so my 1999 1500 conversion van failed Smog on emissions.
This could be for any number of reasons of course but I'm wondering where to start.
HC and NO were the culprits
@15mph HC was 138 (max allowed 46) and NO was 2112 (max allowed 488) so gross polluter status
@25mph HC was 43 (max allowed 29) and NO was 1269 (max allowed 741)

should I start with the less expensive EGR Valve before going for the expensive catalytic converter? (dread to think how much that might be)
any other tips?

 

 

Last time the truck had a tune-up? Oil Change? How old are the O2 sensors?? What type of plugs are in it? Which engine?

 

apologies for the lack of info

It's the 5.2 V8
Oil change was last year (maybe 800 miles ago), new spark plugs were fitted too (doesn't mean they haven't since failed of course and I suppose they are cheap enough that I could replace again)
as far as I know, O2 sensors have never been replaced so could be the originals

 

Could be just old/worn ignition parts. (cap, rotor, plugs, wires), or a vacuum leak..... Pull a couple plugs, see what they look like.

 

Are they the original cats?
How many miles on them?

 

views from another forum suggest it's the Plenum gasket....another common failure point. A bit of a pain to change that, but I will get started on it this week.