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Q: i recently went to the emisions facility, and i failed due to an oxygen sensor, on 2001 Volvo V40

i recently replaced one on my saab 95, but was curious on my 2001 volvo v40, the car that did not pass, 1st question is how many o2 sensors are on my car i found two in the engine, but is there one by the manifold and exhaust system. also can i clear the codes my self or diy project will it pass if i replace it or should i replace it then go to a mechanic to clear the codes which is a better bet, or do i just replace it my self, also how do you know which one is the bad o2 sensor
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There's no O2 sensor in the engine. They are only in the exhaust system.
You need to have the trouble codes retrieved from the computer. Post the codes here and we can help you more.
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.

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
Emission Control
Emission Test Failure
Engine Performance
Check Engine Light (MIL)
I went through the exact same scenario with my V40. Replaced O2 sensors several times. Finally, it was found that a software update was required. That solved the problem completely.
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