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I am the original owner of this CA car, and have been gradually having higher HC readings on smog checks. It failed in 2016, cleaned out the Air flow components; it passsed, but this increased the NO readings.
It failed again last month, high HC readings at 15 mph (allowed 57, read at 81); at 25 mph, (40 allowed, 40 measured). The rest of the readings are fine. I have never replaced O2 sensor, nor any other fuel/exhaust system components. As I said, over the last 6-8 years, the HC have gradually been getting higher. Any ideas what would cause this type of gradual increase?

  • Welcome to Motor Vehicle Maintenance & Repair! I'd suggest, if you've never replaced the O2 sensors, this is a really good place to start. They are the primary means by which the ECM tunes itself. Replace, then reset the ECM to retune itself and you should see a marked improvement. – Pᴀᴜʟsᴛᴇʀ2 Jul 1 '18 at 16:12
  • Sometimes this is increasing oil consumption, my old 91 gmc pick up did this, wound up being the valve stem seals were bad. I would change the O2 sensor, it may get it to pass. – Moab Jul 1 '18 at 16:12

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