Diagnostic Trouble Code (DTC) P0420 DTC P0420 - Catalyst Low Efficiency

Diagnostic Trouble Code (DTC) P0420

DTC P0420 - Catalyst Low Efficiency


Features of the Circuit

  • Here a Three-Way Catalytic Converter (TWC) is used  to control exhaust emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Nitrogen Oxide (NO).
  • The key function of the catalyst inside the converter is to boost a chemical reaction which oxidizes the HC and CO present in the exhaust gas, transforming them into harmless water vapor and carbon dioxide(CO2) simultaneously reducing NO, and converting it into nitrogen(N).
  • The catalytic converter is  specially equipped   to store oxygen.
  • The Engine Control Module (ECM) is developed to monitor this process using a Heated Oxygen Sensor (HO2S) located in the exhaust stream past the TWC.
  •  The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn highlighting the catalyst's ability to convert exhaust emissions effectively and efficiently.
  •  The ECM monitors the catalyst efficiency by allowing the catalyst to heat up and wait for a stabilization period while the engine is idling, finally refilling and removing fuel instantaneously while monitoring the reaction of the HO2S.
  •  If the catalyst operates flawlessly, the HO2S response to the extra fuel is slow compared to the Oxygen Sensor (O2S).
  •  When the HO2S response is near that of the O2S, the Oxygen storage capacity of the catalyst is considered poor, and the Malfunction Indicator Lamp (MIL) will be switched on.


Time for setting the DTC

Task boosted after DTC sets

When will DTC clear?

1) Vehicle speed varies between 60km/h(37.3mph) and 76km/h(47.2mph).



1) The Malfunction Indicator Lamp (MIL) will glow after three successive trips with a fail.

1) The MIL will turn off after four consecutive ignition cycles in which the diagnostic runs without a fault.

2) There is Closed loop stoichiometry existing.

2) The ECM will begin to store conditions which were present when the DTC failed.


2) Disconnecting the ECM battery feed for 10 seconds

3) Engine Coolant Temperature (ECT) is > 70°C(158°F)

3) This information will be recorded in the Freeze Frame buffering Failure Records.


3) The scan tool can be used to remove all existing DTCs.


4) Engine speed ranges between 1,760rpm and 2,530rpm

4) A history DTC will be stored.


4) A history DTC will clear after 40 consecutive warm-up cycles without a fault.

5) The manifold absolute pressure > 76 kpa



6)  The activity of the heated oxygen sensor boosted by lambda controller stimuli remains > a threshold.




7) DTC(s) P0107, P0108, P0117, P0118, P0122, P0123, P0131, P0132, P0133, P0137, P0138, P0140, P0141, P0171, P0172, P0300, P0336, P0337, P0341, P0342, P0351, P0352, P0404, P0405, P0405 and P0562 are not sets.






Diagnostic Assistance

  • A change in the engine load may stop the catalyst test.
  •  The engine load (i.e. A/C, coolant fan, heater motor) must never be tampered   while a catalyst test is in progress.
  • An intermittent   problem may be created by a poor connection, rubbed-through wire insulation, or a wire that is damaged inside the insulation.
  • Any circuitry, that is believed to create the intermittent complaint, should be thoroughly inspected for the following conditions:
  1. Broken Locks
  2. Improper mating
  3. Improperly formed
  4. Backed-out terminals
  5. Damaged terminals
  6. Poor terminal-to-wire connection 



DTC P0420 - Catalyst Low Efficiency







Ensure an EOBD System Check.

Task Over?

 Follow Step 2

Follow "On-Board Diagnostic System Check"


  1. Insert a scan tool to the Data link Connector (DLC).
  2. Turn ON the ignition.

Can you see any component DTC set?

Follow  applicable DTC table

Follow Step 3


  1. Undertake a Visual check of the following:

a)       Exhaust system for a leak.

b)       Heated Oxygen Sensor (HO2S).

Task Over?

Follow Step 4

Follow Step 5


Work on the exhaust system as needed.

Job Accomplished?

Follow Step 6



Change the Three Way Catalytic Converter (TWC).

Job done?

Follow Step 6



  1. Unset all DTCs taking a Scan Tool.
  2. Start the engine and idle at normal operating temperature.
  3. Operate the vehicle within the specified conditions for setting this DTC

Do you see the scan tool confirming that this diagnostic has been operated and the test passed?

Follow Step 7

Follow Step 2


Check if any additional DTCs are set.

Are any DTCs displayed that have not been diagnosed?

Follow Applicable DTC table

 The system of your vehicle is fine.