🚗 Diagnosing Catalytic Converter Issues the Right Way
At Accomplished Auto, we believe in fixing the root of the problem – not just the symptom.
One of the most misunderstood parts of your car’s emissions system is the catalytic converter (or "cat" for short). It plays a critical role in cleaning up the exhaust gases before they exit your vehicle. But when a warning light pops up for something like a “catalyst efficiency” code (P0420 or P0430), it doesn’t always mean the catalytic converter itself has failed.
In fact, many times it’s doing its job just fine, and something else upstream – like how your engine is running – is causing the problem. That’s why we take the time to test properly, not just replace parts.
🔍 Why a Proper Diagnosis Matters
Some repair shops may jump straight to replacing the catalytic converter, which can cost you over a thousand dollars – unnecessarily. But at Accomplished Auto, we follow a structured 1-hour diagnostic process designed to confirm the true cause.
Here’s what that looks like (don’t worry – we’ll keep it simple):
✅ Step 1: A Careful Inspection
We start with a full visual check of your vehicle’s systems:
Are there any exhaust leaks that could throw off sensor readings?
Is there damage or blockage in the intake or exhaust system?
Are there any other warning lights or codes related to oxygen sensors, misfires, or fuel mixtures?
These problems can fool the computer into thinking the catalytic converter has failed – when really, it’s reacting to a separate issue.
✅ Step 2: Road Testing and Sensor Review
Next, we take the car on a test drive and use specialized tools to look at how the engine is behaving:
We monitor fuel delivery and airflow to ensure everything is balanced.
We observe how your oxygen sensors react when we force the engine into a rich or lean state (by slightly changing how much air or fuel it gets).
We analyze special hidden data (called Mode 6) that shows how well the converter
is performing on its own internal tests.
A good catalytic converter will smooth out the oxygen readings after combustion. A failing one will show both sensors (before and 
after the cat) swinging wildly in sync – like the cat isn't even there.
🚦 Sometimes the Converter Is the Problem – But That’s Not the End of the Story
If after all of our testing we confirm that the catalytic converter has truly failed, we’ll recommend a replacement – but we don’t stop there.
Catalytic converters rarely fail on their own. They’re built to last, but only if:
The engine burns fuel cleanly
The air/fuel mixture is balanced
There are no hidden misfires or oil consumption issues
If we replace the cat without fixing the underlying cause, you might be right back in the same situation a year later – and that’s not acceptable to us.
That’s why we may recommend additional services like:
Replacing old or worn spark plugs
Cleaning out carbon buildup from the intake
Fixing a fuel or air delivery imbalance
Adjusting valve timing or clearance, if applicable
These services aren’t "add-ons" – they’re protections. They help ensure the new catalytic converter will last, and they’re often required by the converter manufacturer to keep your warranty valid.
🛡 Our Promise: Long-Term Peace of Mind
When we install a catalytic converter at Accomplished Auto, it’s backed by a 3-year / 36,000-mile warranty. But we go further than just honoring parts – we help protect your investment.
By properly diagnosing the cause, and correcting anything that could lead to early failure, we ensure:
Your check engine light stays off
Your emissions system works cleanly
Your warranty remains fully valid with the converter maker and with us
💡 The Bottom Line
If your car has a catalytic converter warning, don’t assume it’s toast. There are many reasons a converter might seem bad when it’s not. At Accomplished Auto, we invest time, tools, and experience into making the right call.
Replacing a cat isn’t cheap – and it shouldn’t be done lightly. Let us test it properly, make sure your engine is healthy, and give you the confidence that once it's fixed, it stays fixed.