DTC Advanced Diagnostics: P0420
DTC P0420:Catalyst System Efficiency Below Threshold

General Description


The powertrain control module (PCM) monitors the catalyst system for low conversion efficiency. The catalyst conversion efficiency is determined by measuring the oxygen storage capacity (OSC). The measured OSC is normalized to the OSC of the best performing unacceptable (BPU) catalyst. To evaluate this parameter, air/fuel ratio (A/F) sensor (sensor 1) and secondary heated oxygen sensor (secondary HO2S (sensor 2)) are implemented on the side of the TWC. The oxygen stored by the catalytic converter during a lean mode operation is partly or completely used up in ensuing rich mode to oxidize the residual hydrocarbons, carbon monoxide, and nitrogen oxides. The OSC of the catalyst, and hence its oxidization efficiency, diminishes with age.
When the enable conditions are met, the enrichment phase is started. This phase continues until the catalyst is purged of oxygen. When the exit conditions are met, the lean phase begins. During this phase, the OSC is measured. The oxygen storage is integrated until the secondary HO2S (sensor 2) voltage is less than a specified value. When the lean phase exit conditions are met, the diagnostics is evaluated. If the measured OSC is a specified value, the PCM detects a malfunction and stores a DTC.

Monitor Execution, Sequence, Duration, DTC Type
Execution
Once per driving cycle
Sequence
None
Duration
0.5 second or more
DTC Type
Two drive cycles, MIL on

Enable Conditions
Condition
Minimum
Maximum
Time not in fuel cut-off operation
10 seconds
–
Outside air temperature
-54 °F (-48 °C)
–
Engine coolant temperature [ECT sensor 1]
-54.47 °F (-48.04 °C)
–
Catalyst temperature
842 °F (450 °C)
–
Engine speed [Engine Speed]
1,200 rpm
4,520 rpm
Barometric pressure [Baro Sensor]
739 hPa (555 mmHg, 21.8 inHg)
–
Vehicle speed [Vehicle Speed]
0 mph (0 km/h)
–
Relative air charge
13 % to 22 %
–
Other
Exhaust gas mass flow is stable for at least 1 second
Actual secondary HO2S (sensor 2) heater temperature is around set point temperature
Following table A must be met for more than 0.08 kg (0.18 lbs) of integrated exhaust gas mass flow
Canister not purging

Table A
Condition
Minimum
Maximum
Catalyst bed temperature
842 °F (450 °C)
1,562 °F (850 °C)
Exhaust gas mass flow
20 kg/h (44 lbs/h)
200 kg/h (441 lbs/h)
Commanded lambda
0.97
1.03
Other
Catalyst bed temperature is stable
[ ]: HDS Parameter

Malfunction Threshold
The number of OSC measurements is 3 or more and the averaged OSC value of the catalyst is less than 1.

Possible Cause
NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
TWC deterioration (aging, poisoning, and/or overtemperature)

Confirmation Procedure
Operating Condition
1.
Start the engine. Hold the engine speed [Engine Speed] at 3,000 rpm without load (in neutral) until the radiator fan comes on.
2.
Drive the vehicle at constant vehicle speed with part load.
Drive the vehicle in this manner only if the traffic regulations and ambient conditions allow.

Diagnosis Details
Conditions for setting the DTC
When a malfunction is detected during the first drive cycle, a Pending DTC is stored in the PCM memory. If the malfunction returns in the next (second) drive cycle, the MIL comes on and a Confirmed DTC and the freeze data are stored.

Conditions for clearing the DTC
The MIL is cleared if the malfunction does not return in three consecutive trips in which the diagnostic runs. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool Clear command.