Mitsubishi FUEL Manual page 37

Code No. P0420 Catalyst malfunction
Range of Check
D The engine speed is 3,000 r/min or less.
D During driving
D During air/fuel ratio feedback control
Set Conditions
D The ratio between the oxygen sensor (rear) and the oxygen sensor (front)
output frequencies reaches 0.8 per 12 seconds on average.
Check the exhaust manifold. (Are there any cracks?)
MUT-II Data list
59 Oxygen sensor (rear)
D Transmission: 2nd gear <M/T>, L range <A/T>
D Drive with wide open throttle
OK: 600 - 1,000 mV
MUT-II Data list
11 Oxygen sensor (front)
OK: 600 - 1,000 mV when racing suddenly
MUT-II Data list
11 Oxygen sensor (front)
D Transmission: 2nd gear <M/T>, L range <A/T>
OK: Changeover between 0 - 400 mV and 600 - 1,000
mV occur 15 times in 10 seconds.
Replace the oxygen sensor (rear).
Check the trouble symptoms.
Replace the catalytic converter.
Check the trouble symptoms.
Replace the engine-ECU <M/T> or engine-A/T-ECU <A/T>.
MPI <4G6> -
OK
OK
OK
OK
NG
NG
Troubleshooting
Probable cause
D Malfunction of catalyst
D Malfunction of the oxygen sensor (front)
D Malfunction of the oxygen sensor (rear)
D Malfunction of engine-ECU <M/T>
D Malfunction of engine-A/T-ECU <A/T>
NG
Repair
NG
Check the oxygen sensor (rear) system <sensor 2>
(Refer to P.13A-24, INSPECTION PROCEDURE FOR
DIAGNOSTIC TROUBLE CODE P0136.)
NG
Check the oxygen sensor (front) system <sensor 1>
(Refer to P.13A-21, INSPECTION PROCEDURE FOR
DIAGNOSTIC TROUBLE CODE P0130.)
NG
Replace the oxygen sensor (front).
13A-37