DTC P0420: Diagnostic Trouble Code Breakdown
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P0420: A DTC Breakdown

Over time, you get used to certain codes, when they set, what cars they occur on more often and the mileage at which they occur. Many times, when a car comes in with the check engine light on, we like to guess what it might be based on these factors. Even though we know we may not be right, it’s sometimes just a fun challenge.

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One of the guesses that we are often right about is DTC P0420 Catalyst System Efficiency Below Threshold, and in this DTC breakdown we’re going to take a closer look at the code.


DTC P0420 is stored when the readings of the pre- and post-cat oxygen sensors indicate that the catalytic converter is not working as efficiently as it should be in accordance with pre-programmed specifications.


Each catalytic convertor will have an oxygen sensor before it and after it. When the engine is warmed up and running in closed loop, the control unit monitors the reading of each sensor. Since the catalytic converter reduces the harmful emissions from the exhaust, when it is working properly the readings between the two sensors will differ. At a predetermined threshold of the sensor readings approaching similarity, the control unit interprets that the converter is not working efficiently and will store DTC P0420.


More to Know:

It’s easy to blame the converter, especially if the vehicle has over 100,000 miles on it, and the truth is that the converter is quite often the cause of the problem in these situations. It’s an expensive mistake to make however, and there are a number of other things that may cause this code to set including a failed oxygen sensor, damaged oxygen sensor wiring, a poor connection at the oxygen sensor connector, leaking exhaust, a leaking fuel injector, high fuel pressure or cylinder misfire and oil contamination can cause the problem as well. Always perform a complete diagnosis prior to recommending a repair.



Although there are many possible causes for this code, some of which will exhibit noticeable driveability symptoms, the majority of the time your customer will not experience or realize a symptom aside from MIL illumination.


If there are any other trouble codes stored, always consider the possibility of their relation to DTC P0420. Check for any exhaust leaks, no matter how small, repair any driveability or fuel/ignition problems and always look at basic operating parameters including coolant temperature.

Use a scope to look at oxygen sensor operation, comparing the waveforms of front and rear sensors, as well as confirming oxygen sensor heater operation.


Before You Begin:

Always look for manufacturer information and TSBs. As with all computer-related diagnostics, the possibility of reprogramming or an electrical problem exists. Manufacturer information can save you a lot of time and trouble.

Also, research the system you are working on and determine the logic behind how it works prior to looking at any data or beginning any diagnostics. There are some systems that differ from what has been described here and fully understanding the system before you begin is the only way to properly diagnose it.

Additional Tips:

If you determine the converter is the cause of the problem, use caution when selecting a new one. OEM or direct fit aftermarket converters are generally the best choice. In some cases, aftermarket converters, especially inexpensive or universal ones, will not perform adequately to keep efficiency readings above threshold. It could save the customer money but cost you in the long run.


As an added service to your customer, prior to replacing a converter, make a call to a local dealer and check the vehicle warranty with the service department, just to make sure it’s not still covered.

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