- A P0420 or P0430 code points to low converter efficiency and needs diagnosis. The converter may need replacing, but the code alone doesn't confirm it.
- Converters rarely fail on their own; early failure has a cause, such as misfires, oil burning or rich running.
- If you replace a converter without fixing the cause, the new one can fail early too, and you pay twice.
- No check-engine code does not prove a converter is working well, especially with a low-cost aftermarket part.
- A converter job calls for a complete diagnosis. Ask your technician whether a new O2 sensor should be part of it, to protect your investment and the work.
- Driving with a failing converter, or without one, can harm your engine and your health. Get it diagnosed promptly.
- Ask a qualified mechanic to diagnose before you buy. Miller CAT makes converters and Cat Shields, and this guide is not repair advice.
How do you know if your catalytic converter is failing?
The clearest sign of a failing catalytic converter is a check-engine light with a P0420 or P0430 code. Other signs are a code that keeps returning after a replacement, engine misfires, oil burning, rich running, a previous low-cost converter, and very high mileage. If you're seeing that light right now, it's normal to worry about the cost. The best first step is a proper diagnosis, so the money you spend fixes the real problem.
Converters rarely fail on their own; early failure has a cause. So each sign below is a prompt to find out why, not just to swap the part. Finding the cause is how you avoid paying for the same repair twice.
A catalytic converter is a ceramic or metal honeycomb coated with platinum, palladium and rhodium. Those metals convert carbon monoxide, unburned hydrocarbons and nitrogen oxides into less harmful gases.
7 signs your converter may need attention
1. A P0420 or P0430 code
P0420 (bank 1) and P0430 (bank 2) mean your vehicle's computer measured catalyst efficiency below its threshold. It compares the upstream and downstream oxygen (O2) sensor signals to judge how much oxygen the converter stores.
The code points to low converter efficiency and needs diagnosis. It doesn't say why efficiency dropped. The honest answer is that the converter may need replacing. But the same code can come from other problems, including a worn O2 sensor, so confirm the cause with a complete diagnosis before you spend on a part.
2. The code returns after a replacement
If the light comes back after a new converter, it's frustrating, and it's worth slowing down. Look at the part and the cause. Without enough catalyst, efficiency drops below the threshold quickly and the code returns. Ask your technician what caused the first failure, and whether the O2 sensors that judge the converter were checked or replaced.
3. Engine misfire
A converter is more likely to be damaged from the inside by an engine misfire than by outside heat, according to Miller CAT's engineering position. A misfire is worth fixing for its own sake and for the converter downstream.
4. Excess oil consumption
Oil burning is the other common inside-out cause Miller CAT points to. If your engine uses more oil than it should, tell your technician before you approve a converter replacement.
5. Rich running
An engine running rich (too much fuel) is a known upstream problem that can damage a converter. A converter replaced without fixing it is likely to be damaged again.
6. A low-cost converter was installed before
Low-cost converters often carry little to no precious metal, so they fail early and can hide the engine problem that caused the original failure. A new part may clear the code for a while while the engine problem keeps doing damage. If this is your situation, you're not alone. A low price is tempting when a repair bill is stressful, but it can cost more in the end.
The code can stay off because the computer judges efficiency from oxygen-sensor signals, not from tailpipe emissions. So a clear dashboard doesn't prove the converter is working. Our post on the hidden danger of cheap catalytic converters explains the mechanism.
7. Very high mileage
A converter can simply age out at very high mileage. Treat this as a possibility your technician confirms, and only after the causes above are ruled out.
| Sign | What it points to | Ask your technician |
|---|---|---|
| P0420 / P0430 code | Low converter efficiency | What is causing it? Is it the converter, a sensor or a leak? |
| Code returns after replacement | Low-quality part or unfixed cause | Is the replacement a quality converter? Was the root cause fixed? |
| Misfire | Possible internal converter damage | Can you check ignition and misfire first? |
| Oil burning | Possible internal converter damage | Why is the engine using oil? |
| Rich running | Possible internal converter damage | Can you check fuel mixture? |
| Past low-cost converter | Little to no precious metal | Is the current converter a quality part? |
| Very high mileage | Possible age-related decline | Have other causes been ruled out? |
When is it time to replace or upgrade?
Replace when your technician confirms low converter efficiency and the cause is fixed or ruled out. Then choose a quality converter, because a low-cost part can bring the same problem back. Also ask whether a new O2 sensor should be part of the job. The sensors tell the computer how the converter is performing, so an old or faulty one can undermine a new converter. A complete diagnosis tells you what to replace and protects your investment. The wise path is simple: diagnose, fix the cause, then install a quality part once. That is how you keep the total cost down.
Miller CAT manufactures catalytic converters as well as Cat Shields, so we know how to keep them operating. Browse converters at millercat.com and confirm your vehicle's fitment before ordering.
Miller CAT doesn't give repair or installation-procedure advice. A qualified mechanic should do the diagnosis.
Protect the converter you install
Converter theft is still in the news. In October 2026, Patch reported that police in Berkeley, California said more catalytic converters had already been stolen this year than in all of 2025. (Patch, October 2026)
A Cat Shield makes theft much harder and slower, but no product can guarantee against theft. It can be removed with the security bit for work such as an O2 sensor or converter replacement, then reinstalled. See how a catalytic converter shield saves you money and why repair costs are rising.
