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Mechanic Martyn
Mechanic Martyn, Mechanic
Category: Car
Satisfied Customers: 1002
Experience:  Advanced trade certificate in Automotive engineering
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Suzuki Forenza: MIL..P0420..catalytic did the o2 sensor

Customer Question

my suzuki Forenza failed the state emissions test, because the MIL commanded-on light failed. the code given was P0420. I brought a new o2 sensor and the catalytic converter passes as did the o2 sensor. I went to the local suzuki dealership to ask them to reflash the ecm with the new software, but they said I could do it myself by dicounnecting th negative battery post; that only clears the code and the inspection still failed. I don't know whatto do now!
Submitted: 6 years ago.
Category: Car
Expert:  Mechanic Martyn replied 6 years ago.

A code P0420 may mean that one or more of the following has happened:

  • Leaded fuel was used instead of unleaded
  • An oxygen sensor is not reading (functioning) properly
  • The engine coolant temperature sensor is not working properly - often overlooked - does the gauge indicate correctly
  • Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe - even small leaks can alter the mix enough to throw off the O2 sensor
  • Retarded spark timing
  • The oxygen sensors in front and behind the converter are reporting too similar of reading

So what to do

  • Check for exhaust leaks at the manifold, pipes, catalytic converter. Fix if leaking.
  • Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
  • Inspect the downstream heated oxygen sensor (HO2), replace if necessary - check to see if your model has one.
  • Replace the catalytic converter if none of these other things work.

regards Mechanic Martyn