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Doug
Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8598
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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My car failed emissions. My service engine light has been

Customer Question

My car failed emissions. My service engine light has been on. Every time I had it checked It reads the catalyst efficiency is below threshold. Replaced the catalyst and still light comes on. Is it the sensor messed up? Its a '02 Mitibishi Lancer
Submitted: 1 year ago.
Category: Mitsubishi
Expert:  Doug replied 1 year ago.

Hi,

On Mitsubishis it is pretty much never the sensor if the onlyfault you are getting is the catalyst efficiency fault.
The computer uses two O2 sensors, one before and one after the catalyst on the exhaust manifold. The computer compares the fuel content between the two sensors to determine if the catalyst is working or not... if the rear sensor mimics the front sensor too closely, it deems the catalyst defective since it is no longer reducing the HC content in the exhaust.
While on other vehicles (specifically American cars) it is not uncommon to see an oxygen sensor set this fault in error, the way Mitsubishi (and most Delco-driven electronics) operate this is not the case. If you see a catalyst fault on one of these, it is pretty much always a catalytic converter... if the O2 sensor is at fault it will just about always accompany an O2 sensor fault as well.

It is important to note that the catalyst efficiency faults are set by the front catalyst, the one built into the exhaust manifold. They are not set by the large under-car catalyst that auto parts stores like to push since they are semi-universal. Th under car catalyst is not monitored in any way and can not cause the light to come on... only the manifold catalyst can do so, requiring a new manifold.
It is also important to note that Mitsubishi did have a software update for the computer that reduces the likelihood of the catalyst failing... it will not correct the code once the catalyst is depleted, but it will change the the fueling behavior to make the catalysts last a bit longer. They are still consumable devices (most catalysts have a life expectancy of their precious metals to be 80-90k miles) but the change in computer operation makes them a little more durable.