P0420 HONDA Catalyst System Efficiency (Bad Cat??)

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?

P0420 Failed Catalytic Converters — Failed Catalytic Converters

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.

Using Fuel Trim Data to Diagnose a P0420 code (Rich or Lean Condition)

Response:

So, this is your original content? I did not realize you created the video; That’s awesome!

I was looking at your scan PIDs in the video and noticed that one was front O2 and it was hanging at 600mv; that must be a calculated value in the scan tool itself then. It might be interesting to look at the fuel trims to see if the PCM is commanding a lean correction in response to a rich mixture. If your trims are centered then there probably is not a mixture problem, but if fuel trims are maxed out lean I think I would investigate a little to find out why.

Air/fuel sensors are sent a bias voltage from the PCM; I believe the exact voltage is different for different manufacturers. The output voltage from the sensor varies above and below that median (lambda). Lambda goes negative when the mixture is rich, and positive when the mixture is lean. It is supposedly a much more precise measurement of fuel mixture than a traditional O2 sensor can provide.

I read somewhere that you can convert lambda value (if it shows on your scan tool) to actual air/fuel ratio measurement by multiplying lambda by 14.1

Response:

Thanks for the info. Ya, the video was a little vague. Where the customer didn’t have any problems with CEL before we put the new cat in. The old exhaust was rotted out. That was the original reason for replacement. SO I didn’t spend a whole lot of time on diagnostic. I justed cleared the light and had the customer drive it for a few days. If the CEL comes back on, I will do a new, much more in-depth video. Been close to a week now though. Oddly enuff I have had new catalytic converters set p0420 code within a day of installation then code never come back again. I’ve never heard off breaking in a new cat. but in these very few causes it certainly seemed that way. I’m anxiously waiting to see if this car returns. I’ll be sure to update you. It was an aftermarket cat, which I’m not too fond of.

Yeah, aftermarket CATS scare me also. I have learned the hard way to never use aftermarket CATS on a Subaru, for example. The expensive OE cats on Subarus seem to have barely enough catalyst material for them to pass the cat monitor, so what do you think a cheaper aftermarket one will do? The last time I tried an aftermarket CAT on a Subaru it was from a US manufacturer called Eastern Catalytic, which was supposed to be a top quality brand according to my sales rep. I spent hours on the phone with their tech support and even had several long phone conversations with one of their engineers when the second CAT they sent was setting P0420 faults. They never did figure out what the problem was, but after I installed an OE Subaru one the fault was gone and never returned. Of course, I never got paid for any of those hours I had to spend chasing the problem after I installed their product. If sell an aftermarket cat because the customer insists on one, I have them sign a disclaimer on the RO that states converter related fault codes will NOT be covered under our repair warranty if the MIL should come on afterward.

P0420 codes and Exhaust Leaks

By the way, you didn’t say where it was rusted, but if the original CAT you replaced was rusted through and leaking, should there not have been a Catalyst fault stored because of the air entering the exhaust stream before the repair? If not the customer may have cleared the fault code before you got the car; the first thing many people do if their MIL pops on is stop in at Autozone and have their memory wiped. Maybe this is a pre-existing condition they never bothered to tell you about. Of course, that never happens, right?

Response:

Diagnosing a P0420

Why Does a Catalytic Converter Fail?

Did a Misfire Cause my Catietic Converter to fail?

O2 sensor and Air Fuel sensors Look the same but work Differently.

Using Fuel Trim Data to Diagnose a P0420 code (Rich or Lean Condition)

Leaking Exhaust or Bad Catietic Converter?

After Market Catiletic Converters VS OEM

P0420 codes and Exhaust Leaks

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