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cip0420


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                                                                                                                                  DI-77
                                                 DIAGNOSTICS        -   ENGINE
                                                                                                                                  DI6VC-04




    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 behind 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 behind 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 Heated Oxygen                                                                   Waveform of Heated Oxygen
                                                                 Normal Catalyst
        Sensor before Catalyst                                                                      Sensor behind Catalyst




                                                                                                                                   FI7081




   DTC No.                      DTC Detecting Condition                                                  Trouble Area
              After the engine and the catalyst are warmed up, and while the
                                                                                S Gas leakage on exhaust system
             vehicle is driven within the set vehicle and engine speed range,
                                                                                S Open or short in heated oxygen sensor circuit
   P0420     the waveforms of the oxygen sensors (bank 1 sensor 1 and
                                                                                S Heated oxygen sensor
             bank 1 sensor 2) have the same amplitude
                                                                                S Three-way catalytic converter
             (2 trip detection logic)




 2002 ECHO (RM884U)

                                                                                 Author :                     Date :              247
DI-78
                                             DIAGNOSTICS   -   ENGINE

CONFIRMATION ENGINE RACING PATTERN

          Engine Speed
                                                                  (c)               (d)
  2,500  3,000 rpm




                               (b)
               Idling
           IG SW OFF
                         (a)
                                    Warmed up              3 min. or so            Check        Time
                                                                                                         FI7132




(a)   Connect the TOYOTA hand-held tester to the DLC3, or connect the probe of the oscilloscope between
      terminals OX1, OX2 and E1 of the ECM connector.
(b) Start engine and warm it up with all accessories switched OFF until water temp. is stable.
(c) Race the engine at 2,500 - 3,000 rpm for about 3 min.
(d) After confirming that the waveforms of the oxygen sensor, bank 1 sensor 1 (OX1), oscillate around 0.5
      V during feedback to the ECM, check the waveform of the oxygen sensor, bank 1 sensor 2 (OX2).
                                           HINT:
       OX Signal Waveform (Oscilloscope)
                                            S    If there is a malfunction in the system, the waveform of the
   1.0 V                                         oxygen sensor, bank 1 sensor 2 (OX2), is almost the
                                                 same as that of the oxygen sensor, bank 1 sensor 1
                                                 (OX1), on the left.
                                            S    There are some cases where, even though a malfunction
     0V
                                                 exists, the MIL may either light up or not light up.

             200 m sec. /Division         FI6514




INSPECTION PROCEDURE
HINT:
Read freeze frame data using TOYOTA 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 warmed up or not, the air-fuel ratio 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 ).



 2002 ECHO (RM884U)

                                                                        Author :           Date :            248
                                                                                            DI-79
                                  DIAGNOSTICS   -   ENGINE



  NO



  2     Check gas leakade on exhaust system.


                                          NG        Repair or replace exhaust system.


  OK



  3     Check oxygen sensor (bank 1 sensor 1) (See page DI-48 ).


                                          NG         Repair or replace oxygen sensor (bank 1 sen-
                                                    sor 1).


  OK



  4     Check oxygen sensor (bank 1 sensor 2) (See page DI-57 ).


                                          NG         Repair or replace oxygen sensor (bank 1 sen-
                                                    sor 2).


  OK



 Replace three-way catalytic converter.




2002 ECHO (RM884U)

                                                         Author :          Date :            249



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