P0420low severityGenerally driveable

P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)

Written in plain English and reviewed by the eli5cars editorial team · Last reviewed June 2026

What it means in plain English

Your exhaust goes through a catalytic converter — a box of precious metal mesh that converts toxic gases (CO, NOx, hydrocarbons) into safer ones before they leave the tailpipe. Oxygen sensors before and after the cat check whether the cat is doing its job. P0420 means the sensor after the cat sees exhaust that looks almost like the exhaust before the cat — the catalytic converter isn't cleaning the exhaust efficiently enough.

Most likely causes — ranked

#1 Worn or degraded catalytic converter (internal substrate failure)

most likely

Fix: The catalytic converter itself has reached end of life — the precious metal coating on the internal honeycomb structure is spent or damaged. Replacement is the fix. Use an OEM-spec or CARB-compliant aftermarket cat. On most US cars this is a shop job requiring lifting the vehicle and cutting/welding exhaust flanges.

DIY ~$250Shop ~$1200

catalytic converter repair guide

#2 Failed downstream (post-cat) oxygen sensor

likely

Fix: The downstream O2 sensor reads the exhaust after the cat. If the sensor is lazy or dead, it may report that the cat is underperforming when the cat itself is fine. An O2 sensor can be tested with a live data scan (it should show steady, low voltage after the cat compared to the switching upstream sensor). Replacement is a straightforward DIY job with an O2 sensor socket.

DIY ~$50Shop ~$200

oxygen sensor repair guide

#3 Exhaust leak upstream of the catalytic converter

possible

Fix: A leak in the exhaust manifold or the pipe between the manifold and cat introduces oxygen into the exhaust stream, causing both O2 sensors to read similarly — mimicking a failing cat. Listen for a ticking or popping sound from the engine bay on cold starts. A shop can pressure-test the exhaust or use a smoke machine to locate leaks.

DIY ~$30Shop ~$300

#4 Engine burning oil or coolant (contaminating the catalytic converter)

possible

Fix: If the engine is consuming oil or coolant and the smoke enters the exhaust, it coats and poisons the catalytic converter's substrate. Fix the underlying engine issue first (valve seals, PCV system, head gasket) — replacing the cat without fixing the contamination source will destroy the new cat quickly.

DIY ~$100Shop ~$1500

Check engine light on? Get a free code scan

If your check engine light is on, most auto-parts stores (AutoZone, O'Reilly, Advance) will read the code for FREE — no purchase required, just walk in. Bring the code back here and look it up at eli5cars.com/obd2 for the plain-English explanation.

Note: a scan only helps when there's an active fault code. For purely mechanical symptoms (noises, vibrations, leaks), a scan may show nothing — the suspects listed above are your starting point.

Want to read the codes yourself?

First, the free option: AutoZone, O’Reilly, and Advance Auto Parts read your codes for free — no purchase needed. That’s the cheapest way to get the code to look up here. If you’d rather have your own so you can check anytime (and clear a code after a fix), the Veepeak OBDCheck BLE is an inexpensive, reliable plug-in reader that pairs with a free phone app and works on any car sold since 1996.

See the Veepeak OBD2 scanner on Amazon

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Frequently asked questions

Can I drive with a P0420 code?

Yes — P0420 is an emissions fault, not a safety or drivability fault. The car will run normally. It won't pass an emissions test with P0420 active. You should address it within a few months, not a few days.

Will P0420 cause my car to fail inspection?

Almost certainly yes. OBD2 emissions tests read your ECM for active fault codes. P0420 is an emissions-related code and will cause a fail in virtually every US state that does OBD2-based inspection.

Is the $50 oxygen sensor fix worth trying before the $1,200 catalytic converter?

Yes, always try the O2 sensor first — it costs $50 and takes an hour. If the code returns after a full drive cycle with a new downstream sensor, the catalytic converter is most likely the issue. A shop can also perform a live catalyst efficiency test to confirm before recommending a replacement.

Does adding a can of 'catalytic converter cleaner' fix P0420?

Occasionally, on very mildly contaminated cats. Products like Cataclean can help restore a slightly fouled substrate, but they will not fix a substrate that has physically degraded or been poisoned by oil or coolant. Worth $20 as a first experiment; don't count on it working.