Service manual

G420F(FE) Service Manual Chapter 9. Advanced Diagnostics 304
Table 2. Diagnostic Fault Codes (Flash Codes) cont’d.
DFC PROBABLE FAULT FAULT ACTION *
CORRECTIVE ACTION
FIRST CHECK
842
LiqPostO2NotActive
Post-catalyst O2 sensor
inactive on gasoline, open
O2 sensor signal or
heater leads, defective
O2 sensor.
(1) TurnOnMil
(2) DisableLiqPost O2Ctrl
Check that Post-catalyst O2 sensor
connections are OK. O2 (return) Pin 4
to SECM Pin B1 O2 Pin 2 (HEATER
GND) to SECM (DRVG GNG)
Pins A16, B17 O2 Pin 1 (HEATER
PWR) to Post O2 Heater Relay. Relay
pin 87.
This relay only turns on after engine
has been running for some time and
SECM has calculated that water
condensation in exhaust has been
removed by exhaust heat.
Post O2 Heater Relay has SECM
(DRVP + 12V) applied to the relay coil
power. The relay coil ground is
controlled by SECM Pin A20 to
activate the relay to flow current
through the post O2 heater. Verify O2
sensor heater circuit is operating by
measuring heater resistance (2.1  ±
0.4 ) O2 Pin 2 (HEATER GND) to
Pin 1 (HEATER PWR)
843 Reserved
851
LiqO2FailedLean
Pre-catalyst O2 sensor
indicates extended lean
operation on gasoline
(1) TurnOnMil
(2) DisableLiquid O2Ctrl
Check for vacuum leaks. Low gasoline
fuel pressure, perform gasoline
pressure test. Injector problems, e.g.
plugged, defective injector
852
LiqPostO2FailedLean
Pre-catalyst O2 sensor
indicates extended lean
operation on gasoline
(1) TurnOnMil
(2) DisableLiqPost O2Ctrl
Correct other faults that may
contribute to 852 (e.g. faults pertaining
to Injectors, MAP, IAT, Pre-Cat O2,
Post Cat O2 sensor.
Look for leaks in exhaust, catalytic
converter, HEGO sensors; repair
leaks. Check all sensor connections
(see fault 842 corrective actions).
871
LiqO2FailedRich
Pre-catalyst O2 sensor
indicates extended rich
operation on gasoline
(1) TurnOnMil
(2) DisableLiquid O2Ctrl
High gasoline fuel pressure, perform
gasoline pressure test Injector
problems, e.g. leaking, defective
injector
872
LiqPostO2FailedRich
Pre-catalyst O2 sensor
indicates extended rich
operation on gasoline
(1) TurnOnMil
(2) DisableLiqPostO2Ctrl
Correct other faults that may
contribute to 872 (e.g. faults pertaining
to Injectors, MAP, IAT, Pre-Cat O2,
Post Cat O2 sensor.
Look for leaks in exhaust, catalytic
converter, HEGO sensors; repair
leaks. Check all sensor connections
(see fault 842 corrective actions).
(*) Fault actions shown are default values specified by the OEM.