[Northers]

Hi can anyone help me with this please?.....
My 1997 1.25 zetec fiesta failed its MOT on emmissions.
All 3 readings were sky high!
I renewed the plugs, air filter, fuel filter, changed the oil and filter, filled the tank with fresh fuel and a dose of injector cleaner and drove it for 100 miles (mostly motorway).
On retest only the CO test is failing (just out).
I am now checking for air leaks and noticed that when I 'blip' the throttle there is a definite 'sucking' sound from around the manifold. Is this normal? If not is there anyway to check for leaks without taking the manifold off?
Any other suggestions of what the problem may be?

Any help would be most welcome!

Thanks...Andy

[Hometune]

The sound is normal. Your cat has failed.

[Northers]

Thanks
Do you think it might be worth checking/replacing any of the sensors before going straight for the CAT?
I also noticed that the engine 'runs on' slightly when changing gear. In other words the revs rise slightly when lifting off the accelerator and pressing the clutch and then drop more slowly than you'd expect. Could the throttle body or idle valve be stcking?

Thanks for your help...Andy

[Hometune]

What sensors are you going to 'check' and how do you plan doing this? Unless you are an experienced diagnostic technician with a quality scanner I cannot see how you can 'check' anything. :rolleyes:
I have answered your query based on what you have written here.
Therefore, if the lambda reading is within limits (which you have said it is) and the hydrocarbons are within limits (which you have said they are) then the CO reading above the limit indicates the cat has failed. Cats are only lifed to 50,000 miles, not forever.
I'd guess the actual CO reading is somewhere near 0.60% at fast idle. And that means only one thing - the cat is dead.
The 'running on' could be a slow reacting idle speed control valve, a faulty speed sensor in the gearbox, poor contact between the throttle position sensor and the wiring loom or the throttle butterfly is not seating correctly due to carbon build up.

[Northers]

Thanks for your advice,
I decided to check the oxygen (lambda) sensor anyway before shelling out for the CAT. I found a usefull tip on another site which said that I can connect a voltmeter across the sensor and it should 'swing' between 0.2 and 0.8 volts. Mine was stuck around .023 and .024 volts which I guessed would tell the ECU that the mixture was weak and cause more fuel to be supplied.
I changed the sensor (£40 from PARTCO) and the car has now passed all emmissions test...so the new CAT can wait for another year (or another owner!)...hope this helps anyone else with the same problem.

[Hometune]

I would like to accept this explanation but from the details supplied but this is not correct.
If the lambda sensor reading on the MOT was within limits then the oxygen sensor was doing its job.
The hydrocarbons would have been over limit if the car had been overfuelling
The CO was too high because the CAT is at the end of its life.
This car passed its test because the CAT was probably a lot hotter when retested and this allowed it to get below the 0.3% limit.
There is no mention of when the oxygen sensor was tested. Was the engine hot, cold or in between? How long had the engine been idling when this test was done? The engine and more importantly, the CAT, have to be hot and up to temp before testing takes place. If the engine was idling for 30 secs it may have gone 'out of loop' and the ECU would not be controlling fuelling.
There is no measurement of short term fuel trims which is the ONLY proper way to accurately assess the sensor. And, was it ever considered whether the airflow sensor was the real culprit forcing the oxygen sensor to give a low fixed reading?
Sorry, but I do not accept the explanation.