i recently went to the emisions facility, and i failed due to an oxygen sensor,

i was looking at the two under the hood, they look fine , but my question is where would i find the others, and if i do with some help. How many are there.the code was po133 sensor circut low responce

Customer Concern: Malfunction Indicator Light (MIL) lamp is on. P0133 (ECM-28) is set. The front O2 sensor has been replaced. Tests/Procedures: 1. Review Tech-Net Note (TNN) 25-15. 2. Check for any exhaust, intake and vacuum leaks. Check the Fuel Pressure Regulator (FPR) for leakage into the vacuum line. 3. With a scan tool, check the O2 sensor operation for both front and rear sensors. If the rear O2 sensor seems to almost mimic the front sensor but at a slightly slower rate, recommend resetting Engine Control Module (ECM) fuel adaptation before condemning any parts. 4. Verify that the adaptor cable is installed for the front O2 sensor per TNN 25-16. 5. If unable to find any fault, recommend the Engine Control Module (ECM) software to be upgraded. This is per TNN 25-15. After downloading, the ECM adaptation will need to be reset. See TNN for the procedure. Tech Tips: Anytime a fuel trim or O2 sensor code is set, the adaptive data will likely have to be reset after repairing or you risk the code returning. Unhooking the battery usually does not work but has occasionally been successful. If you do, remove both battery cables and touch together for as long as you can. 8+ hours seems to work the best. Reset Engine Control Unit (ECU): Needed dealer scan tool to reset adaptations. Problem Description: P0133, cleaered codes and light on again in 3kms. O2 sensors scoped and are good condition, fuel trims ok, regulator ok. Tried ECM adaption 1st. No more code appearing. DEALER ONLY scan tool. tried launch, vedis no feature available, Vehicle Description: 2001 Volvo S40 1.9L, Eng Des B4204T3, USA/Canada Category: Emission Control Emission Test Failure Engine Performance Check Engine Light (MIL)