If your catalytic converter starts to go bad, it can lead to all sorts of performance issues, from sluggish acceleration to failed smog tests. Knowing the symptoms of a failing catalytic converter is key to catching problems early. In this post, we’ll break down the top signs of a bad cat, and what you should do if you notice these signs.
Catalytic Converter Basics
In previous articles we've covered the basics of all-things-cats:
- What Is A Catalytic Converter?
- What's The Difference Between CARB-Compliant and CARB-Exempt Cats?
- What Kind Of Cat Do You Need?
Now, let’s dive into the catalytic converter failure warning signs - when your cat starts failing, it’ll let you know with some clear catalytic converter failure symptoms. Check out the links below to find cats of all types, and read on to learn what problems to watch for.
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Top Symptoms Of A Failing Catalytic Converter
A bad cat can mess with your ride’s performance and emissions compliance. However, OE catalytic converters are designed to last the life of the vehicle. Be sure you take the time to diagnose exactly what damaged the original converter before replacement. Replacing a converter without addressing what caused the original to fail means your replacement may also fail similarly. Here are the top signs of a failing catalytic converter to keep on your radar:
1. Check Engine Light
The most common clue your cat’s in trouble is that pesky check engine light. Your car’s ECU and OBD-II system monitors the cat’s efficiency. If it’s not cleaning exhaust gases properly, the ECU will throw codes, commonly P0420 or P0430, among others. These codes often mean your cat’s clogged, damaged, or just worn out.
- What You’ll Notice: A glowing check engine light on your dash.
- What to Do: Grab an OBD-II scanner (or head to an auto parts store) to read the code. If it’s P0420 or P0430, your cat could be the culprit, but check O2 sensors or exhaust leaks first to rule out other issues.
2. Sluggish Performance or Poor Acceleration
A failing cat can choke your exhaust system, causing backpressure that strangles your engine’s performance. If your ride feels like it’s lost its pep, it could be a clogged catalytic converter.
- What You’ll Notice: Slow acceleration, trouble hitting high speeds, or a general “bogged down” feel, especially under heavy throttle.
- What to Do: Have a mechanic check for a clogged cat. They might use a back pressure test or inspect the cat visually to confirm it’s restricting exhaust flow.
3. Rotten Egg Smell
Ever catch a whiff of a sulfur-like, rotten egg stench coming from your exhaust? That’s a classic sign of a failing catalytic converter. When the cat can’t properly convert hydrogen sulfide (a byproduct of fuel combustion) into odorless sulfur dioxide, you get that nasty smell.
- What You’ll Notice: A funky, eggy odor from your tailpipe, especially when idling or accelerating.
- What to Do: Don’t ignore this - it could mean your cat’s failing or your engine’s running too rich (burning extra fuel), overloading the cat. Get it checked to avoid further damage.
4. Rattling or Noisy Exhaust
If your cat’s internal core (that honeycomb structure) starts breaking apart, it can rattle like a tin can full of marbles. This usually happens when the cat’s overheated, been damaged by road debris, or just worn out from high mileage.
- What You’ll Notice: A rattling or clunking noise from under your car, especially when starting up or idling.
- What to Do: Crawl under your ride (safely) or have a mechanic inspect the cat. A broken core means it’s time for a catalytic converter replacement.
5. Poor Fuel Economy
A clogged or failing cat can mess with your engine’s ability to breathe, forcing it to work harder and burn more fuel. If your gas mileage has tanked without a clear reason, your catalytic converter's health could be to blame.
- What You’ll Notice: You’re hitting the gas station more often, with lower MPG than usual.
- What to Do: Pair this symptom with others (like a check engine light or sluggish performance) and have your cat tested. A restricted exhaust flow is a common culprit.
6. Overheating or Dark Exhaust Smoke
A severely clogged cat can cause your engine to run hotter than normal, as exhaust gases can't flow freely. In bad cases, you might see dark or excessive smoke from your tailpipe, signaling the cat’s not doing its job.
- What You’ll Notice: Higher engine temps (check your gauge) or unusual smoke from the exhaust.
- What to Do: Stop driving immediately to avoid engine damage. Have a mechanic inspect the cat and exhaust system ASAP.
Why These Symptoms Matter
Ignoring catalytic converter warning signs can lead to big headaches and failed smog tests in states where they're required (like California, Colorado, or New York). Additionally, a bad cat pumps out more pollutants, contributing to smog and air pollution and other negative environmental impacts - no one wants that.
Catching these catalytic converter failure symptoms early can save you time, money, and a whole lot of frustration. Regular maintenance - like fixing misfires, using quality fuel, and checking O2 sensors - can keep your cat alive longer. Some likely culprits behind catalytic converter problems include:
- Clogging: Unburned fuel, oil, or coolant from engine issues (like a bad head gasket) can clog the cat’s honeycomb core, reducing its efficiency.
- Overheating: Running your engine too rich (too much fuel) or misfiring can overheat the cat, melting or cracking the core.
- Physical Damage: Hitting road debris, speed bumps, or curbs can crack the cat’s casing or internals.
- Age and Mileage: Most cats last 100,000 miles or more, and some even make 200,000 miles. But they will wear out at higher mileage.
- Contamination: Leaking oil or coolant from engine problems can coat the cat’s core, making it less effective.
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