Catalyst Efficiency Below Threshold: Diagnose It Like a Tech

Before you buy a $1,200 catalytic converter: the O2 sensor test that resolves half of these codes.

Diagnosis goes wrong when people replace parts in the wrong order. Work through the checks sequenced by cost and probability — free tests first, expensive parts last.

Step 1 — Confirm with a scanner. A $25 OBD-II reader turns guesswork into data. Note every stored code before clearing anything.

Step 2 — Run the free checks. Visual inspection, swapping components between positions, and listening tests rule out half the causes at zero cost.

Step 3 — Replace and verify. Once the fault follows a component, replace it, clear codes, and drive a full warm-up cycle to confirm the fix holds.