Owner's book · not a shop
Open HoodKnow the car. Then the window. Factory specs in plain English — don't authorize until you can say this at the counter.
Fix Finder · 19 aisle tickets
Walking the aisle…
P0420 · Diagnose first
The downstream oxygen sensor looks too much like the upstream one. The computer thinks the catalytic converter on bank 1 is not cleaning the exhaust. That can be a dying brick — or an exhaust leak, a lazy upstream sensor, or a misfire that poisoned it.
Do not buy a catalytic converter from P0420 alone. A converter is $800–$2,500. A sensor or a clamp is not. Graph both O2 sensors and leak-check the exhaust before anyone orders a brick.
DIY
You can read the code and listen for an exhaust tick. Graphing sensors and a smoke / soapy-water leak check is shop work.
Tools: Any $20 OBD-II scanner · Eyes and ears under the car
In the aisle
The parts person does not start with a SKU. They start with the engine and whether it is 2WD or 4WD. Answer from the bay ticket when we have it.
Which engine?
They will ask — not on the hook yet
Bank 1 and the converter change with the engine family. V6 vs I4 is a different box.
Federal or California (CARB) emissions?
They will ask — not on the hook yet
Converters are emissions-family parts. The wrong one will not pass a smog check.
2WD or 4WD?
They will ask — not on the hook yet
Usually does not change the cat. Still ask so they do not pull an exhaust for a different chassis.
Parts aisle
Search for year / make / model not on the hook. Not stock. Not TecDoc.
Sensors · bin SENS-1
Only if the graph is lazy. Not automatic from the code.
Query: upstream oxygen sensor bank 1
Sensors · bin SENS-2
The monitor sensor. A failed heater or a lazy signal can fake a dead brick.
Query: downstream oxygen sensor bank 1
Exhaust · bin EXH-4
A leak in front of the downstream sensor looks like a dead cat.
Query: exhaust manifold gasket
Hold
Exhaust · bin HOLD
HOLD. Diagnose first. Do not throw this part from P0420 alone.
Query: catalytic converter