Check engine light and oxygen sensors

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?

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).

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.

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

wrote in message news: snipped-for-privacy@4ax.com...

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)

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