Toyota 1 MZ–FE Manual page 534

EG2–534
DTC P0420 Catalyst System Efficiency Below Threshold
CIRCUIT DESCRIPTION
The ECM compares the waveform of the oxygen sensor located before the catalyst with the waveform of
the oxygen sensor located after the catalyst to determine whether or not catalyst performance has deteri–
orated.
Air–fuel ratio feedback compensation keeps the waveform of the oxygen sensor before the catalyst re-
peatedly changing back and forth from rich to lean.
If the catalyst is functioning normally, the waveform of the oxygen sensor after the catalyst switches back
and forth between rich and lean much more slowly than the waveform of the oxygen sensor before the
catalyst.
But when both waveforms change at a similar rate, it indicates that catalyst performance has deteriorated.
Diagnostic Trouble Code Detecting Condition
DTC No.
After the engine is warmed up and the vehicle
driven for 5 min. at 32 – 80 km/h (20 – 50
mph), the waveforms of the heated oxygen
P0420
sensors, bank 1, 2 sensor 1 and bank 1 sensor 2
have the same amplitude.
HINT: Only on U.S. vehicles does the MIL light up when a malfunction is detected.
1MZ–FE ENGINE – CIRCUIT INSPECTION
Three–way catalytic converter
Open or short in heated oxygen sensor circuit
Heated oxygen sensor
Trouble Area