6.
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.
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.
The ECM sets P0420 if the average of caculated oscillation size of rear HO2S signal during predetermined duration is higher than the predetermined threshold value.
Item | Detecting Condition | Possible cause |
Monitoring Strategy | ● Oxygen storage capacity ● * Monitoring amplitude of HO2S,compared to modelled signal amplitude. | ● Warm-up catalytic converter deteriorated. |
Threshold value | ● Malfunction Criteria : 1.75 X NMOG standard > 0.65 -> 50% of HC Conversion Eff.Of MCC | |
Enable Conditions | ● Engine speed MT 1200∼2520rpm ,AT 1000-1920rpm | |
● Engine load (rl, curve) MT 16-35%,AT 18-40% | ||
● Calculated(Modelled) Catalyst temp. 470∼650℃(878~1202℉) | ||
● Fuel system status Closed loop control | ||
Diagnostic Time | ● 110 sec | |
MIL on condition | ● 2 driving cycle |
If any codes relating to Oxygen(HO2S) sensor.MAP, injector, a P0171,P0172 are present, do all repairs associated with them before proceeding with this troubleshooting procedure.
For front HO2S
When decelerating suddeniy from 4000rpm : 200mV or less
When engine is suddenly raced : 600~1000mV
1500rpm : The fluctuation between 400mV or less and 600~1000mV
For Rear HO2S
Part load(Approx. 2000rpm) : 500~1000mV
This figures show the waveform 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 ① should be below 0.2V.
The switching time marked ② from lean to rich should be within 200ms,if longer than 200ms HO2S may be not good.
The switching time marked ③ 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.
