reset your fuel mixture. just did mine last week.
do the mix first, if it does not stay , then go on and check your o2 sensor.
my mixture was just like yours, only had to turn the adjuster bout one eighth counter clock wise
you should have the smog shop reset the mixture to get it right. if you lean it a little to much and your nox will fail.
i work as a smog tech so i do have some experiance on this
good luck
rich, from quebec

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88 300E White, sold 9-04
88 190E Black 2.3 sold
90 300CE White, Grey sold 12-24-04
92 400E White,Grey Sold 5-06
90 300CE 91K Signal Red,Black Sold 2007
1989 300CE Smoke silver,Tan Sold 1-08
1993 300E 3.2 M104 motor
1991 300CE White,Grey
1994 E420 beautiful car,gone to new home
1994 E320 Smoke an Parchment.Beautiful car