Catalytic Converter Cleaner

Your two steps: try one bottle over a full tank of driving. If the check engine light is still on after that, a shop has to find the cause.

The cleaner works when carbon is the only thing in the converter's way. Once the check engine light is on, the car's computer stores a short trouble code describing what it detected. CRC, which makes one cleaner of this kind, says its product clears light to moderate clogs. It can sometimes make P0420 and P0430 go away; those codes mean the converter is not cleaning exhaust well enough.

The computer checks the converter through oxygen sensors, small exhaust parts that measure the gases flowing by. You'll usually see results within 1 to 3 driving cycles when the cleaner works. A driving cycle means one trip begun from a cold start, with both town streets and highway in it.

These problems are beyond what a cleaner can fix:

• a converter coated by burning oil, leaking coolant, silicone, or lead (the coat is permanent)
• insides that melted after misfires overheated the converter with fuel that had not burned
• cracks in a converter, or interior parts that are broken

Any of these means the converter needs replacing. Looking at the converter won't tell you which it is. If a full tank with the cleaner changes nothing, let a shop diagnose it.

No single brand is best: what caused the trouble matters more than the label. Cleaners target carbon only, and they do the most good when used early, while the converter is still sound.

Choose a brand people know, and do exactly what its directions say. A dealer service team also recommends one bottle at 5,000 to 7,500 mile intervals to keep carbon from building up.

Begin about a week ahead, since the directions call for more than one full tank of driving before the test. CRC's method has several steps; each gives the cleaner time to move through the engine and exhaust:

1. Start with a tank about half full, then pour in the whole bottle.
2. Do 20 to 40 miles of everyday driving.
3. Fill the tank right up, then drive until it is nearly empty.
4. Refuel. Check that the check engine light is off and the self-checks below are done, then go to the test.

At the station, the inspector connects to OBD (on-board diagnostics), the computer found in every car built since 1996. It looks at the readiness monitors, the self-checks a car runs on its own pollution parts. That driving also gives these checks time to finish. Resist clearing the code on your own right before testing; it wipes those checks, and you'd need roughly 200 more miles. Most states allow 1 unfinished check on a 2001 or newer car, and 2 on a 1996 to 2000 car. Before you go, a code reader or a shop can tell you whether they're done.

A cleaner takes off carbon; it cannot fix a broken part. When the converter really is failing, work in this order:

1. Track down the real cause first. P0420 can also trace back to an exhaust leak or a failing oxygen sensor. Expect to pay about $61 to $90 for a shop diagnosis, far less than the converter itself.
2. Get any misfire fixed. With a misfire, raw fuel lands in the exhaust, burns there and can wreck even a brand-new converter.
3. Replace a damaged converter. Cleaning won't save a melted or broken one.
4. Find out if a repair waiver is possible. After the required emissions-repair spending, many states let a still-failing car stay registered for a while without passing.

The amount you must spend before a waiver depends on your state:

• Ohio: $450
• New York: $450
• Colorado: $715
• Georgia: $1,176

On top of the amount, states set their own conditions. Arizona offers no waiver when a catalytic converter fails. If a converter is missing or tampered with, it fails the parts check, no waiver applies, and it has to be put back.

The price depends on your car. These are published estimates, and a local shop may charge more or less:

The converter prices come from CRC's guide. RepairPal gives an average of $2,280 to $2,480 for replacing a converter.