The Prius has two different converter stories, and it helps to know which one you're dealing with. There's the converter that fails from the inside over the miles, and there's the one that gets cut off the car in a parking lot. Both light up the dash. Both need a real look before you spend.

The Converter That Wears Out

The catalyst inside the converter is a coated honeycomb that scrubs the exhaust as it flows through. Over the miles that coating degrades, and on a high-mileage Prius a little oil consumption speeds it along by fouling the substrate. When the converter stops cleaning the way it should, the downstream oxygen sensor sees it and the ECU sets a P0420. That code means low catalyst efficiency, nothing more specific. You might notice a few things alongside it:

• A drop in fuel economy, since the engine runs richer to compensate
• A sulfur, rotten-egg smell from the tailpipe
• A rattle underneath from a broken-up honeycomb
• Sluggish acceleration if the converter is restricting flow
• The check engine light with that efficiency code

The Stolen Converter

Prius converters are a common theft target because of the precious metals in them, and if yours was cut off, you'll know it the second you start the car. It roars loud, like the exhaust is wide open, because past the manifold it is. That's not a diagnosis so much as a confirmation. From there it's a replacement plus a shield or cage to make the next one harder to take.

Why P0420 Is Not the Whole Answer

Here's the trap. The same P0420 can come from a lazy downstream oxygen sensor reading slow, not a bad converter at all. Replace the converter on the code alone and you've spent big on the wrong part while the sensor still lies. The upstream cause matters too. Oil burning or a misfire will kill a fresh converter the same way it killed the last one.

How We Diagnose It

When a Prius comes in with a converter code, we read the codes and watch the upstream and downstream oxygen sensor voltages live to see whether the converter is actually storing and converting. We compare their switching behavior, check fuel trims for a rich condition, and look for oil consumption or misfires upstream. On a theft, we confirm what's gone and talk shielding. Then you get the real cause and the cost before anything gets bolted on.

Test First, Then Replace

A converter is an expensive part, so it's worth knowing it failed before you buy one. Bring your Prius to Driven Auto Care of Castro Valley and we'll test the converter, read the sensors, and find the real story. Call to set up an exhaust diagnostic.