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I have a 2005 Elantra (205K miles). I replaced the second o2 sensor (downstream; after cat), added a defouler to second o2, replaced cylinder # 2 injector, air filter, tune-up. If the car is at idle in park, the second o2 data looks normal. That is, it straddles .450 nicely. If I start driving, the second o2 sensor stays virtually stuck (very little fluctuations) @ around .800 volts. I think I did have a leaky injector since cylinder #2 spark plug looked fouled but I replaced that injector. Before the injector replacement, my long term fuel trim was about -20. After the injector replacement, its staying solid at -3.9 and still throwing both codes.

Any help and how to test would be greatly appreciated.

  • What about the short term and s1 o2s? .450 sounds low for cl and .800 seems too high. What engine? How about the maf? When is the code setting? What does freeze frame say? – Ben Aug 1 '18 at 0:36
  • Short term seems to fluctuate between -4.0 and 0.0. Sometimes it goes positive. Hard to believe but this Hyundai came with a wideband (lambda) o2 sensor. When cruising, the value seems to over around 1.900 to 2.000 volts but from what I've learned about this type of sensor, these values are specific to the individual car (unlike standard o2 sensors e.g. .450 = stoichometry). Equivalence ratio is about 0.975. It seems pretty close to 1.0. Its a 2.0, California emissions. Code is set in about 30 miles of driving. – Paul Aug 2 '18 at 1:06
  • That sounds about right for hyundai b1s1 afr sensors iirc. Or are you saying there’s a afr sensor on the downstream? What market are you in? When is the code setting? .450 on the downstream sensor still seems too low to me on usdm cars i’d expect .650-.700 at idle and cruise. What happens if you remove the anti fouler? – Ben Aug 2 '18 at 2:10
  • Wideband only on upstream. At idle, the downstream oscillates between.100 and .700 (which I’ve read it’s suppose to do). However, when I drive it, it climes and pegs out at around .780 volts (I also get no activity detected code). What baffles me is if I give it gas while parked, it also oscillates like it should. Before I put the defouler, the data looked the same. – Paul Aug 2 '18 at 16:08
  • Rear o2 should remain steady at idle or cruise oscillation means the cat is bad or theres an air leak could be just a pin hole but enough to effect the reading. What kind of fuel pressure are you seeing? – Ben Aug 2 '18 at 16:27

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