Honda Civic Service Manual page 523

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lTo420l Th" """n toot indicates Diagnostic Troubte code
(DTc) p0420: catatyst sysrem efficiency below threshold.
Description
This system evaluates the catalyst's capacity by means of the Ho2s {Primary and secondary)
output during stable driving
conditions. ll deterioration has been detected during two consecutive driving cycles. the
MIL comes on and DTC P0420 will
be stored.
NOTE: lf some of the DTCS tisted below are stored at the same time as DTC P0420, troubleshoot
those DTCS first. then
recheck for DTC P0420
P0137, P0138: Secondary HO2S (Sensor 2)
Po14'1:Secondary
H02S {Sensor 2) Heater
Possibl€ Cause
. TWC Deterioration
. Exhaust system leakage
Troubleshooting Flowchart
I
The MIL has been reported on.
DTC P0420 is stored
Problem verilication:
1. Do the ECM/PCM Reset Proce
dure.
2 . S t a r t t h e e n g i n e H o l d t h e
engine at 3,000 rpm wath no
load {in Park or neutral) until
the radiator fan comes on.
3. Connect the SCS service con
4 . T e s t d r i v e 4 0 - 5 5 m p h f o r
a p p r o x , t w o m i n u t e s T h e n
decelerate for at least 3 sec_
o n d s w i t h t h e t h r o t t l e c o m '
pletely closed- Then reduce
the vehicle speed to 35 mph,
a n d t r y t o h o l d i t u n t i l t h e
readiness code comes on
Intermittent Iailure, system is OK
at this time.
l s D T C P 0 4 2 0 i n d i c a t e d ?
Check the TWC {see section 9l
lf necessary, replace the TwC