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I have just bought 2000 ford focus with 121k miles on it. when i took it to mechanic for oil change they said that oil pan and transmission pan have a small leak but nothing major. after driving it 60 more miles check engine light came on. at auto-zone on the computer it showed P0420 code:
defenition
catalyst efficiency low bank 1
explanation
the oxygen sensors monitor the Catalytic converters ability to store oxygen
probable cause
a1. catalytic converter detective
a2. engine misfire or running condition
a3. large vacuum leak
a4. engine oil leakage into exhaust-valve guide seals, pistol rings

what is wrong with it? will it be expensive to fix it? and are repairs necessary?  

start with number 3....

large vacuum leak...        check for vacuum leaks..  including the power brake booster hose..  the PCV hose...  the crankcase breather hose..    the vacuum hoses to the purge valve under the hood..    feel the hoses .. if they are soft and mushy ..  they are probably bad..

with a scan tool.. like the harbor freight 98614..  i would plug that into the OBD2 port.. look at the oxygen sensors voltages..
i would look at the voltage swings in GRAPH display mode..

O2S1B1...

O2S2B1

what... the heck do those mean...

O2 is oxygen sensor..

S1 or S2 is sensor location  one or sensor location 2..    sensor 1 is near the exhaust manifold..  sensor 2 or 3 is behind the catalytic converter...    this monitors that the engine fuel control is sending the proper fuel mixture to keep the cat operating properly..  


B1, B2...   bank one.. bank two.  ... are which side of the engine  the sensor is on..   this can vary from model to model..

its totally possible that the catalytic converter has failed...   but instead of just throwing a new cat on it.. as they are really expensive... the idea it to find out why the cat failed..   Catalytic converters are PASSIVE..  they don't fail on their own.. if the fuel ratio is off..  that can damage the cat... stick a new one on.. don't fix the original problem..  the new cat will be damaged...  

next i would look into the ignition system .. make sure that the coil is not cracked...

that the spark plug wires are intact...

that the EGR back pressure transducer has been replaced with a black plastic version.. and the EGR vacuum control solenoid had been replaced...





you did not say if you have the single overhead cam motor or the dual over head cam motor...

can you post the 8th digit of the Vin number to identify which motor you have???

it will either be a 3 for a single over head cam motor or a P for a dual overhead cam motor..  

thank you!
if i take it to mechanic how expensive will it be to do this? labor and details?