If your check engine light is on and the code reads P0420, you’re likely looking for answers. The P0420 Honda Accord 2007 is a common diagnostic trouble code that signals a problem with your car’s emissions system.
This code specifically points to the catalytic converter not working as efficiently as it should. It’s a problem you shouldn’t ignore, as it can affect your car’s performance and lead to a failed emissions test. This guide will explain what it means, why it happens, and give you clear steps on how to diagnose and fix the issue.
P0420 Honda Accord 2007
The P0420 code stands for “Catalyst System Efficiency Below Threshold (Bank 1).” In simpler terms, your car’s computer (the Engine Control Module, or ECM) has detected that the catalytic converter on your 2007 Honda Accord is not cleaning the exhaust gases as well as it’s supposed to. The “Bank 1” part refers to the side of the engine that contains cylinder number one, which on most inline 4-cylinder engines like yours, is the only bank.
Your Accord has two oxygen sensors (O2 sensors) monitoring the catalytic converter: one before it (upstream) and one after it (downstream). The ECM compares the readings from these two sensors. If the downstream sensor’s signal starts to look too similar to the upstream sensor’s, it means the converter isn’t doing its job of changing the exhaust gases, and the ECM triggers the P0420 code.
What Does the Catalytic Converter Do?
Before we go further, it’s helpful to understand the part at the center of this code. The catalytic converter is a key part of your exhaust system. Its job is to convert harmful pollutants in the exhaust gas into less harmful substances before they leave the tailpipe. It uses precious metals like platinum, palladium, and rhodium as catalysts to facilitate chemical reactions:
* It turns carbon monoxide (a poisonous gas) into carbon dioxide.
* It helps burn unburned fuel (hydrocarbons).
* It reduces nitrogen oxides (which contribute to smog).
When it fails, these pollutants are released directly into the atmosphere, and your car will not pass an emissions inspection.
Common Symptoms of a P0420 Code
You might notice one or more of these signs along with the illuminated check engine light:
* Check Engine Light: This is the primary indicator. The light may be steady or, in some cases, flashing if a severe misfire is also present.
* Reduced Fuel Economy: A failing converter or related issue can cause your Accord to use more gas than usual.
* Lack of Power or Poor Acceleration: You might feel like the car is sluggish or “robbed” of power, especially when trying to accelerate.
* Rotten Egg Smell from Exhaust: A failing catalytic converter can sometimes produce a strong sulfur smell.
* Excessive Heat Under the Car: A clogged converter can overheat, creating noticeable heat under the center of the vehicle.
Most Common Causes of P0420 in a 2007 Accord
While a bad catalytic converter is the direct cause the code points to, it’s often a symptom of another problem. Replacing the converter without fixing the root cause will lead to the new one failing prematurely. Here are the typical culprits, starting with the most likely:
1. A Failed Catalytic Converter: After 15+ years and high mileage, the internal structure can simply wear out, melt, or become clogged with debris.
2. Faulty Oxygen Sensor (O2 Sensor): The downstream sensor (Sensor 2, after the converter) is most often suspected. A lazy or incorrect sensor can send false data to the ECM, tricking it into thinking the converter is bad.
3. Exhaust Leaks: A leak before or between the oxygen sensors (like at the exhaust manifold, a cracked pipe, or a bad gasket) can allow outside air in. This skews the oxygen sensor readings and can cause a P0420.
4. Engine Misfires or Running Rich: If the engine is dumping too much unburned fuel into the exhaust (from bad spark plugs, ignition coils, a leaky fuel injector, or a faulty engine sensor), it can overheat and destroy the catalytic converter. This is a very common root cause.
5. Fuel System Issues: A leaking fuel injector or a faulty fuel pressure regulator causing a rich condition.
6. Faulty Engine Sensors: Problems with mass airflow (MAF) or coolant temperature (ECT) sensors can cause the engine to run rich.
How to Diagnose a P0420 Code Step-by-Step
Important Safety Note: Always let the exhaust system cool completely before working on it. Wear safety glasses and use proper jack stands if lifting the car.
Tools You Might Need: A basic socket set, O2 sensor socket, a scan tool or code reader that can show live data, and a vacuum gauge or smoke machine for leak testing (optional but helpful).
Step 1: Check for Other Codes
Use your OBD2 scanner to read all stored codes. A P0420 accompanied by misfire codes (P0300-P0304), fuel trim codes (P0171, P0172), or other O2 sensor codes is a major clue. Always address any other codes first, as they are likely the root cause.
Step 2: Inspect for Obvious Exhaust Leaks
With the engine cold, visually and audibly inspect the entire exhaust system from the manifold back. Look for:
* Sooty black marks at joints
* Rust holes or cracks in the pipes
* Loose or missing bolts at flanges
* Damaged or blown-out gaskets
Start the engine and listen for a ticking or hissing sound, especially near the manifold and ahead of the catalytic converter. You can carefully (avoid moving parts) feel around joints for puffing air.
Step 3: Look at Live Oxygen Sensor Data
This is the most effective diagnostic step if you have a scan tool that can display live data. You need to graph or watch the voltage readings from Bank 1 Sensor 1 (upstream) and Bank 1 Sensor 2 (downstream).
* Get the engine to normal operating temperature.
* Observe the upstream sensor’s voltage. It should fluctuate rapidly between roughly 0.1V and 0.9V.
* Now observe the downstream sensor. A properly working catalytic converter will “smooth out” the signal. The downstream voltage should be fairly stable, usually hovering around a midpoint (e.g., 0.5-0.7V) with slow changes.
* The Problem: If the downstream sensor waveform starts to mimic the rapid up-and-down pattern of the upstream sensor, it confirms the catalytic converter is not storing and releasing oxygen properly—it’s inefficient. This strongly points to a bad converter.
Step 4: Check the Rear O2 Sensor
While live data is best, you can do a basic test. Unplug the electrical connector for the downstream O2 sensor (it’s usually on the exhaust pipe under the car, behind the converter). With it unplugged, the ECM will use a default value. Clear the code and drive the car for a day or two. If the P0420 code does not return, the downstream O2 sensor itself was likely faulty. If the code comes back quickly, the converter is probably the issue.
Step 5: Perform a Backpressure Test (for a clogged converter)
This requires a pressure gauge. You’ll need to remove the front oxygen sensor and screw in the gauge’s adapter.
* Start the engine and let it idle. Note the pressure reading.
* Slowly increase and hold engine RPM to around 2500. Note the reading again.
* Excessive backpressure (typically over 1.5 psi at idle or over 3 psi at 2500 RPM) indicates a clogged or restricted catalytic converter.
How to Fix the P0420 Code
The fix depends entirely on your diagnosis. Follow this logical repair order.
Fix 1: Address Underlying Engine Problems
If you found misfire or rich condition codes, fix those first. This may involve:
* Replacing spark plugs and ignition coils.
* Fixing vacuum leaks.
* Cleaning or replacing the mass airflow sensor.
* Replacing a leaky fuel injector.
After repairs, clear the codes and drive for a few days to see if P0420 returns.
Fix 2: Repair Exhaust Leaks
Replace any bad gaskets (like the exhaust manifold gasket or donut gasket). Weld or replace cracked pipes. After fixing leaks, clear the code and test drive.
Fix 3: Replace an Oxygen Sensor
If your live data or unplug test pointed to the sensor, replace the downstream O2 sensor. Use an OEM or high-quality direct-fit sensor. Remember to apply anti-seize compound only to the threads if it’s not pre-applied (and avoid getting it on the sensor tip). Clear codes and test.
Fix 4: Replace the Catalytic Converter
If all other causes are ruled out and live data confirms converter failure, replacement is necessary. You have a couple options:
* OEM (Honda) Catalytic Converter: Most reliable and guaranteed to work, but very expensive.
* EPA-Compliant Aftermarket Converter: Must be labeled for use on your 2007 Honda Accord. This is the legal and recommended aftermarket choice. It will be less expensive than OEM.
* “Universal” or “Direct-Fit” Non-Compliant Converters: These are often cheaper but may not meet EPA emissions standards for your state and could cause you to fail inspection. They are illegal for street use in many areas.
Replacement is a significant job. It involves unbolting or cutting the old converter out and welding or clamping a new one in place. Rusted bolts and nuts are a major challenge. Professional installation is often recommended.
Cost to Fix P0420 on a 2007 Honda Accord
Costs vary wildly based on the root cause and whether you do the work yourself.
* Downstream Oxygen Sensor: Part = $60 – $150. Professional labor = $100 – $200. Total: ~$160 – $350.
* Catalytic Converter (Part Only): EPA-compliant aftermarket converter = $300 – $800. OEM Honda converter = $1,000 – $1,500+.
* Catalytic Converter (Professional Replacement): With aftermarket part and labor = $700 – $1,200+.
* Spark Plugs/Ignition Coils: If they are the cause, this is a much cheaper fix, often under $300 for parts and labor.
Can You Drive With a P0420 Code?
You can typically drive for a short while, but it’s not advisable long-term.
* If the light is steady: The car is likely drivable, but you may see worsening fuel economy and performance. There’s also a risk of the converter becoming completely clogged, which can leave you stranded.
* If the light is flashing: This indicates a severe misfire that is dumping raw fuel into the converter. Stop driving immediately to prevent destroying the converter. Have the car towed to a repair shop.
* You will fail your next emissions test until the problem is fixed.
FAQ Section
Q: Can a P0420 code be caused by a loose gas cap?
A: No, a loose gas cap typically triggers an evaporative emissions code like P0455. It will not cause a P0420.
Q: Will a P0420 code clear itself?
A: Sometimes, if it was triggered by a temporary condition. However, if the code is persistent and the monitor runs again, it will likely come back. It’s best to diagnose the issue properly.
Q: Can I clean my catalytic converter instead of replacing it?
A: There are “catalytic converter cleaner” additives, but they are generally ineffective for a converter that is already failing and triggering a P0420. They might help with minor buildup in a slightly degraded unit, but don’t count on it as a fix.
Q: Is it okay to use a spacer or “non-fouler” trick on the O2 sensor to bypass the P0420 code?
A: This is a temporary, band-aid fix that involves moving the downstream sensor out of the exhaust stream to trick the ECM. It is not a real repair, it’s illegal for street use as it bypasses emissions equipment, and it may not work on all vehicles or pass a visual inspection.
Q: My Accord has a P0420 but runs fine. Should I worry?
A: Yes. While it may drive okay now, the underlying issue will not improve and will likely get worse, leading to more expensive repairs down the road. It’s also polluting more than it should.
Dealing with a P0420 Honda Accord 2007 code requires a methodical approach. Start with the simple and inexpensive checks—like looking for exhaust leaks and checking live O2 sensor data—before assuming the catalytic converter is bad. Often, the fix is less scary than the code itself. By following this guide, you can save money and get your reliable Accord back to running cleanly and efficiently.