When your Check Engine Light comes on with a P0420 code, it’s easy to shrug it off—especially if the car still seems to run normally. Many drivers assume they can deal with it later. Here’s the truth: P0420 is one of the most misunderstood and most ignored trouble codes—and ignoring it can turn an already expensive repair into something far worse. If you plan on keeping your car long-term, this is not a repair to delay.

What P0420 Actually Means

P0420 = Catalyst System Efficiency Below Threshold (Bank 1). This means your catalytic converter—the component that reduces harmful exhaust gases—isn’t working well enough anymore. But the catalytic converter itself usually isn’t the real problem. Something else upstream damaged it.

Why Ignoring a P0420 Code Is a Costly Mistake

1. Catalytic converters are already expensive

A converter replacement often ranges from $1,000–$3,000+. That’s a serious repair bill—but delaying it will only make the total cost higher. Underlying issues such as misfires, oil consumption, rich fuel conditions, failed oxygen sensors, or exhaust leaks can destroy a new converter if not corrected first.

2. A P0420 masks new problems

When your check engine light is always on because of a P0420, you won’t notice if new issues develop. Driving with your “warning system” disabled is like removing the batteries from your smoke alarm.

3. Engine performance and fuel economy suffer slowly

Over time, a failing converter or faulty sensors can cause reduced fuel efficiency, sluggish acceleration, rough idle, and extra strain on the engine.

4. You will fail emissions testing

In California and many other states, P0420 results in automatic smog failure. You cannot renew registration until the issue is repaired.

5. Delaying the repair can cause engine damage

If the root cause is raw fuel entering the exhaust, it overheats the converter and may melt it internally, block exhaust flow, overheat the engine, or damage valves and pistons. A small repair can turn into a major engine job.

If You Plan to Keep Your Car, Don’t Put This Off

A catalytic converter replacement is not cheap. But if you want to keep your vehicle, taking care of the P0420 and fixing the underlying cause now is far less expensive than waiting. Driven Auto Care diagnoses the real issue before replacing parts to ensure the repair lasts.

Our diagnostic process includes:

- Fuel trim and O2 sensor analysis

- Misfire and ignition tests

- Exhaust leak checks

- Converter temperature profiling

- Oil consumption evaluation

Don’t let a “small” code become a big repair. Visit drivenautocare.com to schedule a Check Engine Light diagnostic. We’ll pinpoint the cause, protect your engine, and keep you safely on the road.