the code p0420 come out when i check my car with the ob2 it says input/turbine speed sensor A circuit Gear Ratio incorrect what is this and how can i fix it

 

are you sure its for a a speed sensor i think p0420 is a cat code

 

po420 is cat efficiency below threshold bank 1 if thats the case its your cat or your secondary o2 sensor thats throwing it

 

Yeah, a P0420 code is a catalyst efficiency below threshold code which is usually an o2 sensor issue....

 

Beg to differ, I threw P0420 a few months back, went to muffler shop and got a stock-replacement-high-flow cat welded in, been gone since.

It usually indicates exactly what it says: That the catalytic converter is dead (mine was alllll burnt up after 240K miles!), not necessarily that the oxygen sensor is bad.

 

I've had plenty of cars come in the shop and it was the o2 sensor, so everyone will encounter differences... I didnt say always, just I've had may fair share that the o2 was bad with that mil showing...

 

My 2001 CRV threw this code after 139K miles. I replaced the cat with a NAPA factory replacement, the primary O2 sensor with a factory replacement, and the secondary O2 sensor with a Walker universal O2 sensor which I had to splice in because it had no plug (universal). After resetting the faults, the MIL continues with the code reading PO420. I suspect the universal sensor because the voltage reads either 0.00v or 1.25v....nothing in between. The voltage for the primary sensor bounces around from .120v to .720v, which I think is normal. I plan to replace the secondary universal sensor with an OE sensor. I'll follow up with my results after the ECM monitors reset .

 

I know this is old, but I have a 2000 honda civic dx with the same issue as OP. I should replace the o2 sensor? If I don't replace it, and I just drive could that fix the issue with the drive cycles? My mechanic fixed my oil leak and I started to get the same issues as OP. I never had a check engine light before in my 6 years of getting the car... It's very frustrating...

 

i was just wondering about this weighing the options of a full tune up or just get a new cat .... and if i get new cat new o2 top and bottom ?

 

should i just do both o2 or which one