Nissan Repair: 1986 King Cab carbon monoxide at idle, nissan king cab, smog check

Question
1986&1/2 Nissan King Cab, 6 cylinder, TBI, 132,000 mi.  New plugs, rotor, rotor cap, coil, fuel filter, air filter last October when cutout problem on acceleration.  Turned out to be fuel pump, so replaced.  Has run great since.  Now new inspection requirements to check emmissions.  All great except carbon monoxide at idle.  No problem at high speed.  Checked computer codes - no O2 sensor error, but I still wonder.  Hydrocarbons are okay at idle, O2 is .3%.  Only one error code indicated - 31 (ECU), but no further explaination in my repair manuals, except the possibility of the ECU receiving a sensor value out of range.  I can't afford the shotgun approach to fix this.  Does this really sound like an ECU problem, or would you suspect one or more of the sensors, controls or the catalytic converter?  Thanks.  - John A.

Answer
John,

Clear the code and get CD2 fuel treatment for smog check and run the truck half empty and then change the oil and take it in for smog check.  Remember, that you need to get the vehicle nice and warm before inspection so take it out on the highway and run at speed for ten minutes minimum before you get the inspection.  Clean oil helps reduce the carbon monoxide.