My dads 1992 sierra saph 2.0dohc failed the mot today on emissions

the c/o level was 4.413, the max alowed is 3.500

any ideas how we can get it within the required limits please?

ive suggested he replaces the plugs and use some red-ex.

anything else we can do?



the mot tester mentioned to him that it may be down to a faulty idle control valve.
could that be the cause?



many thanks in advance

Mark