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Toyota Sienna Service Manual: Catalyst System Efficiency Below Threshold

DTC P0420 Catalyst System Efficiency Below Threshold (Bank 1)

DTC P0430 Catalyst System Efficiency Below Threshold (Bank 2)


The ECM uses the sensors mounted in front of and behind the three-way catalyst (TWC) to monitor its efficiency. The first sensor, an Air Fuel ratio (A/F) sensor, sends pre-catalyst A/F ratio information to the ECM. The second sensor, a heated oxygen sensor (O2S), sends post-catalyst information to the ECM.

The ECM compares these 2 signals to judge the efficiency of the catalyst and the catalyst's ability to store oxygen. During normal operation, the TWC stores and releases oxygen as needed. The capacity to store oxygen results in a low variation in the post-TWC exhaust stream.

If the catalyst is functioning normally, the waveform of the heated oxygen sensor slowly switches between RICH and LEAN. If the catalyst is deteriorated, the waveform will alternate frequently between RICH and LEAN. As the catalyst efficiency degrades, its ability to store oxygen is reduced and the catalyst output becomes more variable. When running the monitor, the ECM compares sensor 1 signals (A/F sensor) over a specific amount of time to determine catalyst efficiency. The ECM begins by calculating the signal length for both sensors (for the rear oxygen sensor, the ECM uses the output voltage signal length). If the oxygen sensor output voltage signal length is greater than the threshold (threshold is calculated based on the A/F sensor signal length), the ECM concludes that the catalyst is malfunctioning. The ECM will turn on the MIL and a DTC will be set.


DTC Detection Condition

Trouble Area


Oxygen Storage Capacity (OSC) value is smaller than standard value under active air-fuel ratio control (2 trip detection logic)
  • Gas leakage from exhaust system
  • A/F sensor (bank 1 sensor 1)
  • HO2 sensor (bank 1 sensor 2)
  • Exhaust manifold (TWC)


OSC value is smaller than standard value under active air-fuel ratio control (2 trip detection logic)
  • Gas leakage from exhaust system
  •  A/F sensor (bank 2 sensor 1)
  •  HO2 sensor (bank 2 sensor 2)
  •  Exhaust manifold (TWC)



Related DTCs P0420: Catalyst Deterioration
P0430: Catalyst Deterioration
Required Sensors / Components (Main) TWC
Required Sensors / Components (Related) A/F sensor, heated oxygen sensor, intake air temperature sensor, mass air flow meter, crankshaft position sensor and engine coolant temperature sensor
Frequency of Operation Once per driving cycle
Duration Approximately 30 seconds
MIL Operation 2 driving cycles
Sequence of Operation None


The monitor will run whenever these DTCs are not present P0011, P0012 (VVT System 1-Advance, Retard), P0021, P0022 (VVT System 2- Adavance, Retard), P0031, P0032, P0051, P0052 (A/F Sensor Heater Sensor 1), P0037, P0038, P0057, P0058 (O2 Sensor heater Sensor 2), P0100, P0101, P0102, P0103 (MAF Sensor), P0115, P0116, P0117, P0118 (ECT Sensor), P0120, P0121, P0122, P0123, P0220, P0222, P0223 , P2135 (TP Sensor), P0125 (Insufficient ECT for Closed Loop), P0136, P0156 (O2 Sensor 2), P0171,P0172 (Fuel System), P0300, P0301, P0302, P0303, P0304, P0305, P0306 (Misfire), P0335 (CKP Sensor), P0340 (CMP Sensor), P0351, P0352, P0353, P0354, P0355, P0356 (Ignitor), P0500 (VSS), P2196, P2198 (A/F Sensor (Rationality)), P2A00, P2A03 (A/ F Sensor (Slow Response))
Battery voltage 11 V or more
IAT -10C (14F) or more
ECT 75C (167F) or more
Atmospheric pressure 0.75 or more
Idle OFF
Engine RPM Less than 3200 rpm
A/F sensor Activated
Fuel system status Closed loop
Engine load 10 to 70%
All of the following conditions are met Conditions 1, 2 and 3
1. Mass air flow rate 5 to 60 g/sec
2. Front catalyst temperature (estimated) 600 to 750C (1112 to 1382F)
3. Rear catalyst temperature (estimated) 100 to 900C (212 to 1652F)
Rear HO2S monitor Completed
Shift position 4th or more


Oxygen Storage Capacity (OSC) of catalyst Less than 0.046 g (0.000101 lb)




HINT: Perform the operation with the engine speeds and durations described below prior to checking the waveforms of the A/F and HO2 sensors. This is in order to activate the sensors sufficiently to obtain the appropriate inspection results.

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