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 represents 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 calculated oscillation size of rear HO2S signal during predetermined duration is higher than the predetermined threshold.
DTC Detecting Condition
Item | Detecting Condition | Possible Cause |
DTC Strategy | ? Check catalyst oxygen storage capacity by evaluation downstream O2 sensor fluctuations | ? Exhaust gas leaks ? Faulty rear HO2S ? Faulty three way catalyst converter |
Enable Conditions | ? Coolant temperature >74?(165?) ? 5< Vehicle speed(km/h) <180 ? Engine speed < 3400rpm ? 200 < Mass air flow(mg/rev.) < 700 ? Canister load < 0.5 ? 400?(752?) = Catalyst temp. model = 900?(1652?) ? Lambda control active & Stable driving condition ? Downstream O2 sensor operative readiness ? No opening / closing of Canister Purge Valve ? No relevant failure ? 11V < Battery voltage < 16V |
Threshold Value | ? Average malfunction index >0.3 |
Diagnostic Time | ? 50 Lambda Controller Cycles |
MIL On Condition | ? 2 Driving Cycles |
Signal Waveform
Scan Tool Display for HO2STest Condition | Scan Tool Parameter |
O2 SNSR VOL.-B1/S1 | O2 SNSR VOL.-B1/S2 |
Normal Value when circuit is normal | Idle after warm up | Signal is switching from rich(above 0.45V) to lean(below 0.45V) a minimum of 3 times in 10 seconds. | above 0.7V |
HO2S(B1S1) signal circuit open | Approx. 0.43~0.45V | - |
HO2S(B1S2) signal circuit open | - | Approx. 0.43~0.45V |