I own a 97 subaru legacy 2.5 gt and i got that code po420 catalyst efficiency below threshold .what does that mean and what can I do ?thanks

You should post this in the new generation section

There are a few causes of P0420 codes. The big three are aftermarket header, exhaust leak, and faulty O2 sensor. It could also be a bad cat. How many miles on the car? When was the last time, if ever, were the O2 sensors changed?

i just bought the car about a month ago .I also had a code of missfire #3 fixed that .the car has 183,000 miles and the moter 60,000 I have no idea what the guy did to it .there was a exust leak i fixed that .not sure if that was it .well i realy dont think that is it .it has lost alittle power.I am going to change the o2 sencers when it warms up .hopeing it is that .can the car lose power from the o2 sensor.thanks

You might have a bad cat.

 

~Howard

 

I got this same code a few weeks ago after washing the motor to clean out coolant from a head gasket change. I thought the worse and post on here. If you search theres more threads about the PO code. I cleared mine and (with fingers crossed) it hasn't come back. So I would suggest just clearing it.

i changed both o2 s .un did the batt and no light yet .and I have more power and runs better .