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

What happens if you don't fix it

Symptoms you can actually notice

• Check engine light steady, no flashing
• Slightly rough idle
• Failed emissions test
• Sulfur or rotten-egg smell at the tailpipe

What the computer is actually complaining about

The downstream oxygen sensor sits behind the converter and watches what leaves it. The computer compares that slow, almost flat signal against the fast switching upstream sensor. When the two start reading alike, the converter has stopped storing oxygen the way it should. Two drive cycles of that and the light comes on.

Most likely causes, cheapest check first

What you need in your hands

• OBD-II scanner with live data

  Any $25 Bluetooth reader plus a free phone app shows B1S1 and B1S2 voltage. You do not need a shop-grade tool for this.

• Infrared thermometer Optional

  $20 at a hardware store. It reads converter inlet and outlet temperature without touching anything hot.

• Penetrating oil and an O2 sensor socket Optional

  Soak the threads the night before. A 22mm O2 socket with a slot for the wire is about $12.

Test it yourself, step by step

Do these in order. Each step tells you what to do, what to look at, what counts as fine, what counts as broken, and what you will actually see.

1. 1 Plug the scanner into the trapezoid port under the dash on the driver side, right above your left knee. Key to ON, engine running and warm. Open live data and select both oxygen sensor voltages.

   Look at

   Watch how fast each voltage moves. The upstream sensor should switch constantly. The downstream one should hold almost still.

   Fine if

   B1S1 swings between roughly 0.1V and 0.9V about eight times per second, and B1S2 stays near 0.6V to 0.8V with only small drift.

   Broken if

   B1S1 sits flat near 0.45V or switches fewer than three times per second. A dead upstream sensor mimics a dead converter and costs a tenth as much.

   What you'll see Two voltage traces on screen. The top one jumps up and down. The bottom one looks like a nearly straight line.

2. 2 Open the hood and look down the back of the engine where the exhaust pipe leaves the manifold. Follow the pipe rearward to the converter, a fat metal can about the size of a loaf of bread under the floor on the passenger side.

   Look at

   Listen along the pipe and flanges for a steady tick or hiss while the engine idles. Look for black soot streaks at the joints.

   Fine if

   No ticking, no hiss, no soot at any joint between the manifold and the converter.

   Broken if

   A tick that speeds up with the engine, or a soot streak at a flange. Air entering before the converter makes the rear sensor read lean and fakes a P0420.

   What you'll see A soft even exhaust note. Any sharp tick means you found a leak, and a leak is a $0 fix compared with a converter.

3. 3 Point the infrared thermometer at the pipe right before the converter, then at the pipe right after it. Hold it two inches away and wait for the number to settle. Do this with the engine at full operating temperature.

   Look at

   The temperature difference between the outlet pipe and the inlet pipe, read a few seconds apart at the same distance.

   Fine if

   The outlet reads 30F to 100F hotter than the inlet. A working converter burns leftover fuel and gets hotter on the way out.

   Broken if

   Outlet and inlet read within 20F of each other, or the outlet is cooler. That points at the converter itself.

   What you'll see Two numbers a few seconds apart. Write them down before you talk to any shop.

4. 4 Still on live data, switch to fuel trims and misfire counters. Let the engine idle for two minutes, then hold 2500 rpm for one minute with the hood open.

   Look at

   Long-term fuel trim on bank 1, and whether any misfire counter is climbing.

   Fine if

   Long-term trim stays inside plus or minus 10 percent, and misfire counters stay at zero.

   Broken if

   Trim climbs past plus 15 percent, or a misfire counter ticks up. Fix the lean condition or the misfire first. A converter cannot survive either one.

   What you'll see A trim percentage that holds steady. Numbers that keep climbing while you watch mean the engine is still feeding the problem.

5. 5 If step 1 failed, replace the upstream sensor. Reach down behind the engine to the pipe, unplug the four-wire connector, then break the sensor loose with the 22mm socket. Thread the new one in by hand before you put a wrench on it.

   Look at

   Whether the code comes back after the repair. Watch the pending codes screen, not only the stored codes screen.

   Fine if

   P0420 stays off through two cold starts and about 60 miles of mixed driving.

   Broken if

   The code returns within 30 miles with the new sensor in place. At that point the converter is the remaining suspect.

   What you'll see The old sensor comes out with a sharp crack. The new one turns in by hand for the first three turns. If it fights you, the threads are crossed, so back it out and start again.

What the repair costs

Reference range compiled from 3 source type(s), as of 2026-11. This is not a quote — actual cost depends on your vehicle, region, and which part turns out to be the real cause.

Basis: editorial reference range, not a live parts quote · Data as of 2026-11

How to confirm it's really fixed

✓ Clear the code, then do two full warm-up cycles with a cold start between them. If P0420 stays off after about 60 miles of mixed city and highway driving, the repair held. If it comes back, the converter is the next part to test, not the next part to buy.

Common questions