Toyota Celica 2005 Repair Manual page 394

DTC P0420
MONITOR DESCRIPTION
The vehicle is equipped with two heated oxygen sensors. One is mounted upstream from the TWC (Three−
Way Catalytic) converter (Front Oxygen Sensor, "sensor 1"), the second is mounted downstream (Rear Oxy-
gen Sensor "sensor 2"). The catalyst efficiency monitor compares the sensor 1 and sensor 2 signals in order
to calculate TWC ability to store the oxygen.
During normal operation, the TWC stores and releases oxygen as needed. This results in low oxygen varia-
tions in the post TWC exhaust stream as shown below.
DTC
After the engine and the catalyst are warmed up, and while the
vehicle is driven within the set vehicle and engine speed range,
P0420 the waveforms of the oxygen sensors (bank 1 sensor 1 and
bank 1 sensor 2) have the same amplitude (2 trip detection
logic).
Waveform of
Heated Oxygen Sensor
(before Catalyst)
Waveform of
Heated Oxygen Sensor
(before Catalyst)
2005 CELICA (RM1131U)
DIAGNOSTICS
Catalyst System Efficiency Below Threshold
(Bank 1)
Detecting Condition
Normal Catalyst
Abnormal Catalyst
− ENGINE (1ZZ−FE)
S Front exhaust pipe (catalyst)
S Gas leakage in exhaust system
S Heated oxygen sensor (bank 1 sensor 1)
S Heated oxygen sensor (bank 1 sensor 2)
393
Trouble Area
Waveform of
Heated Oxygen Sensor
(after Catalyst)
Waveform of
Heated Oxygen Sensor
(after Catalyst)
DI−195
DIC6X−02
A20596