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Repair Information for P0420 Honda code. Learn what does P0420 Honda Catalyst System Efficiency Below Threshold Bank 1 means and how to diagnose p0420 code

Diagnosing a P0420

Tests/Procedures: 1. Verify that the fuel trims on both banks are good at idle and when driving the vehicle. It is possible to have the fuel trim at 20% and not set a fuel trim code and cause the catalytic converter to not function properly.

2. If the fuel trim is only off on one bank, look for a faulty Air/Fuel (A/F) Ratio sensor, a vacuum leak or a fuel delivery problem on the bank the fuel trim is off on.

3. If the fuel trim is good and within 10% plus or minus, drive the vehicle at a steady throttle, steady cruise and monitor the rear O2 sensor operation. If the rear O2 sensor is switching rich to lean, the catalytic converter is not storing oxygen correctly. When the catalytic converter functions correctly, the rear O2 sensor should not switch. If the rear O2 sensor switches at a steady cruise while driving, the catalytic converters are failing.

4. Since this vehicle uses Air/Fuel (A/F) Ratio sensors instead of front O2 sensors, if the vehicle has high mileage, it is always wise to replace the front A/F Ratio sensors if the catalytic converters are faulty and need to be replaced. In some cases, the A/F Ratio sensors can cause a false code P0420.

On the vehicle in the video, The only reason the cat was replaced on this car was due to rust and it set the p0420 the day after. And here is the follow-up video to the first on. A complete guide to diagnosing a p0420 on a Honda CRV

Below are some very useful comments left on my subreddit post. That could definitely be useful in diagnosing a p0420 code.

Why Does a Catalytic Converter Fail? When a CAT fails an important part of the repair is determining why it failed in the first place and correcting the problem so the new one does not have a repeat failure. CATS have no moving parts and no consumable components, so they should last the life of the vehicle; if a CAT fails there is always some kind of external cause involved. This can be physical damage from an impact or vibration, poisoning with a foreign substance such as fuel system cleaners or oil additives, or from an engine burning oil or coolant. A very common cause of CAT failure is fuel contamination from a misfire condition or an engine that is constantly running rich; any unburned fuel exiting the engine hits the CAT and burns there, which raises temperatures inside the CAT high enough to melt and damage the internal ceramic honeycomb causing it to crumble apart. This is usually visible if you look inside the old cat when it is removed. Did a Misfire Cause my Catietic Converter to fail? That is probably what is happening in the car in the video. If you watch the front O2 it is at a pretty constant 600+ mv, which is a rich condition; a normal front O2 should be switching frequently above and below 450mv, more like the rear O2 is doing. The sensor is alive and operational, when he snaps the throttle open you can see the voltage spike low momentarily from all the extra air entering the engine when the throttle is opened. That engine is dumping unburned fuel down the exhaust, and that is the likely cause of both of the converter failures although the mechanic in the video does not mention nor check it. Odds are if he installs a third one it too will be back with a P0420 failure in a few weeks. That engine probably has high fuel pressure, a dripping injector, a vacuum leak (which drops MAP so the ECM thinks the engine is under load), a faulty thermostat causing the engine to run too cold, or an out of calibration intake air temp or coolant temp sensor causing the rich fuel mixture, or engine misfire that is killing the CATs.