P0420
Catalyst System Efficiency Below Threshold (Bank 1)
Published · Reviewed and updated
In plain English: The catalytic converter on bank one isn't cleaning the exhaust as well as the computer expects — or the sensor watching it is lying.
You can keep driving in the short term. It won't strand you. But you will fail an emissions test, and if the cause is a misfire or a rich condition, you're actively cooking the converter while you wait.
A diagnostic starting point: Catalytic converter genuinely worn out. Start by asking: Are there any other codes stored alongside this one? Illustrative parts budgets span $15–$2500 USD, with diagnosis, labor and taxes extra.
Quick answers about P0420
- What does P0420 mean?
- The catalytic converter on bank one isn't cleaning the exhaust as well as the computer expects — or the sensor watching it is lying.
- What should I check first for P0420?
- One starting point is Catalytic converter genuinely worn out. Most common past a hundred thousand miles, and the default answer if everything else checks out clean. The code alone does not identify a failed part; follow the procedure for your vehicle.
- Can I drive with P0420?
- You can keep driving in the short term. It won't strand you. But you will fail an emissions test, and if the cause is a misfire or a rich condition, you're actively cooking the converter while you wait.
- How much does it cost to fix P0420?
- Illustrative parts budgets in this guide span $15–$2500 USD. Diagnosis, labor and taxes are extra. So the spread on this one code runs from about thirty dollars to well over two thousand. That is exactly why you test in the order we just went through instead of starting with the converter.
- How do I diagnose P0420?
- Start with: Are there any other codes stored alongside this one? If you have a misfire code or a fuel trim code sitting next to it, stop. Fix that one first, clear everything, and drive it. Those cause this code, and about a third of the time the P0420 never comes back.
Watch the P0420 videos
Full walkthrough — 8:11. Watch on YouTube
What P0420 actually means
There are two oxygen sensors on this exhaust path. The upstream sensor sits before the converter and swings up and down constantly as the fuel trim hunts. The downstream sensor sits after it. A healthy converter stores and releases oxygen, which smooths that signal out, so the downstream sensor should sit lazy and nearly flat, usually somewhere around zero point six to zero point seven volts.
The computer runs a catalyst monitor that compares the two. When the downstream signal starts copying the upstream signal's switching, that means the converter has stopped storing oxygen. Cross the calibrated threshold on two consecutive drive cycles, and you get P0420.
Symptoms
- Check engine light, on steadySteady, not flashing. A flashing light is a misfire and that's a different, more urgent problem.
- Failed emissions or smog testThis is the one that usually forces the repair, because it's an automatic fail in most states.
- A mild drop in fuel economyUsually a couple of miles per gallon, and often blamed on something else entirely.
- A sulfur or rotten egg smell from the tailpipeThat's unburned sulfur compounds passing straight through a converter that isn't reacting anymore.
- A rattle from under the floor on cold startThat is the ceramic honeycomb inside the converter broken loose. If you hear it, the converter is mechanically finished.
Possible causes and what to check
This is a practical diagnostic order, not measured failure rates. Follow the tests and priorities specified for your vehicle; a code alone does not identify a failed part.
Downstream O2 sensor — healthy converter vs failed converter
Illustrative graph, not a recording from your vehicle or a universal service limit. Confirm the test conditions, units and acceptance criteria in the applicable service procedure.
Put your scan tool on live data and watch two sensors at once, with the engine fully warm and holding steady around two thousand RPM.
This is a healthy converter. The upstream sensor is switching hard between about zero point one and zero point nine volts, which is normal. The downstream trace stays lazy and nearly flat.
This is a failed converter. The downstream sensor has started mirroring the upstream sensor almost exactly. The converter is no longer buffering oxygen at all.
If your downstream trace is flat, your converter is fine and you're looking at a sensor or a leak. If it mirrors, the converter is the problem.
How to diagnose it, in order
-
Are there any other codes stored alongside this one?If you have a misfire code or a fuel trim code sitting next to it, stop. Fix that one first, clear everything, and drive it. Those cause this code, and about a third of the time the P0420 never comes back.Yes → Fix those codes first, clear, and re-driveNo → go to step 2
-
Any exhaust leak between the engine and the converter?Listen at cold idle for a ticking that fades as it warms, and check the manifold, the flex pipe and every gasket. A leak upstream feeds false air to the sensors and produces this exact code with a perfectly good converter.Yes → Repair the leak, clear, re-drive before condemning anythingNo → go to step 3
-
Are long term fuel trims within plus or minus ten percent?Check them at idle and at cruise. If they're outside ten percent in either direction, the engine is feeding the converter a mixture it can't process, and that's the actual fault.Yes → Diagnose the fuel trim fault firstNo → go to step 4
-
Is the downstream O2 sensor responding at normal speed?Force it rich and lean by snapping the throttle and watch how fast the downstream sensor reacts. A lazy sensor that takes more than about a second to move is the fault itself, not a witness to it.Yes → go to step 5No → Replace the downstream O2 sensor
-
Does the downstream trace mirror the upstream switching?This is the decisive test, and it's the graph you just saw. Warm engine, steady two thousand RPM, both sensors on screen. Flat downstream means the converter is still working.Yes → Converter has failed — replace itNo → Converter is fine — re-examine sensor and leaks
Work it in this order and you won't buy a converter you didn't need.
What you need to do this yourself
- A scan tool that reads live data — Choose vehicle-compatible access to the live data, freeze frame and monitor results required by the diagnostic procedure. Some checks also need bidirectional control.
- Digital multimeter — Use the specified meter and test method for electrical checks. Follow circuit diagrams and avoid applying an unsuitable test load to a computer driver.
- Vehicle-specific service information — Confirm the procedure, safety precautions, tools and acceptance limits for the exact year, model and engine.
Some links may be affiliate links. They cost you nothing extra and do not change which tools are recommended — the diagnostic order on this site is the same either way.
Illustrative repair budgets
These are rough US planning examples, not quotes or measured repair averages. Parts and labor are separate; diagnosis, taxes, difficult access and additional repairs can increase the total.
| Repair | Parts (USD) | Labor time |
|---|---|---|
| Exhaust leak repair (gasket or flex pipe) Cheapest possible outcome, and genuinely common. |
$15–$90 | 1.0 h |
| Downstream oxygen sensor Usually one bolt and a connector, if it isn't seized. |
$40–$140 | 0.5 h |
| Catalytic converter, aftermarket direct fit Buy one that is certified for your state, or you'll be back here. |
$200–$700 | 1.5 h |
| Catalytic converter, OEM Required on some newer vehicles for the monitor to ever pass. |
$900–$2500 | 1.5 h |
So the spread on this one code runs from about thirty dollars to well over two thousand. That is exactly why you test in the order we just went through instead of starting with the converter.
P0420 video summary · 1:00
A brief introduction to the code and a diagnostic starting point. Use the full guide and vehicle-specific service information for the complete procedure.
Written diagnostic walkthrough
Current written guidance organized by topic. This text may include corrections made after a video was published and is not a verbatim transcript of the archived video.
Meaning and driving guidance
Code P0420. Catalyst System Efficiency Below Threshold (Bank 1).
In plain English: the catalytic converter on bank one isn't cleaning the exhaust as well as the computer expects — or the sensor watching it is lying.
You can keep driving in the short term. It won't strand you. But you will fail an emissions test, and if the cause is a misfire or a rich condition, you're actively cooking the converter while you wait.
How it is measured
Here's what your engine computer is actually measuring.
There are two oxygen sensors on this exhaust path. The upstream sensor sits before the converter and swings up and down constantly as the fuel trim hunts. The downstream sensor sits after it. A healthy converter stores and releases oxygen, which smooths that signal out, so the downstream sensor should sit lazy and nearly flat, usually somewhere around zero point six to zero point seven volts.
The computer runs a catalyst monitor that compares the two. When the downstream signal starts copying the upstream signal's switching, that means the converter has stopped storing oxygen. Cross the calibrated threshold on two consecutive drive cycles, and you get P0420.
So the code is not a part failure report. It's a performance measurement that came back under spec.
Symptoms
What you'll usually notice, beyond the light itself.
Check engine light, on steady. Steady, not flashing. A flashing light is a misfire and that's a different, more urgent problem.
Failed emissions or smog test. This is the one that usually forces the repair, because it's an automatic fail in most states.
A mild drop in fuel economy. Usually a couple of miles per gallon, and often blamed on something else entirely.
A sulfur or rotten egg smell from the tailpipe. That's unburned sulfur compounds passing straight through a converter that isn't reacting anymore.
A rattle from under the floor on cold start. That is the ceramic honeycomb inside the converter broken loose. If you hear it, the converter is mechanically finished.
Possible causes
Here is a practical order for checking the possible causes. These are diagnostic priorities, not measured failure rates.
Check 1: Catalytic converter genuinely worn out. Most common past a hundred thousand miles, and the default answer if everything else checks out clean.
Check 2: Failed or lazy downstream oxygen sensor. A slow sensor reports a signal that looks like a dead converter. This is the cheap one, and it's why you test before you buy.
Check 3: Exhaust leak ahead of or near the converter. A leak pulls outside air into the stream and skews both sensors. Fix the leak first or you'll chase this forever.
Check 4: Engine running rich or lean. Bad fuel trim overloads the converter. If you also have a fuel trim code, that one gets fixed first, always.
Check 5: Misfire or oil and coolant contamination. Raw fuel or burnt coolant poisons the catalyst material permanently. Fix the source or you'll kill the new converter too.
Check 6: Aftermarket converter that doesn't meet spec. Cheap universal converters frequently set this code within months, even when brand new.
Use the test results to choose the repair. A position in this list does not prove that a part has failed.
Illustrative diagnostic pattern
This illustrative graph explains a diagnostic pattern. Use the vehicle-specific procedure to interpret it.
Put your scan tool on live data and watch two sensors at once, with the engine fully warm and holding steady around two thousand RPM.
This is a healthy converter. The upstream sensor is switching hard between about zero point one and zero point nine volts, which is normal. The downstream trace stays lazy and nearly flat.
This is a failed converter. The downstream sensor has started mirroring the upstream sensor almost exactly. The converter is no longer buffering oxygen at all.
If your downstream trace is flat, your converter is fine and you're looking at a sensor or a leak. If it mirrors, the converter is the problem.
Diagnostic steps
So here is the order to actually test this, cheapest and most likely first.
Step 1. Are there any other codes stored alongside this one? If you have a misfire code or a fuel trim code sitting next to it, stop. Fix that one first, clear everything, and drive it. Those cause this code, and about a third of the time the P0420 never comes back.
Step 2. Any exhaust leak between the engine and the converter? Listen at cold idle for a ticking that fades as it warms, and check the manifold, the flex pipe and every gasket. A leak upstream feeds false air to the sensors and produces this exact code with a perfectly good converter.
Step 3. Are long term fuel trims within plus or minus ten percent? Check them at idle and at cruise. If they're outside ten percent in either direction, the engine is feeding the converter a mixture it can't process, and that's the actual fault.
Step 4. Is the downstream O2 sensor responding at normal speed? Force it rich and lean by snapping the throttle and watch how fast the downstream sensor reacts. A lazy sensor that takes more than about a second to move is the fault itself, not a witness to it.
Step 5. Does the downstream trace mirror the upstream switching? This is the decisive test, and it's the graph you just saw. Warm engine, steady two thousand RPM, both sensors on screen. Flat downstream means the converter is still working.
Work it in this order and you won't buy a converter you didn't need.
Illustrative repair budgets
These are illustrative US parts budgets with labor shown separately. Diagnosis, taxes, access and additional repairs can change the total.
Exhaust leak repair (gasket or flex pipe): 15 to 90 dollars in parts, plus about 1.0 hours of labor. Cheapest possible outcome, and genuinely common.
Downstream oxygen sensor: 40 to 140 dollars in parts, plus about 0.5 hours of labor. Usually one bolt and a connector, if it isn't seized.
Catalytic converter, aftermarket direct fit: 200 to 700 dollars in parts, plus about 1.5 hours of labor. Buy one that is certified for your state, or you'll be back here.
Catalytic converter, OEM: 900 to 2500 dollars in parts, plus about 1.5 hours of labor. Required on some newer vehicles for the monitor to ever pass.
So the spread on this one code runs from about thirty dollars to well over two thousand. That is exactly why you test in the order we just went through instead of starting with the converter.
Sources and application limits
This legacy guide has not yet had a page-specific source list added. Check the factory service information for your exact year, make, model and engine before acting on a test or specification.