How Do You Know Which Catalytic Converter Is Bad

Dan Ferrell writes about practise-it-yourself car maintenance and repair. He has certifications in automation and control applied science.

The catalytic converter can lose efficiency or get plugged upwards.

Photo courtesy of Ahanix1989 on Wikimedia

Signs of a Bad Catalytic Converter

Ordinarily, when the catalytic converter runs into trouble, the Cheque Engine Light (CEL) will come on to alarm you. The computer may store a problem lawmaking (like P0420 through P0423) in memory. A problem code doesn't necessarily mean the expensive converter needs to exist replaced; you demand to do some troubleshooting before you decide.

These may be signs of a bad catalytic converter:

• The engine loses power.
• The engine stalls when idling.
• A scent of rotten eggs wafts effectually the vehicle.
• You hear a rattling sound coming from underneath the vehicle.

These symptoms bear witness up when your converter loses efficiency, the internal substrate disintegrates, the housing becomes partially restricted, or the converter becomes plugged up.

What Does a Catalytic Converter Do?

A catalytic converter reduces the post-obit harmful frazzle emissions:

• hydrocarbons (HC)
• carbon monoxide (CO) (two-fashion converter)
• oxides of nitrogen (NOx) (three-way converter)

It's possible that during normal performance, the converter can succumb to historic period and lose the ability to catalyze emissions. But this is rare. Most catalytic converters outlast the engine.

If the converter is damaged, it's more commonly acquired past engine operation problems (misfires, rich air-fuel mixture) or other serious issues that allow contamination (coolant and engine oil) to attain the frazzle system.

A bad catalytic converter can be difficult to diagnose. But the investigation tin be well worth it. Earlier y'all send your car to the shop or decide to supervene upon a potentially expert converter, the following serial of tests will help you troubleshoot the unit of measurement using some special just inexpensive tools.

Locating the Converter(s)

The catalytic converter is located between the exhaust manifold and the muffler. The converter looks similar a small muffler with a pipe connected at each end.

Depending on your vehicle, you may take one or 2 converters. Also, many Chrysler and Mitsubishi models use a mini-converter between the exhaust manifold and the main catalytic converter. The smaller converter heats up fast, so it begins reducing emissions as soon as a cold engine is started.

To locate the converter, if necessary, check the Haynes vehicle repair transmission for your particular make and model.

Troubleshooting Your Converter: Four Tests

To make up one's mind whether your catalytic converter is clogged or has lost efficiency, apply the post-obit four tests. For some tests, you lot'll demand a special tool, which you tin can find at almost motorcar-parts stores or online.

Apply 2 or more of these tests to ostend your findings.

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A vacuum gauge tin can help you lot test for a restricted exhaust arrangement.

Photo in the public domain

1. Converter Brake: The Vacuum Test

An exhaust-system restriction will crusade a loss of engine ability, just then tin many other problems. If you suspect a plugged-up converter, you need to test the organisation. This test requires a vacuum gauge; if you don't have one, consider buying one. A vacuum guess is a useful diagnostic tool that will come up in handy in other repair projects.

1. Set the transmission to park (automated) or neutral (manual), and utilise the parking brake.
2. Disconnect the vacuum hose at the brake ability booster and connect the vacuum gauge to the hose or to another direct intake manifold port.
3. Start the engine and let it idle for about 15 to 20 minutes then that it reaches operating temperature.
4. At idle, vacuum reading should be betwixt eighteen and 22 in-Hg (inches of Mercury).
5. Increment and hold ending speed to near 3000rpm. Vacuum reading should drop but should go dorsum to the previous level in a few seconds; otherwise, information technology's likely the exhaust system is blocked. Proceed to the side by side step.
6. Repeat the examination, but this time, snap the throttle valve open nigh iv times, rising engine speed to about 2500rpm. Take note of the vacuum reading with each snap of the throttle.
7. If you see that judge needle dropping steadily to near zero, about probable the catalytic converter or muffler is restricted.
8. To confirm a possible brake, loosen the back pipage from the catalytic converter just enough to permit gas to flow through.
9. Repeat footstep 6. If vacuum doesn't drop considerably this fourth dimension, you've establish the blockage (back pipage or muffler). Otherwise, go to the next stride.
10. Loosen the header exhaust pipage from the converter but plenty to allow exhaust gas to menstruation through.
11. Repeat step half-dozen and take annotation of your vacuum readings. If your vacuum readings prove normal vacuum, the converter is restricted or plugged up.

Tip: A Fast Way to Exam for a Restricted Catalytic Converter

Loosen the catalytic converter from the header piping or exhaust manifold. This allows gases to catamenia through. If you notice an improvement in engine performance, you are on the correct track. If necessary, you can follow the same procedure to test for a possible restricted muffler. All the same, double-check your findings using one or more of the methods outlined before deciding that you actually need to supplant a restricted converter or muffler.

A restricted exhaust organisation will rob engine ability.

Photo in Public Domain

2. Converter Restriction: The Dorsum Pressure Test

Just like the vacuum examination to a higher place, a back pressure level examination can help yous diagnose a clogged exhaust organization, but this test is done directly at the exhaust system to detect a plugged catalytic converter or muffler or pipage.

This unproblematic test requires a back pressure gauge. Yous may detect the guess at your local auto parts store or online.

To acquit the examination, remove the oxygen sensor (front or rear). Sometimes, the oxygen sensor is difficult to remove because the exhaust arrangement operates at loftier temperatures, causing the sensor to stick to the threaded bore it mounts on. (If yous run into this problem, refer to the department below, How to Unstuck a Stuck Oxygen Sensor.)

1. Outset the engine and let information technology idle for about 20 minutes--or caput over to the highway for a 20-minute ride and and then go back to your garage.
2. Locate and remove the front end (pre-catalytic) oxygen sensor. You may need to heighten your auto with a floor jack and safely support it with a couple of jack stands, then block the rear wheels with a couple of wooden blocks.
3. So, install the pressure gauge in identify of the oxygen sensor.
4. With the gauge in identify, start the engine and take annotation of the approximate pressure reading.
5. Normal backpressure should be close to zilch: iii psi at virtually.
6. When increasing engine speed to about 2500 rpm, you should get a pressure reading betwixt 1 and three psi. Your mileage may vary, depending on your item model. Nevertheless, a reading of around 8 psi or higher ways y'all are dealing with a chock-full converter, exhaust pipe, or muffler.
7. If you need to exam the back pipage or muffler, supercede the oxygen sensor and remove the catalyst monitor (rear oxygen sensor) and install the back pressure judge in its place.
8. Repeat step 6.
9. Take a pressure reading at idle and at higher engine speeds.
10. If back pressure is normal this time, you have a clogged catalytic converter; if back pressure is still high, y'all have a clogged muffler or back pipe.

A goad monitor (oxygen sensor) tin can detect an inefficient catalytic converter.

Photo courtesy of Hans Haase on Wikimedia

Tip: How to Loosen a Stuck Oxygen Sensor

Oxygen sensors tend to seize to the frazzle pipes or manifold due to the high temperature at which they take to operate. This makes them difficult to remove because forcing the sensor loose can damage the boss threads.

If yous find information technology hard to loosen the oxygen sensor, follow these steps:

1. A few hours earlier you programme to remove the sensor, employ a quality penetrating oil to the base of operations of the sensor.
2. If time allows, apply the oil again a couple more times, an 60 minutes or ii apart.
3. When you're gear up to remove the sensor, park your car on a flat surface, lift the forepart of your vehicle, and safely support information technology with jack stands, if y'all demand to, to access or reach the sensor. Block the rear wheels and set the parking brake.
4. Start the engine and let it idle for about ten minutes to raise the exhaust temperature. This volition aid to loosen the sensor.
5. Be careful when starting to piece of work on the sensor itself: the exhaust pipes and manifold will be at a high temperature at this point and y'all can seriously burn down your pare.
6. Disconnect the negative (black) battery cable from the battery.
7. Unplug the sensor electrical connector, and loosen and remove the sensor using an oxygen sensor socket. If you don't have the socket, use a half dozen-indicate wrench that can safely catch the sensor.
8. If the sensor still feels stuck, just plow the sensor counterclockwise a few degrees until yous can't turn it anymore, then utilise penetrating oil and re-tighten the sensor.
9. Repeat the previous stride until the sensor becomes loose.

An infrared thermometer can help yous diagnose a bad catalytic converter.

Photo courtesy of Ra Boe on Wikimedia

3. Catalyst Efficiency: The Temperature Examination

When catalytic converter efficiency degrades to the point that it stops catalyzing exhaust gases properly, the catalyst monitor (rear oxygen sensor) will report it to the computer.

Following a pre-configured procedure, the computer will then turn on the check engine low-cal (CEL) or malfunction indicator light (MIL) and store a trouble code, unremarkably P0420 through P0424.

Nevertheless, you tin test your catalytic converter efficiency using a suitable kitchen thermometer, or better notwithstanding, an infrared thermometer like this one (recommended).

1. Offset the engine and let information technology idle for about twenty minutes to bring it upwards to operating temperature. Or accept your car to the highway and and then come back home.
2. Park your car on a flat surface, gear up your transmission to Park (automatic) or Neutral (transmission).
3. If you don't have easy access to the front or rear of the catalytic converter, heighten your vehicle using a floor jack and safely support information technology on jack stands.
4. Have a temperature reading of the inlet pipe that connects to the front of the catalytic converter, and have notation of the reading.
5. And then, have a temperature reading of the outlet pipe that connects to the rear of the catalytic converter and have note of the reading.

If the catalyst has stopped working, the forepart temperature volition be about the same equally or higher than the rear temperature.

In a expert catalytic converter, the rear of the converter will be 20+ degrees hotter than the front.

Tip: Don't Ignore the Check Engine Light

The check engine light (CEL) or malfunction indicator calorie-free (MIL) is your best friend when dealing with emission problems. Whenever this light comes on, check for trouble codes. And if the light is flashing, the light is warning you of a malfunction that will seriously impairment your converter if not fixed. Don't ignore the light.

Test the condition of the substrate in a monolithic-type converter using a rubber mallet.

Photograph courtesy of Ildar Sagdejev on Wikimedia

4. Catalyst Substrate Condition: The Tap Exam

Pellet catalytic converters use lilliputian ceramic chaplet coated with catalyst. The pellets tin atomize considering of overheating and plug the unit.

Other vehicles use a "monolithic" converter," a ceramic honeycomb coated with goad. This substrate is firmly held inside the converter. Only if the converter takes a direct hit from a hard object on the route or during an blow, the substrate can crack or break into pieces. Besides, operating nether overheating conditions for a prolonged time can break the internal structure.

To test the concrete status of a monolithic type converter, you can employ a condom mallet, or a regular hammer wrapped in a piece of cloth.

1. Heighten the forepart of the vehicle (if necessary to access the converter) and support it with jack stands.
2. Chock the rear wheels and utilize the parking brake.
3. Tap the converter with the rubber mallet.
4. If it rattles, the internal substrate has collapsed and you need to replace the converter.

Ignition system issues can adversely affect the catalytic converter.

Photo courtesy of Allumage_batterie.svg: Frédéric MICHEL on Wikipedia

How to Protect Your Converter

A catalytic converter is a reliable and efficient vehicle emission system device and tin outlive your car'southward engine. Notwithstanding, engine system problems, lack of proper engine maintenance, and driving mostly short trips tin can take a price on a converter's service life.

Contamination—oil, coolant, or unburned fuel leaking from the cylinders—can likewise destroy a catalytic converter.

To avert converter impairment, follow your manufacturer's maintenance schedule for the fuel, ignition and emission systems. Also, make needed repairs when problems like the post-obit prove upward:

1. Set up any arrangement problem that causes the engine to routinely operate using a "rich" air-fuel mixture. Unburned fuel will cause the catalytic converter to overheat and, eventually, to destroy itself.
2. Supplant old or fouled spark plugs that can foreclose proper combustion of the air-fuel mixture.
3. Fix ignition system problems caused by poor maintenance or component failure.
4. Use fuel and oil treatments to fight carbon buildup inside cylinders and around valves. Such buildup can lead to engine oil leaks into the combustion chamber or bad pinch.
5. Fix cylinder gasket issues that allow oil or coolant to leak into the combustion chambers. Meet the next section for clues to leaks into the combustion chambers.
6. Avoid driving merely short trips (nether 20 minutes) that prevent the catalytic converter from reaching operating temperature and called-for off buildup. If necessary, take your motorcar for a spin on the highway once a week to help the converter (and engine oil) burn buildup, contamination and other impurities.
7. Avoid driving with a cylinder misfire condition. Misfires allow unburned fuel to laissez passer to the catalytic converter.
8. Don't run the engine below operating temperature. This also volition continue the converter from doing its task. One reason an engine will run below operating temperature is a stuck-open thermostat.

Black fume tin be an indication of too much fuel consumption and bug for your catalytic converter.

Photograph courtesy of Ilya Plekhanov on Wikimedia

Signs of Combustion Chamber Leaks

Usually, you tin can tell when you have a serious cylinder leak problem by looking at the smoke coming out the tailpipe:

• When the engine is called-for oil, you'll run across bluish-gray fume.
• A rich condition volition produce black fume.
• A coolant leak into the combustion chamber will crusade thick white smoke (but don't confuse this with normal condensation).

Although a converter tin can succumb to age, most unremarkably its failure can be blamed on unburned fuel, contamination (oil or coolant), or overheating. Declining to address any of these problems on time volition brand the catalyst burn itself into uselessness or become plugged. Then brand necessary repairs pronto. And, to help extend your catalytic converter service life, follow the maintenance schedule as described in your vehicle repair transmission.

This article is accurate and truthful to the all-time of the author's knowledge. Content is for informational or entertainment purposes but and does non substitute for personal counsel or professional person communication in business organization, financial, legal, or technical matters.

Questions & Answers

Question: What if my auto didn't smell like rotten eggs, but is otherwise as described in this article, is it still the catalytic converter that went bad or would it exist the muffler?

Respond: If the engine doesn't take much power, endeavour loosening the header pipage, earlier the cat. Then, tighten the pipe and loosen after the cat. See if there's a difference in engine operation.

Question: My malfunction indicator low-cal was on, simply when the store did a diagnostic, they said I needed my catalytic converter changed. Should I replace it?

Answer: Did you feel any of symptoms of a bad catalytic converter? Did they give you the reasons why you lot need to change information technology? Was the check engine low-cal flashing? What was the trouble code stored in the computer's retentivity?

You may want to test the true cat yourself past applying i or ii of the procedures described in the post, or get a second opinion from a reputable store showtime.

Question: What is wrong with my 2010 Ford F150, 4.6l. It reads codes P0171, P0174, P0300. I've replaced the pump commuter module, and it has 48lbs of fuel pressure level. Why won't stay running longer than a couple of seconds?

Answer: These other posts may help you lot diagnose the problem:

https://axleaddict.com/auto-repair/Problem-Code-P0...

https://axleaddict.com/auto-repair/P0300-Random-Mi...

Question: How much does it cost for a mechanic to bank check the catalytic converter?

Answer: Prices vary. It depends on the shop, but expect to pay about $100 or more.

Question: Would a clogged cat cause the loss of power from the engine and the upstream sensor?

Reply: A quick test is to loosen the exhaust pipe, betwixt the manifold and catalytic converter, to let the engine to breathe. If you experience the engine regains its power, it'south possible the cat is clogged. Also, yous can apply a vacuum gauge; at idle, yous get a reading between xv-22 in-Hg (inches of mercury).

Question: No power loss, no scent, merely the engine light on and been diagnosed with fault codes. Garage said aging cat. What could exist the problem?

Answer: When the catalytic converter is clogging yous may feel a loss of power. Quondam the true cat element may brainstorm to atomize. You may hear a rattling when tapping the true cat with a mallet. You tin test the cat with a vacuum estimate or pressure level guess. You tin can borrow the tools at a local automobile parts shop. Also, the guys in the motorcar parts store can download the trouble codes for you and tell you what they mean.

Question: I have a Ford Ranger. The converter was rattling then lost power. I replaced back converter, light went off so came back. Truck has 2 more converters upfront I didn't supersede. Cat code all the same comes up and sometimes a misfire lawmaking comes up before long after. Is information technology possible the ii I didn't replace can crusade it to misfire? Should I supplant all the converters?

Answer: You may want to utilise a vacuum gauge to troubleshoot for backpressure earlier replacing whatever components. This tin tell you lot if i of the other cats is chock-full. But you should besides check for a misfire. https://axleaddict.com/auto-repair/Use-a-Vacuum-Ga...

https://axleaddict.com/car-repair/Engine-Misfires...

Question: I go on getting the P0420 lawmaking. I tin can't afford repairs at this fourth dimension. The car is running nifty with none of the symptoms described in this commodity. My registration renewal is a few months away, at which time I tin go funding for any repairs for emissions failure. Tin can I impairment anything past waiting to practise the repair?

Respond: Y'all may discover a loose of ability; over time, information technology may crusade other driveability issues that may turn into serious problems. The lawmaking basically means the catalytic converter efficiency is lower than it should be. Merely this doesn't necessarily mean the true cat is bad. First, you may desire to cheque the oxygen sensor(south). This other mail may help:

https://axleaddict.com/machine-repair/How-to-Check-an...

Inspect the exhaust system for leaks or like problems. So, have the cat bank check to make sure information technology is the problem. Hope this helps.

Question: Idling/Manual transmission shudder at depression RPM and in betwixt shifts, cats going bad?

Answer: The problem could be in the clutch assembly (disc, pressure level plate) or pilot begetting. This other mail may help:

https://axleaddict.com/auto-repair/Bad-Clutch-Symp...

© 2017 Dan Ferrell

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