What is the cause of fault code P0420?
3 Answers
P0420 fault code indicates that the catalytic converter system efficiency is below the threshold. This may be due to the use of leaded gasoline damaging the converter, issues with the oxygen sensor, or damage to the exhaust pipe or catalytic converter leaks. Related information is as follows: 1. To resolve this fault, first understand that P0420 (Catalyst System Efficiency Below Threshold Bank 1) means the ECU monitors the efficiency using sensors installed before and after the Three-Way Catalytic Converter (TWC). 2. The effect caused by a catalyst in a chemical reaction is called catalysis, and catalysts are also known as promoters in industry. The composition, chemical properties, and mass of the catalyst remain unchanged before and after the reaction; its relationship with the reaction system is like that of a lock and key, exhibiting high selectivity (or specificity).
I've seen the P0420 trouble code a lot over my years of car repair. It indicates catalytic converter efficiency below threshold - basically means the exhaust treatment system isn't working properly. The most common causes are: a faulty rear oxygen sensor (since it specifically monitors the cat's performance), or the catalytic converter itself getting clogged due to aging/too much carbon buildup. Exhaust leaks (like loose connections) can also trigger it. Poor engine combustion is another frequent culprit - worn spark plugs or clogged fuel injectors allowing unburned fuel to damage the converter. For diagnosis, first check the voltage difference between front/rear O2 sensors via live data. DIYers can listen for exhaust abnormalities or inspect for rust, but a professional shop scan is recommended. Cost-wise, sensor replacement runs a few hundred, while catalytic converter replacement is significantly more expensive.
Last month, my car's P0420 trouble light came on, which really scared me, so I quickly looked it up. It means the catalytic converter efficiency is below threshold, and the core issue could be a faulty downstream oxygen sensor, or the converter itself might be clogged or leaking. Engine problems like unstable oil pressure or a rich fuel mixture can also trigger it, making the car noticeably sluggish and increasing fuel consumption. I tried diagnosing it myself: using an OBD scanner to confirm the code, then listening for any hissing sounds from the exhaust system indicating leaks. After replacing the rear oxygen sensor, the light went off, saving me a lot of money. If you're going the DIY route, I recommend starting with the sensor to avoid blindly replacing the catalytic converter—that thing costs over a thousand bucks. Remember to maintain your car regularly and avoid aggressive acceleration or frequent short trips, as these can prevent issues in the first place.
