2000 toyota 4runner code P0420



I am trying to troubleshoot a 4runner with 135k miles, auto, 3.4 V6 California emissions.

This is the freeze data:

PID Description Value Units

0x02 Freeze frame DTC P0420 -

0x03 Fuel system status Closed-loop, using oxygen sensor feedback to determine fuel mix -

0x04 Calculated load value 34.12 %

0x05 Engine coolant temperature 197.6 F

0x06 Short term fuel % trim - Bank 1 0.78 %

0x07 Long term fuel % trim - Bank 1 3.91 %

0x0C Engine RPM 2287 RPM

0x0D Vehicle speed 68.97 MPH

0x0F Intake air temperature 95 F

So far the front and back Ox sensor was replaced and the MAF cleaned to clear a P0171 code. Thank you for your reply,


Has DTC P0171 stayed clear after a 2 trip minimum? Did DTC P0420 come back after it had been erased? The freeze frame short and long term fuel trim is way out of wack. The PCM is trying its best to lean out the mixture but is close to its limit. You will have to find out where the extra fuel is getting into the engine – think leaking fuel pressure regulator, stuck fuel injector, leaking fuel injector, MAF out of calibration, etc. I would be striving to get the long term fuel trim close to 50% before I would condemn the catalytic converter. Can your scanner read the activity of the A/F and o2 sensors? If you have the equipment, use an dual trace oscilliscope to monitor the A/F and o2 voltage wave forms and see if they are similar in rich-lean-rich crossings.

My research has led me to understand that DTC P0420 will be set if the A/F sensor and the O2 sensor transistion at the same frequency. The DTC description is sort of ambigous regarding the A/F sensor and o2 sensor remaining constantly at one value i.e. the wave forms are the same but both are not transitioning.

Keep us in the loop as you resolve this problem so we can learn more from your experience.


hello, here is an update to the truck’s situation. After cleaning the MAF, the long term fuel trim went down. I went trough the repair records and some maintenance was pending, the air filter was not replaced for almost 60 miles and almost 70 on the fuel filter. This one I have to tell, do not try to remove it without a lot of PB blaster and a very good line wrench - I borrowed a snap-on. Jack stand the driver’s rear tire for clearance and remove the plastic cover. The car feels like brand new and fuel trim looks much better now. I think that the converter is gone, too much raw gas. Thank you for your reply.


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