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I have a 2003,Frontier, v-6, automatic. It is a US car but is currently in Costa Rica. My HC and CO were close to zero at idle. At higher RPM HC was still low, but CO wouldn’t pass. I don’t have the exact numbers because the truck is not now with me. I’m hoping someone might be able to tell me if there are common causes for these types of readings. Next time I go to Costa Rica I plan to bring common parts with me, but am trying to avoid buying things I don’t need. After the failed readings I replaced the spark plugs and CO at higher RPM came down, but not enough to pass. I am planning to get a new PCV valve, O2 sensors and maybe plug wires, but am interested in insights from more knowledgeable people. Thank you!
 

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I have a 2003,Frontier, v-6, automatic. It is a US car but is currently in Costa Rica. My HC and CO were close to zero at idle. At higher RPM HC was still low, but CO wouldn’t pass. I don’t have the exact numbers because the truck is not now with me. I’m hoping someone might be able to tell me if there are common causes for these types of readings. Next time I go to Costa Rica I plan to bring common parts with me, but am trying to avoid buying things I don’t need. After the failed readings I replaced the spark plugs and CO at higher RPM came down, but not enough to pass. I am planning to get a new PCV valve, O2 sensors and maybe plug wires, but am interested in insights from more knowledgeable people. Thank you!
How many miles are on your truck? First inspect every vacum line. Due to the age of your truck, it is common for hoses to become brittle and crack, mostly where they slide onto the fittings. I had quite a few when I inspected mine. If the fuel filter is old it can result in lower fuel pressure which could mess with your 02 sensor. Even with a brand new 02 sensor, it is going to tell the computer to make adjustments to fuel trim to try to get back to desired settings from the factory. Make sure you have a clean air filter and Spray your maf sensor ( dont touch the element inside ) with ONLY maf sensor cleaner. These are basic things you should do anyway and relatively cheap. Thumbs up on the pcv and 02 sensor replacement.
 

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Thanks arjadian! I definitely plan to check all the vacuum lines - especially in Costa Rica due to the elements. Thanks for your comments about the fuel filter. It's an easy fix and I'm guessing it's old.
 

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and....162K miles.
Mine has 150k and I was surprised how bad a few of the hose ends looked. And in my location the temps are fairly cool 9 months out of the year. Be sure to use the search box for lots of info pertaining to your older truck. Thats where I read about the maf cleaning, and also the caution about not touching the element inside. Keep us posted on your progress and good luck.
 
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