Looking at a car my kid wants to buy. 2012 Honda Civic LX. One owner, some service records, seemed to run fine in test drive. No colored exhaust gasses, no dashboard lights, sped on freeway, sudden stops, drove half an hour, no unusual noises, actually very quiet ride. Stopped at auto parts store to check codes, where it was shut off ~15 minutes, and found all of these codes:

  • Exhaust Gas Sensor Monitor Bank 1 - Sensor 1 ($01)
  • Exhaust Gas Sensor Monitor Bank 1 - Sensor 2 ($02)
  • Catalyst Monitor Bank 1 ($21)
  • EGR Monitor Bank 1 ($31)
  • EVAP Monitor (0090*) ($ forgot #)
  • EVAP Monitor (0020*) ($3C)
  • Purge Flow Monitor ($3D)
  • Misfire Cylinder 1 Data ($A2)
  • Misfire Cylinder 2 Data ($A3)
  • Misfire Cylinder 3 Data ($A4)
  • Misfire Cylinder 4 Data ($A5)

  1. Did the cylinder misfires cause the problems for everything else downstream, or were they independent problems at different times?
  2. If I fix whatever's causing misfires, is it likely the other problems will resolve themselves?
  3. Or is it more likely that all of those parts will need replacing?
  • Error codes are usually Pnnnn for emissions, Bnnnn, Cnnnn or Unnnn. Two indicators; check engine indicator with an engine symbol in yellow and a wrench indicator in yellow. Both are usually leds. Turn on the ignition switch, don't start, and observe instrument panel indicators for these two. Turning on ignition initiates power up self tests of every electronic module with indicators. In a few seconds when all pass their self tests, indicators turn off, leaving seat belt, battery, oil and brake light on. Repeat cycling ignition until you verify the check engine indicator turns on and off.
    – F Dryer
    Commented Oct 24, 2023 at 14:46

1 Answer 1


If the vehicle is running good and you're not hearing much of anything out of it, the first thing to do is clear the codes. Then, start it back up and take it for another drive. If when you get done you check the codes, whatever comes back is what you need to worry about.

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