Can I drive with code P0420?

⚠️ Limited driving only — go directly to repair shop

How much does it cost to fix code P0420?

Repair costs typically range from $400 to $2500 depending on the root cause, vehicle make/model, and whether you use a dealer or independent shop.

OBD2 Code P0420: Catalytic Converter System Efficiency Below Threshold (Bank 1)

Reviewed by ASE Certified Mechanics · Last updated September 01, 2026

Share: X

Quick Answer

P0420 = Catalytic Converter System Efficiency Below Threshold (Bank 1)
Severity: Check Soon · System: Powertrain · ⚠️ Limited driving only

The P0420 code means Catalytic Converter System Efficiency Below Threshold (Bank 1). Estimated repair cost: $400 – $2500.

⚠️ LIMITED DRIVING ONLY. Drive directly to a repair shop. Do not take long trips or drive at highway speeds.

📋 Diagnostic Reference

OBD2 Code	`P0420`
Category	Powertrain
Severity	Check Soon
Repair Cost	$400 – $2500
Can You Drive?	⚠️ Limited driving only — go directly to repair shop

🔍 Symptoms

Check Engine Light on
decreased fuel economy
sulfur/rotten egg smell
poor acceleration
failed emissions test

⚠️ Most Likely Causes (in order of probability)

Faulty catalytic converter
oxygen sensor malfunction
exhaust leak
engine misfire causing unburned fuel
contaminated converter

🛠️ Diagnostic Procedure

Step 1: Check for other stored codes first — P0420 is often a symptom not the root cause

Check for other stored codes first — P0420 is often a symptom not the root cause

Step 2: Inspect exhaust system for leaks before the catalytic converter

Inspect exhaust system for leaks before the catalytic converter

Step 3: Monitor O2 sensor voltage with live data — rear O2 should be steady not...

Monitor O2 sensor voltage with live data — rear O2 should be steady not mirroring front O2

Step 4: Perform backpressure test on catalytic converter

Perform backpressure test on catalytic converter

Step 5: Check for engine misfire or rich condition that could damage the new converter.

Check for engine misfire or rich condition that could damage the new converter.

🔧 DIY Diagnosis: Step-by-Step

Follow this diagnostic flow for P0420 (Emissions, EVAP & Catalytic Converter Systems). Start at Step 1 and work your way down — the most likely fixes are listed first.

For EVAP codes (P0440, P0442, P0446, P0449, P0455, P0496): check the gas cap first — tighten until it clicks and clear the code
For catalytic converter codes (P0420, P0430): check for other codes first — a bad O2 sensor or misfire is often the real cause
Perform a smoke test on the EVAP system to find even tiny leaks — this is the most effective EVAP diagnostic method
Monitor O2 sensor data: the downstream sensor should read steady (not fluctuating like the upstream) if the catalytic converter is healthy
Check the EVAP purge valve and vent valve operation with a scan tool — they should click and hold vacuum when activated
For EGR codes (P0401, P0402): remove and inspect the EGR valve for carbon buildup — clean or replace as needed

📊 Common Causes & Repair Costs

For P0420, here are the most likely root causes ranked by how often mechanics see them, plus typical repair costs for each.

Rank	Cause	Frequency	Typical Cost
#1	Loose or faulty gas cap (EVAP codes)	30%	—
#2	Failed EVAP purge or vent valve	22%	—
#3	Faulty or degraded catalytic converter	18%	—
#4	O2 sensor malfunction triggering converter code	15%	—
#5	EVAP system leak (hose, canister, seal)	10%	—
#6	EGR valve carbon buildup	5%	—

Category Repair Cost Range (Emissions, EVAP & Catalytic Converter Systems)

DIY / Best Case:	$10
Independent Shop:	$350
Dealer / Worst Case:	$2500

Costs include parts + labor. Actual price varies by make, model, and your location.

⚠️ Address Soon

Can you drive? Limited — EVAP codes (gas cap, purge valve) are safe to drive with but you may smell fuel. Catalytic converter codes mean reduced efficiency — drive moderately and avoid long trips until fixed. You may fail an emissions test.

ASE Certified Review: This diagnostic guide has been reviewed by an ASE-certified master technician. Diagnostic steps follow industry best practices per SAE J1979 and OEM service procedures. Always verify with a qualified mechanic before performing repairs.

Technician's Notes: The most misdiagnosed OBD2 code. Replace the O2 sensor first — then the catalytic converter if the code returns. Always fix the underlying cause before replacing the converter.

Estimated Repair Cost

Typical repair: $400–$2500 (Parts + Labor) · Costs vary by make/model and location

🔌 Don't Pay $150 for a Code Read — Diagnose It Yourself

Dealerships charge $100-$200 just to scan your car and read the code. A $20 Bluetooth OBD2 scanner gives you the same information in 30 seconds — and it works with free apps on your phone. Read and clear codes yourself before deciding if you need a mechanic.

Check OBD2 Scanner on Amazon →

Affiliate disclosure: We may earn a commission at no extra cost to you.

Data Sources: Diagnostic information compiled from SAE J1979 OBD2 standards, OEM service manuals, and ASE master technician field experience. Reviewed for technical accuracy. Repair cost ranges are national estimates and vary by location. Always verify with a qualified mechanic.

❓ Frequently Asked Questions

Can I drive with an EVAP code (P0440, P0442, P0455)?

Yes, EVAP codes rarely affect drivability. They're emissions-related, meaning you'll fail a smog test but the car runs fine. However, fuel odor and environmental impact mean you should fix it within a few weeks.

How much does it cost to fix a P0420 catalytic converter code?

Don't jump to replacing the converter. First check the O2 sensors ($50–$200 each) and fix any misfire codes. If the converter truly needs replacement: $400–$2,500 depending on vehicle. Always fix the root cause first.

Will tightening the gas cap really fix my EVAP code?

Yes! A loose gas cap is the #1 cause of EVAP codes like P0442 and P0455. Tighten until it clicks at least 2–3 times. If the cap seal is cracked, replace it ($10–$20). Clear the code and drive — if it stays off, you're done.

What happens if I ignore a catalytic converter code?

The converter is already operating below efficiency. Ignoring it means increasing emissions, worsening fuel economy, and potential backpressure buildup that damages the engine. You'll also fail any emissions test.

Can I pass emissions with a P04xx code?

No. All P04xx codes are emissions-related and will cause you to fail a state emissions inspection. You need to clear the code and complete the readiness monitors before testing.