Circuit Description
Three-way catalytic converter (TWC) efficiency is measured by how well it can
store oxygen. The PCM monitors converter efficiency by comparing the voltage values
of the oxygen sensor 1 (O2S-1) and heated oxygen sensor 2 (HO2S-2).
Under normal operating conditions, the O2S-1 should vary between 10 mV and
1065 mV and the H02S-2 should remain relatively steady between 500 mV
and 800 mV. This steady reading of the H02S-2 indicates a correctly functioning
catalytic converter. When all parameters have been met, the PCM will run a 5 second
DTC P0420 diagnostic at idle. The PCM will command rich and monitor the time
it takes the HO2S-2 to go rich. It will then command lean and monitor the time it
takes the H02S-2 to go lean. The longer it takes the H02S-2 to change rich/lean means
the converter is storing oxygen and is functioning properly. If the five second test
fails, the PCM may take several tests during several ignition cycles to set the DTC.
DTC P0420 sets when the PCM has determined that the catalytic converter is
no longer efficient.
Conditions for Setting the DTC
DTC P0420 will set if the PCM has determined the catalytic converter
to be inefficient by using the O2S-1 and HO2S-2 when:
| • | Engine run time is greater than 8 minutes and 40 seconds |
| • | Engine speed is at idle within -75 to 150 RPM of the desired idle
speed |
| • | Engine speed over 1000 RPM for 35 seconds (auto), or 45 seconds
(manual) after achieving closed loop |
| • | Engine has not been at idle for more than 45 seconds. Once off
idle, counter resets. |
| • | ECT is between 70°C-125°C (158°F-257°F) |
| • | IAT is between -21°C-80°C (-6°F-176°F) |
| • | Calculated converter temp is between 501°C-627°C (933°F-1161°F) |
| • | Vehicle speed is less than 5 km/h (3 mph) |
| • | Calculated air flow is less than 8 g/s |
| • | Ignition voltage is greater than 9 volts |
| • | Barometric pressure is greater than 72 kPa |
| • | No cam, CKP, ECT, EVAP, fuel tank pressure, FC relay, fuel injector, fuel
trim, HO2S-2, IAT, idle speed, knock, MAP, misfire, O2S-1, PCM internal fault, TP
or VSS DTCs have been set |
DTC P0420 diagnostic is sampled once at idle once the above conditions
have been met. If that test fails, the PCM will take more samples during the current
ignition cycle or may take more samples up to six ignition cycles. The extra sampling
is to ensure that a converter has truly failed.
P0420 is a (type A) DTC
Diagnostic Aids
Important: Make sure HO2S-2 is tight in the exhaust pipe. A ground path must exist between
the exhaust pipe and engine block. Add a ground wire between the exhaust manifold
and engine head if resistance is greater then 10 ohms. DTC P0420 may
set if no ground path exists.
Make sure no exhaust leaks exist between the O2S-1 and 18 inches after
the HO2S-2. An exhaust leak in that area will cause false HO2S-2 readings and may
set DTC P0420. Repair a leak if one exists.
A misfiring cylinder(s) pushing raw fuel out the exhaust port will damage the
converter over time. Verify that no current or history misfire counts exist in any
failure record.
Repair a currently misfiring cylinder(s) if one exists.
Replace the underfloor catalytic converter.
