# P0420 trouble code Bad gas milage Help!!!



## lemoncoke (Sep 25, 2008)

I have a 2001 Olds Alero that has the trouble code P0420 and it is only getting about 19 mpg. The catalitic converter has been replaced, then the light went on again. Dont know to go from here need some help. I have a scan tool and this is what it reads. P0420 Catalyist System Efficency Below Threshhold (Bank 1)Freeze Frame Data----- Fuel sys 1 --OL , Fuel sys 2-- CL-Fault, Calc Load % 18.04, ECT (*F) 188 , STFT B1 (%) 0.00. If anyone could help it would be most appericated. I got this car to help with gas milage and it doesn't seem to be getting that good of milage, it should get about 26-28 mpg what could be wrong? To get the milage up to where it is suppose to be. HELP please. Thanks Rick


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## Winchester (Aug 27, 2008)

I don't have a similar vehicle but I below to a car club for our two 300Ms. When (bank1) or (bank2) is involved we're usually speaking of the O2 sensor(s). A bad one will reduce MPG.

Best of luck.


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## lemoncoke (Sep 25, 2008)

How do I know if it is the one before,or after the cat converter?


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## Winchester (Aug 27, 2008)

Once again, under the situation for my vehicles, 

The input from the upstream heated oxygen sensor
tells the PCM the oxygen content of the exhaust gas.
Based on this input, the PCM fine tunes the air-fuel
ratio by adjusting injector pulse width.​ 
The downstream heated oxygen sensor input is
used to detect catalytic convertor deterioration.​

So, if mileage is less than to be expected, I put money on the upstream O2 sensor. We have a V-6 so the (bank 1) refers to the side that cylinder 1 is on.​


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