How can I tell if a catalytic converter has been stolen?

If your catalytic converter has been stolen, you’ll usually hear a noticeably louder exhaust, see a missing or cut section of the exhaust under the car, and may notice warning lights or a drop in performance. This article explains the signs to look for, how to confirm theft, what steps to take, and how to reduce the risk in the future.

Common signs to look for

Before checking off specific indicators, it helps to know that theft typically leaves both audible and visual clues, along with possible diagnostic codes from the car’s computer. The following signs are the most commonly reported by vehicle owners and law enforcement.

• Unusually loud exhaust noise, especially when the engine is idling or you accelerate


• A rattle, grinding noise, or metallic clinking coming from the undercarriage


• Visible damage to the exhaust system or a gap where the catalytic converter should be


• Missing catalytic converter or cut exhaust pipes under the vehicle


• Check engine light or diagnostic trouble codes such as P0420 or P0430


• Strong sulfur-like odor (rotten egg smell) near the exhaust, which can indicate catalytic inefficiency or leaks


• Decreased fuel efficiency or reduced engine performance

These cues alone aren’t proof of theft—especially if a converter was failing or a leak occurred—but together they strongly suggest an inline issue with the catalytic converter. A quick professional inspection can confirm what happened.

How theft typically happens and what to look for outside the car

In many cases, thieves target catalytic converters because they contain precious metals. They often operate in public parking areas or at night, and may leave behind visible signs of tampering, such as crushed heat shields, fresh metal-cutting marks, or a missing exhaust component. If you’re inspecting your vehicle after a suspected incident, look for:

• Fresh cuts or visible saw marks on the exhaust piping just forward of the muffler


• Broken heat shields or loose undercarriage panels


• Exposed wiring or sensors that should be connected to the converter

Note that some repairs can mimic theft signs, so a professional diagnosis is essential to determine whether the converter was removed rather than damaged in another way.

What to do if you suspect theft

Before taking action, ensure you and any passengers are safe. Do not drive long distances with a suspected missing catalytic converter, as exhaust leaks can allow carbon monoxide into the cabin.

1. Pull over safely if you’re driving and inspect the undercarriage from a safe location. If you see a missing component or a large gap, avoid continuing to drive the vehicle.


2. Call your local police to file a theft report with specifics such as location, time, and any observations. Obtain a copy of the report for insurance and records.


3. Contact your insurer or roadside assistance. Some policies cover theft-related damage or loss, and a tow may be required to transport the vehicle to a repair shop.


4. Have a qualified mechanic inspect the exhaust system. They can confirm whether the catalytic converter was removed, assess related damage, and quote replacement costs.


5. Document everything with photos and notes, including the area around the exhaust and any serial numbers or marks on the new part if you replace it.

Repair costs vary by vehicle and whether OEM or aftermarket parts are used. Expect a range that can include parts plus labor, and factor in potential sensor or exhaust-damages repairs. Your insurer and local authorities can provide guidance on coverage and next steps.

Prevention tips

To reduce the risk of catalytic converter theft, consider proactive measures that deter thieves and make stealing less attractive or more difficult.

• Park in well-lit, secure areas whenever possible and use monitored or Garage/garage parking when available.


• Install an anti-theft device for catalytic converters, such as a steel shield, locking clamps, or reinforced brackets.


• Engrave or etch the vehicle’s VIN on the catalytic converter to deter theft and aid recovery.


• Consider alarms that trigger if the exhaust system is tampered with or sensors detect movement near the undercarriage.


• Work with a trusted mechanic to ensure proper installation and keep receipts for any aftermarket protective parts.

Combining several prevention methods can significantly reduce risk. If theft has occurred in your area, local law enforcement or consumer safety organizations may have additional tips tailored to your region.

Additional context and next steps

Catalytic converter theft is a recurring concern in many cities, driven by the value of platinum, palladium, and rhodium inside the converter. Owners of older or higher-metal-content models can be especially at risk. If you reside in an area with frequent thefts, staying informed about local crime patterns and collaborating with nearby neighbors or property managers can improve prevention efforts.

Summary

Recognizing the signs of a stolen catalytic converter involves a combination of listening for unusual exhaust sounds, visually inspecting the undercarriage for damage or missing components, and noting any diagnostic codes from the vehicle’s computer. If theft is suspected, prioritize safety, contact police, arrange a tow if needed, and have a professional confirm the theft and guide replacement options. Preventive measures—such as anti-theft devices, VIN marking, and secure parking—can reduce the likelihood of future incidents. Staying informed and prepared helps protect both your vehicle and your peace of mind.

How can I check my catalytic converter?

You can test a catalytic converter using an infrared thermometer to check if the outlet is significantly hotter than the inlet, an OBD2 scanner to read diagnostic trouble codes, or a vacuum gauge to measure exhaust back pressure. A clogged or failing converter may also make a rattling sound, or loosening the exhaust pipe can cause a sudden improvement in engine performance. 
Infrared thermometer test
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1. Warm up the vehicle by driving for at least 15-20 minutes to ensure the catalytic converter is hot. 
2. Using an infrared thermometer, measure the temperature at the inlet (front) of the converter. 
3. Measure the temperature at the outlet (back) of the converter. 
4. A healthy converter should have an outlet temperature that is at least 50°F to 150°F hotter than the inlet temperature. 
5. If the outlet is not significantly hotter, or is cooler, the converter may be failing. 

You can watch this video to learn how to use an infrared thermometer to test a catalytic converter: 54smr diyguy123YouTube · Oct 4, 2023
OBD2 scanner test 

1. Connect an OBD2 scanner to your vehicle's diagnostic port. 
2. Look for any diagnostic trouble codes related to the catalytic converter or oxygen sensors. 
3. Monitor the live data from the upstream and downstream oxygen sensors. 
4. A healthy downstream oxygen sensor reading should be a steady voltage (around 0.6 to 0.8 volts) at idle and when revving, while the upstream sensor fluctuates. 
5. If the downstream sensor's reading oscillates or dips, it could indicate a bad converter. 

Vacuum gauge test

1. Locate a vacuum port on the engine and connect a vacuum gauge.
2. At idle, note the baseline vacuum reading (a typical reading is 16-21 inches of mercury).
3. Have a helper rev the engine to about 2500 RPM.
4. With a healthy exhaust, the vacuum should briefly drop and then return to the baseline.
5. If the vacuum drops and takes a long time to return, or stays low, it can indicate a blockage in the exhaust system, likely from a bad catalytic converter. 

Other diagnostic methods

• Listen for rattles: Tap the converter with a rubber mallet and listen for any rattling sounds, which could indicate broken internal parts.
• Power test: Loosen the bolts before the converter and see if there is an increase in engine power, which would point to a clogged converter. 

Can you tell if your catalytic converter was stolen?

You can tell if your catalytic converter was stolen by a very loud roaring sound when you start your car, a lack of acceleration power, and visual signs of damage or missing parts under your vehicle. Other indicators include a strong exhaust smell inside the car, a check engine light, or failed emissions tests. 
Audio and performance clues 
This video explains the sounds you may hear if your catalytic converter is stolen: 1mThe Enthusiast MechanicYouTube · Jun 5, 2024

• Loud roaring noise: A missing catalytic converter will cause your exhaust to be extremely loud, especially when you start the car or press the gas pedal, as if your car has no muffler. 
• Sputtering acceleration: Your vehicle may feel sluggish, sputter, or have less power than usual. 
• Rough idling: The engine may run unevenly or misfire. 
• Unusual exhaust smell: You may notice strong, noxious, or gasoline-like fumes entering the cabin. 

Visual clues

• Look under the vehicle: Check the area under your car, near the middle of the exhaust system. You will likely see a large gap where the converter was, signs of fresh cuts on pipes, or metal shavings.
• Look for damage: There may be visible damage, such as dents or unusual marks, on the undercarriage or exhaust pipes.
• Missing components: A missing section of pipe or cut wires where an oxygen sensor was connected can also be a sign. 

Warning lights and other indicators
This video demonstrates what a catalytic converter looks like and where it's located under a car: 50s1A Auto: Repair Tips & Secrets Only Mechanics KnowYouTube · Aug 24, 2024

• Check Engine light: The "check engine" light on your dashboard will likely turn on because the vehicle's computer detects the missing component. 
• Failed emissions test: If your vehicle fails an emissions test, a missing catalytic converter is a likely reason. 
• Health issues: In rare cases, some people have reported headaches or other illnesses due to exposure to exhaust fumes in a poorly ventilated area, as noted on this Allstate article.

Will a car run if the catalytic converter is stolen?

If your catalytic converter has been stolen or you bypass it because it's been having problems, your vehicle will still work. However, without a converter, your car lets out harmful emissions, including carbon monoxide and nitrogen oxides.

Will the check engine light come on if the catalytic converter is stolen?

Yes, the check engine light will likely come on if your catalytic converter is stolen because the vehicle's oxygen sensors will detect a malfunction in the exhaust system. The car's engine control unit (ECU) will register a problem with the emissions and turn on the check engine light. 
Why the check engine light turns on

• Sensor malfunction: A stolen catalytic converter means the exhaust system is physically broken, which causes the oxygen sensors to send incorrect data to the car's computer. 
• Emissions mismatch: The sensors will report a mismatch in what the engine is producing versus what the exhaust system is handling, triggering a diagnostic trouble code and illuminating the check engine light. 

Other signs of a stolen catalytic converter

• Loud noise: The most obvious sign is a sudden and extremely loud roaring or rumbling sound from your car's exhaust system. 
• Poor performance: The vehicle may run poorly, such as sputtering, jerking when accelerating, or having difficulty idling. 
• Visible damage: Look under your car to see if the exhaust system has been cut or if a section is missing. 
• Exhaust smell: You may notice the smell of exhaust fumes, which are now poisonous without the catalytic converter to process them. 

What to do if your catalytic converter is stolen

• Do not drive the car: If you suspect theft, avoid driving the vehicle. It is unsafe and continuing to drive could cause more damage. 
• Have the car towed: Have the vehicle towed to a mechanic to prevent further damage and for a proper inspection. 
• Get it replaced: A mechanic will need to replace the catalytic converter to fix the issue and turn off the check engine light.