Honda Civic 1998 Service Manual page 357

Emission Control System
Three Way Catalytic Converter (TWC) (cont'dl
] P0420 I tne scan tool indicates Diagnostic Trouble Code (DTC) PO42O: Catalyst system efficiency below threshold.
Description
This system evaluates the catalyst's capacity by means of the HO2S (Primary and Secondary) output during stable driving
conditions. lf 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 listed below are stored at the same time as DTC P0420, troubleshoot those DTCS first, then
troubleshoot DTC P0420.
P0137, P0138: Secondary HO2S (Sensor 2)
P0141: Secondary H02S (Sensor 2) Heater
Possible Cause
. TWC Deterioration
. Exhaust system leakage
Troubleshooting Flowchart
-
The MIL hes been reoorted on.
-
DTC P0420 b stored.
Problgm verfficationi
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 with no
load (A/T in E or E position,
M/T in neutral) untilthe radia-
tor fan comes on.
3. Connect the SCS service con-
nector.
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,
Intelmitlent trilure, ayrlom ia OK
at thi3 timc.
Ch.ck the TWC {see section 9}.
lf nocessary, roplace the TWC.
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