po135 /po131

hi i have a 1999 elantra when i am driving my car eng light come on code po135 02 sensor bank 1 sensor 1 about 2 min po 131 will come on i took it to my mec he change 02 same ploblem po 131 say low voltage can some one tell me what cause low voltage when my mec check he say it have 13.5 volts. thanks in adv.

Problem could be stemming from engine performance - perhaps the air/fuel ratio is out of whack causing excess of fuel or too lean fuel ratio.

What has been done tune-up wise? Such as air filter, spark plugs, wires, etc...?

If all of that is decent then:
Could be a problem in the wiring/ciruit to the O2 sensors - either a problem/short/damage to wiring or the ECU/ECM is going bad in that area....

I don't know how many O2 sensors these have specifically, but are you sure he changed them all or the right one?

Hope this helps!

-Dale in Idaho