Toyota T100 1996 Service Manual page 474

DI−210
DTC P0420
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 deterio-
rated.
Air−fuel ratio feedback compensation keeps the waveform of the oxygen sensor before the catalyst repeat-
edly 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 cata-
lyst.
But when both waveforms change at a similar rate, it indicates that catalyst performance has deteriorated.
Waveform of Oxygen Sensor
before Catalyst
DTC No.
After engine and catalyst are warmed up, and while
vehicle is driven within set vehicle and engine speed range,
P0420 waveforms of heated oxygen sensors (bank 1 sensor 1, 2)
have same amplitude
(2 trip detection logic)
CONFIRMATION ENGINE RACING PATTERN
Engine Speed
2500 − 3000 rpm
Idling
IG SW OFF
1996 TOYOTA T100 (RM449U)
DIAGNOSTICS
Catalyst System Efficiency Below Threshold
DTC Detecting Condition
(2)
(1)
Warmed up
− ENGINE (5VZ−FE)
Normal Catalyst
S Three−way catalytic converter
S Open or short in heated oxygen sensor (bank 1 sensor 1, 2)
circuit
S Heated oxygen sensor (bank 1 sensor 1, 2)
(3)
3 minutes
Waveform of Oxygen Sensor
after Catalyst
Trouble Area
(4)
Time
Check
DI0UZ−01
FI7081
P25762