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Code PO420... Keeps Coming On!

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when i look up the code "po420" it comes up as a 3 way catalyst low efficiency or some crap. what could be causing this? i reset the computer and in a week it comes back. it also will say "pcm 1599 history" These Crackalacs are driving me nuts

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The PCM tests the ability of the catalytic converter to lower exhaust emissions. The PCM determines the performance of the catalytic converter by calculating the oxygen storage capacity The PCM calculates this data by comparing the oxygen content of the exhaust entering the catalyst (read from the front and rear HO2 sensors) to that leaving the catalyst (read from the post HO2 sensor).

At the appropriate time, the PCM transitions the air fuel mixture from rich to lean or lean to rich depending on the current state of HO2S bank 1 sensor 2 (rich or lean state). Then, the PCM initiates a second, and opposite transition. The PCM compares the amount of time for the fuel control sensors to cross a calibrated threshold to the amount of time for HO2S bank 1 sensor 2 to cross the same threshold. The longer it takes HO2S bank 1 sensor 2 to cross the threshold the higher the oxygen storage capacity. An HO2S bank 1 sensor 2 that crosses the threshold close to the same time as the front and rear HO2 sensors indicates a catalyst with low oxygen storage capacity, and DTC P0420 will set.

Conditions for Running the DTC

No TP sensor, MAF sensor, MAP sensor, IAT sensor, ECT sensor, HO2S, HO2S Heater, Fuel trim, Misfire, EVAP System, EVAP Control, VSS, or Idle Speed DTCs are set.

Engine speed since end of last idle period is more than 800 RPM for 46 seconds.

Idle period is less than 180 seconds (3 minutes).

Actual engine speed is within 100 RPM of desired engine speed (Non-California)

Actual engine speed is within 125 RPM of desired engine speed (California).

Engine run time is more than 7.5 minutes.

The predicted catalyst temperature is between 420°C and 650°C (788°F and 1202°F).

BARO is more than 75 kPa.

ECT is between 71°C (160°F) and 120°C (248°F).

IAT is above -6°C (21°F).

Engine is in closed loop fuel control.

Tests attempted this ignition cycle is less than 12.

Test attempted during idle period is less than 1.

Conditions for Setting the DTC

Catalytic converter efficiency is too low.

Action Taken When the DTC Sets

The PCM will illuminate the malfunction indicator lamp (MIL) during the first trip in which the diagnostic test has been run and failed.

The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC

The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.

The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

The DTC can be cleared by using the scan tool Clear DTC Information function.

Diagnostic Aids

Fuel with a high sulfur content can cause a marginal converter to look bad. Check fuel quality.

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