If you own a Honda, seeing a “Catalyst System Efficiency Below Threshold” warning light can be a real worry. This common Honda engine issue points to a problem with your car’s emissions control system, specifically the catalytic converter.
It’s a message you shouldn’t ignore. While it might not stop your car from running immediately, it can lead to failed emissions tests, reduced performance, and potentially costly repairs down the line. Understanding what this code means, why it happens in Hondas, and what you can do about it is key to getting your vehicle back to health.
Catalyst System Efficiency Below Threshold and Honda
This warning is tied to a specific diagnostic trouble code (DTC): P0420. Your car’s computer, the Engine Control Module (ECM), uses oxygen sensors before and after the catalytic converter to monitor its efficiency. If the downstream sensor readings are too similar to the upstream sensor, the ECM decides the converter isn’t cleaning the exhaust gases properly—its efficiency is “below threshold.”
For Honda owners, this code is a frequent visitor. It’s not always because the catalytic converter itself has suddenly failed. Often, it’s the result of another underlying problem that has damaged the converter over time.
Why is This Code So Common in Honda Vehicles?
Several factors make Hondas, especially popular models like the Accord, Civic, CR-V, and Odyssey, prone to triggering the P0420 code.
– Precise Engine Management: Honda engines are finely tuned for performance and emissions. Their ECMs are very sensitive to even slight changes in exhaust gas composition.
– High-Temperature Operation: Catalytic converters need to be hot to work. Honda’s engine designs and placement can sometimes lead to converters running at very high temperatures for long periods, which can accelerate wear.
– Mileage and Age: Many Hondas are built to last for hundreds of thousands of miles. The catalytic converter is a wear item, and it’s common for it to lose efficiency after 100,000 miles or more.
– Specific Engine Issues: Certain Honda engine families have known problems that directly lead to converter damage.
The Primary Culprit: Underlying Engine Problems
A failing catalytic converter is rarely the first problem. It’s usually the victim. Here are the most common engine issues that cause this Honda fault.
1. Misfiring Engine Cylinders
This is the number one cause. When a spark plug, ignition coil, or fuel injector fails, unburned fuel gets dumped into the exhaust. This fuel then ignites inside the incredibly hot catalytic converter, causing internal melting or breakdown of the precious metals inside. If you’ve had recent misfire codes (like P0300-P0304), they likely started the chain reaction.
2. Faulty Oxygen (O2) Sensors
The sensors themselves can fail. A lazy or incorrect downstream O2 sensor can send false data to the ECM, tricking it into thinking the converter is bad when it’s actually fine. The upstream sensor, if faulty, can also cause the engine to run too rich or too lean, damaging the converter over time.
3. Exhaust Leaks
A leak before the upstream oxygen sensor (like a cracked manifold or leaking gasket) allows outside air to enter. This extra oxygen fools the sensor, making the ECM think the engine is running lean. It then over-corrects by adding more fuel, potentially damaging the converter. A leak between the sensors can also skew readings.
4. Oil or Coolant Consumption
Some Honda engines, particularly older models, are known to burn a bit of oil or have minor coolant leaks. When oil or coolant enters the combustion chamber and is burned, it leaves deposits that coat and clog the honeycomb structure of the catalytic converter, rendering it useless.
Step-by-Step Diagnosis: What to Check First
Before you resign yourself to buying a new catalytic converter, follow these steps. They can save you a lot of money.
1. Read All Codes: Use an OBD2 scanner to pull all stored codes, not just P0420. Codes for misfires (P0300), fuel trim (P0171, P0172), or other O2 sensor faults give you the real starting point.
2. Check for Technical Service Bulletins (TSBs): Visit a Honda dealer website or forum with your model, year, and engine. Honda has issued TSBs and even warranties for catalytic converter issues on specific models.
3. Inspect for Obvious Damage: Look under the car for visible damage to the exhaust system, rust holes, or sooty deposits indicating a leak.
4. Monitor Live Data: With a scanner that can read live data, check the pre-cat and post-cat O2 sensor waveforms. A good converter will show a dampened, slower signal from the downstream sensor compared to the rapidly switching upstream sensor. If they look identical, the converter is likely bad.
5. Perform a Visual/Physical Inspection: If possible, tap the converter gently (when cold). A rattle inside means the internal substrate is broken apart. Also look for dents or areas that have turned blue from extreme heat.
Your Repair Options, From Least to Most Expensive
Option 1: Address the Root Cause
If you find a misfire, bad sensor, or leak, fix that first. Clear the codes and drive the car for a few days. Sometimes, if the converter wasn’t severely damaged, the P0420 code may not return.
– Replace spark plugs and ignition coils if needed.
– Swap out a faulty upstream or downstream oxygen sensor.
– Repair any exhaust leaks.
Option 2: Catalytic Converter Cleaner
This is a low-cost, low-effort try for mild cases. Pour a bottle of quality catalytic converter cleaner into your fuel tank. Drive the car on the highway for a while to get the exhaust system very hot. This can sometimes burn off light carbon deposits. It’s not a guaranteed fix, but it’s worth a shot before major repairs.
Option 3: Replace the Catalytic Converter
If the converter is truly dead, replacement is the only option. You have three choices:
– OEM (Honda) Converter: The most expensive but guaranteed to fit and perform perfectly. Often required for California emissions vehicles.
– Aftermarket “Direct-Fit” Converter: Less expensive, designed to bolt directly in. Ensure it’s EPA-certified for street use.
– Universal Converter: Cheapest, but requires welding and fabrication by a muffler shop. Also must be EPA-certified.
Warning: Never simply remove the catalytic converter (“straight-pipe” it). This is illegal for street use in all US states, will cause your check engine light to stay on, and will make your car fail emissions testing.
Prevention: How to Protect Your Honda’s Catalytic Converter
The best repair is the one you avoid. Keep your Honda’s converter healthy with these habits.
– Fix Check Engine Lights Immediately: A flashing light especially means a active misfire—stop driving and get it towed to prevent immediate converter meltdown.
– Stick to Regular Maintenance: Change spark plugs at the recommended interval. Use good quality fuel.
– Address Oil Consumption: If your Honda is burning oil, investigate the cause (like worn piston rings or valve seals) sooner rather than later.
– Avoid Short Trips: Frequently driving only short distances prevents the exhaust system from fully warming up, leading to moisture and contaminant buildup inside the converter. Take it on a 20-minute highway drive regularly.
Frequently Asked Questions (FAQ)
Can I drive my Honda with the P0420 code?
You can usually drive for a short while, but you should adress it soon. Performance may suffer, fuel economy can drop, and you will fail any required emissions inspection. If the light is flashing, stop driving.
How much does it cost to fix a Honda catalytic converter?
Costs vary widely. An aftermarket direct-fit converter plus installation can range from $800 to $1,500. An OEM Honda converter can cost $1,500 to $2,500 or more with labor.
Will a new oxygen sensor fix a P0420 code?
Only if the sensor is actually faulty. Use live data to diagnose the sensor first. Throwing parts at the problem without diagnosis gets expensive.
Is the catalytic converter covered under Honda’s warranty?
The basic new-car warranty covers it for 8 years/80,000 miles. For certain models and years, Honda has issued extended warranties specifically for the catalytic converter—check with a dealer.
What’s the difference between P0420 and P0430?
P0420 refers to the efficiency of Bank 1 (the engine bank containing cylinder #1). P0430 is the same issue for Bank 2 on V6 engines. The cause and repair are identical.
Dealing with a “Catalyst System Efficiency Below Threshold” warning in your Honda can be frustrating, but it’s a manageable problem. The key is to think of the catalytic converter as a symptom. By methodically checking for underlying issues like misfires or leaks, you might find a simpler and less expensive fix. If the converter itself is indeed failed, understanding your replacement options helps you make a smart, cost-effective decision. With the right approach, you can clear that check engine light and restore your Honda’s efficiency and performance for many more miles to come.