The upstream O2 sensor sits before the catalytic converter. The computer uses it to control the fuel mixture, so it switches back and forth between lean and rich all the time.
The downstream sensor sits after the converter. Its main job is checking whether the converter is working. Here's how to read them together.
What a good converter looks like
A working converter stores oxygen and smooths out the swings. So the upstream sensor bounces back and forth, but the downstream sensor stays fairly steady.
What a bad converter looks like
If the downstream sensor starts copying the upstream one, switching just as much, the converter isn't storing oxygen anymore. With misfires, fuel problems, and exhaust leaks ruled out, the converter has failed.
Things that fake a bad converter
Don't condemn the cat too fast. A few things make the downstream sensor look bad when the converter is fine:
- An exhaust leak after the converter: outside air swings the downstream sensor, so it looks like the converter isn't working
- Misfires or fuel problems: those need to be ruled out first, because they can set the same kind of code
Why converters keep dying
If a car's on its second converter in a year, look for what's killing it. An engine burning oil is a common one. The burned oil coats the catalyst and poisons it. Until the oil burning gets fixed, every new converter will fail.
Secondary air and the upstream sensor
On cars with secondary air injection, the system pumps air into the exhaust during a cold start to heat up the converter. That extra oxygen makes the upstream sensor read lean during warm-up. The computer uses that to confirm the air is flowing. If it never reads lean, the air isn't getting there.
Cheat sheet
- Upstream sensor controls the fuel mixture and switches constantly
- Downstream sensor checks the converter and should stay fairly steady
- Downstream copying upstream = converter not storing oxygen
- A leak after the converter can fake a bad cat
- Oil burning poisons converters, so fix it before replacing the cat
Common questions
What does the downstream O2 sensor do?
It mainly checks whether the catalytic converter is working by watching how much the oxygen level swings after the converter.
How do I know if my catalytic converter is bad?
If the downstream O2 sensor switches just like the upstream one, the converter isn't storing oxygen. Rule out misfires, fuel problems, and exhaust leaks first.
Can an exhaust leak cause a catalyst code?
Yes. A leak after the converter can swing the downstream sensor and make a good converter look bad.
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