When looking for a fuel additive to help clean the exhaust system and lower hydrocarbon emissions before a smog check, 
Cataclean is widely considered the leading and most recognized option on the market.
Top options and alternatives designed to clean fuel systems, oxygen sensors, and catalytic converters include:
Cataclean 120007 Fuel System Cleaner | Walmart& more | 3.8(7.1K)
Cataclean Fuel and Exhaust System Cleaner (120007) – The most popular specialized choice. It is formulated to reduce carbon buildup in the catalytic converter, oxygen sensors, and fuel injectors, with independent testing showing potential hydrocarbon emission reductions.
Royal Purple Max Clean Fuel System Cleaner | Royal Purple& more | 4.6(1K)
Royal Purple Max-Clean Fuel System Cleaner & Stabilizer – A powerful deep-cleaner that revitalizes fuel injectors and combustion chambers, safely clearing out deposits that can trigger high emissions without harming delicate emissions equipment .
Solder It Catalytic Converter Cleaner | 
Solder-It& more | 4.2(53)
Solder-It Catalytic Converter Cleaner and Deodorizer (CAT-1) – A targeted fuel additive designed to clean carbon from oxygen sensors and catalytic elements while eliminating foul sulfur ("rotten egg") odors.
Chevron Techron Concentrate Plus Fuel System Cleaner | CARiD.com& more | 4.6(123)
Chevron Techron Concentrate Plus Fuel System Cleaner – While not marketed explicitly as a converter "restorer," Techron is heavily recommended by mechanics as a premier general cleaner that reliably cleans up sulfur-fouled fuel senders and dirty oxygen/MAF sensors that cause check engine lights and failed tests.
- Check the fuel level: Most of these cleaners require you to pour the entire bottle into a low tank of gas (usually around 1/4 to 1/2 tank remaining) so the concentrated formula mixes effectively.[1][2]
- Drive aggressively / clear the system: Drive the vehicle for about 15 to 20 minutes at highway speeds (higher RPMs) to heat up the exhaust and allow the active cleaning agents to burn off carbon deposits.
- Refuel and test: Fill the tank back up with regular fuel and drive normally for a day or two before heading to the emissions testing facility.
Note: While these additives can successfully lower hydrocarbon readings or clear minor efficiency codes (like a marginal P0420 code) caused by surface carbon fouling, they cannot repair a physically broken, melted, or completely degraded catalytic converter core.
If you'd like to share what specific code or issue your car is having (e.g., a P0420 code, high tailpipe hydrocarbons, or a check engine light), I can help you figure out if a cleaner will do the trick or if a mechanical fix is needed.
