Failed the emissions test: what to do

Turned away at the test station. Here is what exactly failed and which of it is an evening's work.

An emissions failure comes in three kinds, and they should not be confused: they are fixed in completely different ways.

First: monitors not ready

The most annoying failure, because the car is healthy. The unit runs its own checks of the catalyst, the oxygen sensors and the evaporative system, and clearing the codes or disconnecting the battery resets all of them.

The cure is not a repair but kilometres: several varied trips — town, motorway, a full warm-up, a night's standing in between.

Second: the Check Engine lamp is on

In many countries a lit lamp is a failure in itself, whatever the measurements say. Here you have to read the code and fix the cause rather than put the lamp out: a lamp cleared before the test brings us back to the first point — the monitors reset.

Third: the numbers did not fit

CO, HC, lambda — the readings are outside the limits. What usually lies behind that:

1. A tired oxygen sensor. A slow response means the mixture is never right.
2. A worn catalyst. Worth checking second, not first.
3. A rich mixture. Look at the fuel trims: a steady negative value means over-fuelling.
4. An air leak in the exhaust. Extra oxygen before the sensor breaks the whole picture.
5. A cold engine. A measurement on a cold car almost never passes — that is the test procedure, not a fault.

Before the second attempt

1. Read the codes and see whether any are active.
2. Check the readiness monitors — are they all complete.
3. Warm the car up properly on the way there: driving, not idling.
4. Look at the fuel trims at idle and at 2500 rpm.

Three of those four take ten minutes with the app and an adapter, and cost less than a second failed attempt.