Check engine light and oxygen sensors

I have a ’00 Legacy GT and changed both oxygen sensors after getting a Check engine code P0420 (i think) for oxygen sensors. Within a week the check engine light is back- same code. Why would new sensors, fore and aft not keep that "check engine" light off?

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I found my solution on the "net". Apparently if the oxygen reading at the fore and aft sensors is the same, it means that the converter is not efficiently cleaning the exhaust fumes and likely needs to be replaced, resulting in the check engine light. Replacing the sensors probably wasn’t necessary in the first place. Wish I’d known that B-4 I did it.

"scjwonder" wrote: > I have a '00 Legacy GT and changed both oxygen sensors after > getting a Check engine code P0420 (i think) for oxygen > sensors. Within a week the check engine light is back- same > code. Why would new sensors, fore and aft not keep that "check > engine" light off?

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Yeah, I tried to explain that in an earlier thread on this ng, but folks see that $1400 tab for a set of converters and their eyes glaze over. Suddenly the cheaper O2 sensors look a lot better. Oh well look on the bright side. If you haven't changed the O2 sensors before, you were due for them anyway.

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It's because the PO420 code doesn't have anything to do with the O2 sensors per se. That code sets when the sensors determine through correlation that the catalytic converter is not doing it's job. If the O2 sensors themselves have a problem it sets a completely different code(s).

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what is the code if it is the O2 sensors themselves?

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wrote:

There are dozens. Here is the largest group...

P0130     O2 Circuit (Bank 1, Sensor 1) P0131     O2 Circuit Low Voltage (Bank 1, Sensor 1) P0132     O2 Circuit High Voltage (Bank 1, Sensor 1) P0133     O2 Circuit Slow Response (Bank 1, Sensor 1) P0134     O2 Circuit No Activity Detected (Bank 1, Sensor 1) P0135     O2 Heater Circuit (Bank 1, Sensor 1) P0136     O2 Circuit (Bank 1, Sensor 2) (See P2270, P2271) P0137     O2 Circuit Low Voltage (Bank 1, Sensor 2) P0138     O2 Circuit High Voltage (Bank 1, Sensor 2) P0139     O2 Circuit Slow Response (Bank 1, Sensor 2) P013A     O2 Sensor Slow Response - Rich to Lean (Bank 1, Sensor 2) P013B     O2 Sensor Slow Response - Lean to Rich (Bank 1, Sensor 2) P013C     O2 Sensor Slow Response - Rich to Lean (Bank 2, Sensor 2) P013D     O2 Sensor Slow Response - Lean to Rich (Bank 2, Sensor 2) P0140     O2 Circuit No Activity Detected (Bank 1, Sensor 2) P0141     O2 Heater Circuit (Bank 1, Sensor 2) P0142     O2 Circuit (Bank 1, Sensor 3) P0143     O2 Circuit Low Voltage (Bank 1, Sensor 3) P0144     O2 Circuit High Voltage (Bank 1, Sensor 3) P0145     O2 Circuit Slow Response (Bank 1, Sensor 3) P0146     O2 Circuit No Activity Detected (Bank 1, Sensor 3) P0147     O2 Heater Circuit (Bank 1, Sensor 3) P0150     O2 Circuit (Bank 2, Sensor 1) P0151     O2 Circuit Low Voltage (Bank 2, Sensor 1) P0152     O2 Circuit High Voltage (Bank 2, Sensor 1) P0153     O2 Circuit Slow Response (Bank 2, Sensor 1) P0154     O2 Circuit No Activity Detected (Bank 2, Sensor 1) P0155     O2 Heater Circuit (Bank 2, Sensor 1) P0156     O2 Circuit (Bank 2, Sensor 2) (See P2272, P2273) P0157     O2 Circuit Low Voltage (Bank 2, Sensor 2) P0158     O2 Circuit High Voltage (Bank 2, Sensor 2) P0159     O2 Circuit Slow Response (Bank 2, Sensor 2) P0160     O2 Circuit No Activity Detected (Bank 2, Sensor 2) P0161     O2 Heater Circuit (Bank 2, Sensor 2) P0162     O2 Circuit (Bank 2, Sensor 3) P0163     O2 Circuit Low Voltage (Bank 2, Sensor 3) P0164     O2 Circuit High Voltage (Bank 2, Sensor 3) P0165     O2 Circuit Slow Response (Bank 2, Sensor 3) P0166     O2 Circuit No Activity Detected (Bank 2, Sensor 3) P0167     O2 Heater Circuit (Bank 2, Sensor 3)

And here is the group of cat codes...

P0420     Catalyst System Efficiency Below Threshold (Bank 1) P0421     Warm Up Catalyst Efficiency Below Threshold (Bank 1) P0422     Main Catalyst Efficiency Below Threshold (Bank 1) P0423     Heated Catalyst Efficiency Below Threshold (Bank 1) P0424     Heated Catalyst Temperature Below Threshold (Bank 1) P0425     Catalyst Temperature Sensor Circuit (Bank 1, Sensor Circuit 1) P0426     Catalyst Temperature Sensor Circuit Range/Performance (Bank 1, Sensor Circuit 1) P0427     Catalyst Temperature Sensor Circuit Low (Bank 1, Sensor Circuit 1) P0428     Catalyst Temperature Sensor Circuit High (Bank 1, Sensor Circuit 1) P0429     Catalyst Heater Control Circuit (Bank 1) P042A     Catalyst Temperature Sensor Circuit (Bank 1, Sensor Circuit 2) P042B     Catalyst Temperature Sensor Circuit Range/Performance (Bank1, Sensor Circuit 2) P042C     Catalyst Temperature Sensor Circuit Low (Bank 1, Sensor Circuit 2) P042D     Catalyst Temperature Sensor Circuit High (Bank 1, Sensor Circuit 2)

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here's a related but slightly different question: i do not get a cel, but if i hook up a code reader, the "c" test (cat. converter) always flashes, indicating that it didn't complete. this has been going on for months (well over 100 "trips"), and with no (apparent) ill effects. what does this mean?

'02 legacy wagon, 5mt, 168,000 km

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