One of the items on the mot  to be tested is the exhaust emissions, below  is an image of the printouts of  the results, a failure and after repair.

Often the results as displayed mean little or nothing to the vehicle owner, however the results displayed give valuable clues as to where the fault lies.

The left hand section shows the results obtained prior to repair, both the co level and  lambda ratio are outside specification, and the idle co is very high at over 6% but the hc level is within limits, although a little higher than expected, this initially points to a failing lambda sensor. A quick check with the diagnostic kit showed the range the lambda sensor was switching was very narrow, a check at the lambda sensor plug under the bonnet (to eliminate a wiring fault) confirmed this.

Had the catalytic converter failed I would have expected to see the hc level higher.

A new lambda sensor was fitted, an emissions test immediately carried out and gave the results as shown to the right. (normally we would have allowed the vehicle to warm up fully before carrying out the retest but time was running out to finish the job before it was due to be collected)
As can be seen the 1st fast idle was a fail (engine not quite up to temperature) however the 2nd fast idle is well within limits at 0 .12% and the hc level down at 16 parts per million, with the natural idle down to 0.5% and lowering.

Should 4-2-2 testing become the norm this vehicle could be polluting for a further 12 months without the driver being aware of any problem. www.facebook.com/MOTTesting?sk=app_132065096874831


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