Component Location
General Description
The ECM uses dual oxygen sensors to monitor the efficiency of the manifold catalytic converter (warm-up catalytic converter). By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream (front) HO2S is used to detect the amount of oxygen in the exhaust gas before it enters the catalytic converter. A low voltage indicates high oxygen contents (lean air mixture). A high voltage indicates low oxygen contents (rich air mixture). When the catalyst efficiency drops, no chemical reaction takes place. This means the concentration of oxygen will be the same at the rear as it is at the front. The output voltage of the rear HO2S copies the voltage of the front HO2S.To monitor the system, the lean-to-rich switches of the front HO2S to the rear HO2S is counted. The ratio of rear switches to front switches is used to determine whether the catalyst is operating properly. An effective catalyst will have fewer rear switches than front switches, that is, a ratio closer to zero.
DTC Description
The ECM calculates oscillation size of rear HO2S signal which respresents catalyst conversion properties. This oscillation size will determine if catalyst conversion is low due to aging or poisoning from leaded fuel or misfiring. The ECM sets P0420 if the average of caculated oscillation size of rear HO2S signal during predetermined duration is higher than the predetermined threshold. If same error code is set in the next driving cycle, the ECM illuminates the MIL.
DTC Detecting Condition
Item | Detecting Condition | Possible cause |
Monitoring Strategy | ? Oxygen storage capacity | ? Warm-up catalytic converter deteriorated. ? Leak in intake system. ? Faulty HO2S. ? Faulty ECM |
Threshold value | ? Difference of ’amplitude of downstream O2 sensor and ‘amplitude of model signal’ : > 0.55 |
Enable Conditions | ? Engine speed 1800 ~ 3200rpm ? Engine load 15 ~ 40% ? Catalyst temperature(model) 530 ~ 760?(986~1400?) ? Purge factor (high load canister) < 10 ? Closed loop control |
Diagnostic Time | ? 100 sec |
Fail Safe | ? None |
Specification
HO2S FR | Normal Parameter(at 1300 rpm after warming up) |
Rich | Lean |
Signal Voltage | 0.6 ~ 1.0 V | 0 ~ 0.4 V |
Hi-scan Data display | Engine condition |
When deceleration suddenly from 4000rpm | 200 mV or lower |
When engine is suddenly raced | 600 ~ 1000 mV |
HO2S RR | Normal Parameter(at 2000 rpm after warming up) |
Signal Voltage | 500 ~ 1000 Mv |
Schematic Diagram
Signal Waveform
This figures show the wave form measured at terminal 2 of HO2S at idle RPM.
The average voltage of HO2S should be 0.5 V.
When sudden decceleration,the voltage at point ? should be below 0.2V.
The switching time marked ? from lean to rich should be within 200ms,if longer than 200ms HO2S may be not good.
The switching time marked ? from rich to lean should be within 300ms,if longer than 300ms HO2S may be not good.
In case the average voltage of HO2S is over 0.6V.
Possible cause:Injector leakage,faulty Pressure regurator Faulty MAPS,ECTS. |
The rear oxygen sensor output is usually not changeable like front oxygen sensor,however,if it moves like front oxygen sensor,the catalytic converter is broken.