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.

P0420low severityGenerally driveable

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

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

Get a FREE OBD2 scan first — no purchase required

AutoZone, O'Reilly Auto Parts, and Advance Auto Parts all scan your car's computer for free. Walk in, they plug in a scanner, you get a code in under 2 minutes. Then come back here and look up that code at eli5cars.com/obd2 for the plain-English explanation.

Pro tip: Take a photo of the code before they clear it.

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

Most likely causes — ranked

Get a FREE OBD2 scan first — no purchase required

Frequently asked questions