What are the symptoms of P0420?

Common symptoms include: Check-engine light (often the only sign); Failed emissions test; Slightly reduced economy.

What causes a P0420 code?

The most common causes are: Aged or failed catalyst; Oxygen sensor fault; Exhaust leak; Upstream engine issue contaminating the cat.

How do you fix P0420?

Confirm with upstream/downstream sensor data; Check for exhaust leaks; Replace the catalyst if it's genuinely degraded. Worth diagnosing soon — left alone, the underlying issue can grow into a bigger, costlier problem.

OBD-II Code Lookup

P0420Watch

Catalyst System Efficiency Below Threshold (Bank 1)

Emissions system

The catalytic converter (or oxidation catalyst) isn't cleaning the exhaust as efficiently as required — the downstream oxygen/temperature readings indicate reduced conversion.

Worth diagnosing soon — left alone, the underlying issue can grow into a bigger, costlier problem.

Common symptoms

Check-engine light (often the only sign)
Failed emissions test
Slightly reduced economy

Common causes

Aged or failed catalyst
Oxygen sensor fault
Exhaust leak
Upstream engine issue contaminating the cat

How it's diagnosed & fixed

Confirm with upstream/downstream sensor data
Check for exhaust leaks
Replace the catalyst if it's genuinely degraded

Got P0420? Get it diagnosed.

A real technician can interpret P0420 in the context of your truck and symptoms — and tell you what's actually wrong before you buy parts.

Find a Specialist

Related Emissions codes

P0401EGR Flow Insufficient Detected P0402EGR Flow Excessive Detected P0404EGR Control Circuit Range/Performance P0470Exhaust Pressure Sensor Fault

Reference only. A trouble code names a symptom, not the failed part — always confirm with a live diagnosis before repairs. See our Knowledge Base for deeper guides.