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.