be incorporated in the ignition system, and according to the task that cannot be the problem). Basically that's what i worked on and now i have to change the whole project. I have an air leak into the exhaust system upstream the oxygen sensor, which in my hypothetical example affected the lambda reading, making the engine management ECU to inject more fuel (therefore high HCs). I will say that the HCs also affected the upstream oxygen sensor and that partially melted the cat. What sort of HCs would u expect to melt the cat overtime (say maybe over a period of about a year) - and with the driver ignoring the MIL lamp. Just let me know if that makes sense to you, or if theres anything i should change.

On top of that I think you need a little bonus for your help.





I can't give you a quantative value for HCs that would damage the catalyst. That would require "lab type" testing any number I would guess would be just that, a guess. I don't work with actual emissions figures that are the byproduct of the combustion event ever since the introduction of catalytic converters because with a properly functioning catalyst CO & HC are "scrubbed" so I have no idea what the actual values are. The numbers you listed in your original question were well within the scope of a properly running engine if the catalyst were not in the system or ineffective.

In the case of the air leak the most it could do would be to raise the fuel trims to the maximum "trim", or adaptation, which would be +25%. That means the injection duration time would be 125% of the "mapped" duration time in the ECM based on the sensor inputs being: Engine RPM, MAF, CTS, Intake air temperature, Throttle position, Load, Barometric pressure, Ignition timing, Intake camshaft timing, and Knock control. I think that's about it. If I were to "guess" I would say 5-6%???

Also if I understand the scope of the question, I would seriously consider the possibility of incorrect, contaminated or poor quality fuel. In the real world this is often overlooked and I have diagnosed many vehicles where the engine was mechanically sound, and the engine management system was perfect and fuel was the cause of the condition. It's the most sensible answer.

The saturated carbon canister is also a good one. I had a 2003 BMW Z4 that the customer repeatedly "topped off" the fuel to the point where the carbon canister that is meant to store fuel vapors was actually flooded with gasoline. When the purge was activated raw fuel rather than vapor would be drawn into the engine causing a rich misfire. During diagnostic testing I noticed when I was monitoring the live data streams when the vehicle was misfiring that the fuel trims were going +25% or "full rich". My analogy was that when excessive fuel went into the engine it caused a misfire. When the engine is misfiring not only does unburned fuel go through the exhaust, but unburned oxygen. Since the oxygen sensor measures oxygen and not fuel it reports a lean condition to the ECM which in turn adapts the engine to run even richer. This vehicle only displayed the rich miss when cold, but over time this could most certainly damage the catalyst.

So there you have three possibilities. It seems like they are "steering you" into the air leak into the exhaust, but the others are "bonus" possibilities. In the real world it's not always black or white and can't be determined by logic alone. Things must be tested and verified. It's not always the obvious.

Thank You!
The Audi Doctor and 4 other Car Specialists are ready to help you
Customer: replied 4 years ago.

Thanks a million for that Jake, you're a star!!

Thank you Daniel,

Hope things go well for you.


Related Car Questions