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Gregg
A01674
Waveform of
A/F Sensor
Normal Catalyst
Waveform of Heated Oxygen
Sensor behind Catalyst
Abnormal Catalyst
DIAGNOSTICS
− ENGINE
DI
−71
Author: Date:
227
2000 AVALON (RM746U)
DTC P0420 Catalyst System Efficiency Below Threshold
(Bank 1)
CIRCUIT DESCRIPTION
The ECM observes the waveform of the heated oxygen sensor located behind the catalyst to determine
whether the catalyst performance has deteriorated.
If the catalyst is functioning normally, the waveform of the heated oxygen sensor located behind the catalyst
switches back and forth between rich and lean much more slowly.
When the waveform of the heated oxygen sensor located behind the catalyst alternates flutteringly between
rich and lean, it indicates that catalyst performance has deteriorated.
DTC No. DTC Detecting Condition Trouble Area
P0420
After engine and catalyst are warmed up, and while vehicle is
driven within set vehicle and engine speed range, waveform of
heated oxygen sensor alternates flutteringly between rich and
lean (2 trip detection logic).
Front exhauast pipe (front catalyst bank 1)
LH exhaust manifold (front catalyst bank 2)
Center exhaust pipe (rear catalyst)
Gas leakage on exhaust system
Heated oxygen sensor
A/F sensors
DI6T5
−01
A18908
Idling
IG SW off
Warm up 4 min. 16 min.
(64 to 88 km/h)
40 to 55 mph
(56 to 72 km/h)
35 to 45 mph
DI
−72
DIAGNOSTICS
− ENGINE
Author: Date:
228
2000 AVALON (RM746U)
CONFIRMATION DRIVING PATTERN
(a) Connect the hand
−held tester to the DLC3.
(b) Clear DTC.
(c) Warm−up the engine.
(d) Drive the vehicle at 40 − 55 mph (64 − 88 km/h) for 4 minutes.
(e) Drive the vehicle allowing speed to fluctuate between 35 − 45 mph (56 − 72 km/h) for 16 minutes.
(f) Check pending DTC.
NOTICE:
Drive with smooth throttle operation and avoid sudden acceleration.
DIAGNOSTICS
− ENGINE
DI
−73
Author: Date:
229
2000 AVALON (RM746U)
INSPECTION PROCEDURE
Read freeze frame data using the hand
−held tester or OBD II scan tool. Because freeze frame records the
engine conditions when the malfunction is detected. When troubleshooting, it is useful for determining
whether the vehicle was running or stopped, the engine was warmed up or not, the air−fuel ratio was lean
or rich, etc. at the time of the malfunction.
1 Are there any other codes (besides DTC P0420) being output?
YES Go to relevant DTC chart (See page
DI−14).
NO
2 Check gas leakage on exhaust system.
NG Repair or replace.
OK
3 Check A/F sensor (See page
SF−79).
NG Repair or replace.
OK
4 Check heated oxygen sensor (See page
SF−81).
NG Repair or replace.
OK
Replace front and rear catalysts (See page
EC
−12 and EC−15).
Replace all of the front exhaust pipe (front catalyst bank 1), LH exhaust manifold (front catalyst bank
2) and center exhaust pipe (rear catalyst).
Linear Mode
