How to tell if your Honda has a bad catalytic converter

Most Hondas with a failing catalytic converter will show a Check Engine light with codes P0420 (Bank 1) or P0430 (Bank 2 on V6s), feel sluggish, smell like rotten eggs, rattle from the exhaust, or fail an emissions test. You can confirm by scanning for codes, checking oxygen sensor data, and measuring exhaust backpressure or temperature; a qualified mechanic can verify with professional tests before you replace anything.

Why the catalytic converter matters

The catalytic converter turns harmful exhaust gases (HC, CO, NOx) into less harmful emissions. On Hondas, an upstream air-fuel ratio sensor (often called a wideband O2 sensor) measures mixture before the converter and a downstream O2 sensor monitors the converter’s efficiency. Converters can fail from age and mileage, engine misfires, running too rich or too lean, oil or coolant contamination, physical impact, or prolonged short-trip driving that never gets the converter hot enough.

Common signs your Honda’s catalytic converter is failing

The following list outlines the symptoms most owners notice when a converter starts to go bad. Recognizing several of these together strengthens the case for a failing catalytic converter.

• Check Engine light with P0420 (Bank 1) or P0430 (Bank 2 on V6 models)
• Sluggish acceleration, poor high‑RPM power, or a “choking” feel at sustained speeds
• Rotten‑egg (sulfur) smell from the exhaust, especially after heavy throttle
• Rattling from under the car on startup or acceleration (broken catalyst substrate)
• Excessive heat from the converter or it glowing red after driving (dangerous condition)
• Drop in fuel economy compared with your baseline
• Failed or borderline emissions test (elevated HC/CO and sometimes NOx)

One symptom alone isn’t definitive, but a combination—especially emission codes plus performance changes—often points to catalytic converter trouble.

Quick checks you can do at home

These steps can help you narrow down the issue before heading to a shop. Work on a cool exhaust and use eye/hand protection; catalytic converters can reach 1,000°F+ during operation.

1. Scan for codes: Use an OBD‑II scanner. Note P0420/P0430, misfire codes (P0300‑P030x), fuel trim or O2 sensor codes—misfires or rich/lean faults can damage or mimic a bad converter.
2. Listen and smell: With the car idling, listen for a metallic rattle under the floor or near the manifold. Sniff for a sulfur/rotten‑egg odor after a short drive.
3. Temperature check (rough screen): With an IR thermometer after a 15–20 minute mixed drive, the outlet of the converter should generally be hotter than the inlet (often by ~100°F under load). A cooler outlet may indicate poor conversion; an extreme glow suggests overheating. This is a screening test, not definitive.
4. Exhaust flow feel: At the tailpipe (in Park/Neutral with brake applied), have a helper briefly raise RPM to ~2,500. Weak, wheezy flow or a dramatic drop as RPM is held can hint at restriction.
5. Basic visual inspection: Look for recent scrapes/dents, missing heat shields, or fresh black soot around joints that could indicate exhaust leaks upstream of the converter.

These checks can indicate a problem but won’t prove it on their own; they help you decide whether to seek a deeper diagnostic.

Professional diagnosis your mechanic might perform

A shop can confirm converter health with tools and data your home setup may not provide. Here’s what a proper workup typically includes.

• Live data analysis: Compare upstream A/F sensor activity (should switch or reflect commanded mixture) to downstream O2 sensor (should be relatively stable on a healthy cat). If downstream mirrors upstream, efficiency is low.
• Exhaust backpressure test: Gauge installed in an O2 sensor port. Healthy exhaust typically measures under ~1.5 psi at idle and under ~3 psi at 2,500 rpm. Readings above ~3–5 psi indicate restriction.
• Manifold vacuum trend test: At a steady 2,500 rpm, a gradual vacuum drop suggests exhaust restriction.
• Smoke/leak test: Finds exhaust leaks ahead of the converter that can trigger false P0420/P0430 by letting in outside air.
• Misfire and fuel control analysis: Checks plugs, coils, injectors, fuel trims, and compression—root causes that can kill a new converter if left unaddressed.
• Five‑gas emissions analysis (where available): Confirms elevated HC/CO/NOx consistent with low catalyst efficiency.

Combining these methods gives a reliable diagnosis and helps avoid replacing an expensive part unnecessarily.

Problems that can mimic a bad catalytic converter

Before condemning the converter, rule out these common culprits that can set the same codes or cause similar symptoms.

• Exhaust leak before or at the converter (manifold, gaskets, flex pipe, flanges)
• Faulty downstream O2 sensor or upstream air‑fuel ratio sensor
• Ignition misfires (plugs, coils), vacuum leaks, or intake air leaks
• Rich running (leaky injector, failed fuel pressure regulator) or lean condition (unmetered air)
• Oil burning (worn rings, valve seals, PCV issues) or coolant intrusion (head gasket)—contaminates the catalyst
• Recent battery disconnect or code clearing causing monitors to reset; early trips may flag efficiency before the system relearns

Fixing these issues first can restore proper readings—or at least protect a new converter from rapid failure.

What to do if it really is the converter

Once testing confirms low catalyst efficiency or restriction, plan the repair with these considerations in mind.

• Check warranty: In the U.S., catalytic converters are covered by a federal emissions warranty for 8 years/80,000 miles. In CARB states, some PZEV‑rated Hondas carry extended emissions coverage up to 15 years/150,000 miles—verify by VIN and emissions label.
• Use correct part: Many Hondas use a manifold‑integrated converter. In CARB states, you must install a CARB‑approved unit. OEM parts tend to last longer and avoid drivability issues.
• Fix root causes first: Resolve misfires, fuel trim faults, leaks, or oil/coolant consumption before installing a new converter.
• Legal note: It’s illegal to remove or tamper with emissions equipment; replacing with compliant parts is required.
• Security: If converter theft is a risk in your area, consider an anti‑theft shield and secure parking after replacement.

Addressing underlying engine or exhaust issues first helps your new converter last and keeps you compliant with emissions laws.

Costs and timing

Converter replacement cost varies by model and configuration. The list below reflects typical 2024–2025 U.S. shop pricing; local labor rates and parts availability will affect your quote.

• Civic/Accord/CR‑V 4‑cyl manifold‑integrated converter: roughly $900–$2,000 with OEM parts; $500–$1,200 with aftermarket (CARB‑compliant higher)
• V6 models (dual banks): $1,500–$3,000+ if both banks are replaced
• Labor time: 1.5–4.0 hours per bank depending on rust/corrosion and sensor access
• Related parts: new gaskets, hardware, and often new O2 sensors to avoid seized or damaged fittings

After replacement, your shop will clear codes and run drive cycles to complete emissions monitors; you may need 50–150 miles of mixed driving to set readiness before an inspection.

Prevention tips

Good maintenance habits significantly extend catalytic converter life and reduce the risk of repeat failures.

• Keep up with spark plugs, coils, and air filters to prevent misfires
• Fix oil leaks and address oil consumption; use the correct oil and viscosity
• Repair coolant leaks promptly; coolant in the exhaust will poison the catalyst
• Avoid silicone sealants not labeled “sensor‑safe” on the intake/exhaust
• Take periodic longer highway drives to fully heat the converter if you mostly do short trips
• Use quality fuel from reputable stations; avoid long‑term operation on E85 unless your Honda is rated for it

These practices help keep the engine in tune, the converter at proper operating temperature, and emissions in check.

When it’s probably not the cat

If your Honda just started showing efficiency codes after recent work or unusual conditions, consider these scenarios before committing to replacement.

• Recent exhaust or O2 sensor replacement creating a small upstream leak or wiring issue
• Battery disconnect or code clear causing incomplete monitors and premature P0420/P0430 flags
• Cold weather and repeated short trips preventing the converter from lighting off
• Bad tank of fuel causing temporary rich/lean operation; issues resolve after refuel

A quick recheck after repairs, completion of drive cycles, or fixing small leaks often resolves these transient efficiency codes.

Summary

If your Honda has a Check Engine light with P0420/P0430, sluggish performance, a sulfur smell, rattles, or overheating at the converter, you may have a failing catalytic converter. Confirm with OBD‑II data, leak checks, temperature or backpressure tests, and rule out misfires and sensor faults. If replacement is needed, verify emissions warranty coverage, fix root causes first, and install the correct compliant part to ensure long‑term reliability and emissions compliance.

How to check if your catalytic converter is bad?

To check if your catalytic converter is bad, look for symptoms like a rotten egg smell, poor engine performance, a sulfur smell, a check engine light, and rattling noises. You can also perform tests such as using an OBD-II scanner to read fault codes (P0420 or P0430), checking for a temperature difference between the converter’s inlet and outlet, or performing a vacuum test to detect exhaust back pressure. 
This video explains the symptoms of a bad catalytic converter and how to diagnose it: 45sHVAC Mechanic YouTube · Sep 1, 2023
Symptoms to Look For

• Rotten egg smell/Sulfur smell: A strong odor of sulfur or rotten eggs indicates that the catalytic converter is not properly filtering exhaust gases. 
• Poor engine performance: You may experience reduced power, sluggish acceleration, or engine misfires. 
• Check Engine Light: An illuminated check engine light is a common indicator of a catalytic converter issue. You can use an OBD-II scanner to read fault codes starting with P042 or P043, which point to converter problems. 
• Rattling noise: A damaged or broken catalytic converter can produce a rattling sound. 
• Excessive heat: A severely clogged converter can cause the underside of the vehicle to become unusually hot. 
• Failed emissions test: A failing catalytic converter can prevent your vehicle from passing an emissions test. 

Diagnostic Tests You Can Perform

• Exhaust Temperature Test: . Opens in new tabUsing an infrared thermometer, measure the exhaust temperature before and after the catalytic converter. The outlet should be significantly hotter (100-150 degrees Fahrenheit) than the inlet if the converter is working correctly. 
• Vacuum Test: . Opens in new tabConnect a vacuum gauge to a vacuum line on the engine and slowly increase the RPMs. If the vacuum continues to drop rather than rise and level off, it could mean the catalytic converter is clogged. 
• OBD-II Scan Tool: . Opens in new tabUse a scanner to read any stored fault codes from your vehicle’s computer. Codes like P0420 or P0430 often indicate a problem with the catalytic converter. 
• Check for Rattling: . Opens in new tabWith the car safely raised, gently wiggle the exhaust system to listen for any rattling, which signals that the internal materials have broken apart. 
• Oxygen Sensor Disconnection: . Opens in new tabAs a quick test, you can temporarily disconnect an oxygen sensor or loosen a connection upwind of the converter. If the vehicle’s performance improves, a blockage is likely present, though this is illegal in many areas. 

This video demonstrates how to test a catalytic converter for back pressure: 59sAMSOIL INC.YouTube · Nov 21, 2023

How long do Honda catalytic converters last?

A Honda catalytic converter is designed to last for the lifetime of the vehicle, or around 100,000 miles, but its lifespan can be significantly shortened by factors like engine issues, poor maintenance, or fuel system problems. A failing catalytic converter may show signs such as a rotten egg smell, poor engine performance, dropping fuel economy, or a rattling noise, and a professional inspection should be performed if these symptoms occur.
 
Factors affecting lifespan

• Engine problems: Misfiring engines can send too much unburnt fuel into the catalytic converter, damaging it. 
• Contamination: Contaminants from the engine or exhaust system can clog the converter. 
• Physical damage: External damage can also reduce a catalytic converter’s lifespan. 
• Maintenance: Regular maintenance ensures your engine is running efficiently, helping the catalytic converter last longer. 

Signs of a failing catalytic converter

• Rotten egg smell: A sulfurous smell from the exhaust is a key indicator. 
• Reduced engine performance: A decrease in acceleration and power can signal a clogged converter. 
• Poor fuel economy: Dropping gas mileage often points to a failing catalytic converter. 
• Rattling noise: A rattling sound from under the vehicle can mean the internal components are broken. 
• Black smoke: Dark smoke from the tailpipe can be a symptom of a malfunctioning converter. 

What to do if you suspect a problem

• Get it inspected: If you notice any of these symptoms, have your vehicle inspected by a qualified service technician. 
• Address engine issues: Before replacing the converter, ensure any underlying engine problems that may have caused the failure are fixed. 

How much does it cost to replace a catalytic converter on a Honda?

Labor costs are estimated between $145 and $213 while parts are priced between $1,894 and $2,521.

What are four signs of failing a catalytic converter?

If you notice any of these symptoms, it’s best to take your vehicle to a specialist for inspection.

• Rattling Noise. Catalytic converters make a sound if they start to fail.
• Blue Emissions.
• Reduced Engine Performance.
• Low Gas Mileage.
• “Check Engine” Light Comes On.
• Sulfur Smell from Exhaust.
• Failed Emissions Test.