o2 sensors can be working when you check them or be out of range. if its old and can be found at a good price, then change it. some times all the checks you do just don't catch it acting up.
plus a new sensor works a lot faster. don't forget to verify that the new sensor works. even new parts can be dead.
buy the bosch brand . less problems
rich

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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