2008 Mercedes Benz C200 catalysts filter remove

  • 2008 MERCEDES BENZ C200
  • 2.0L
  • 4 CYL
  • 140,000 MILES
I would like to know what would happen if the catalyst is removed?
Do you
have the same problem?
Friday, March 13th, 2015 AT 12:41 AM

1 Reply

You'll be crying over the repair bill.

Starting with all '96 models sold in the U.S, and therefore many other countries, there is now a second oxygen sensor after each catalytic converter, along with the ones in front that have been there since the '80s. When the engine is running properly, the front oxygen sensor(s) will switch from "rich" to "lean" about twice per second. During lean periods, unburned oxygen is stored in the catalyst. During rich periods unburned hydrocarbons mix with that oxygen and are burned. The Engine Computer adjusts the average fuel / air mixture by adjusting the ratio of lean times to rich times. That makes for lowest emissions.

All the rear oxygen sensors do is monitor the exhaust gas leaving the catalytic converters to see if it's staying somewhat lean or somewhat rich. If the converter is working properly, those sensors will switch between rich and lean perhaps once every minute or two. If the converter isn't completely cleaning up the emissions, the rear oxygen sensor will switch between rich and lean a little faster; lets say once per minute. As the converter loses more of its efficiency, the switching rate might increase to three times per minute. If the converter stops doing anything at all, no change will take place in the composition of the exhaust gases, so the front and rear oxygen sensors will switch at exactly the same rate. Before it even gets that bad, the faster rear switching rate is what the Engine Computer looks at to determine the converter isn't doing anything. It will set a diagnostic fault code, "catalytic converter efficiency".

Operation of the front, or "upstream" oxygen sensor works with the Engine Computer and its other sensors to PREVENT excessive emissions. The catalytic converter takes care of those emissions that occur in spite of our attempts to create clean exhaust. The rear oxygen sensor just makes sure that is happening.

Thanks to this system, you can suck on today's cars' tail pipes and live to tell about it. Emissions are mostly carbon dioxide and water vapor. Besides the harmful emissions resulting from removing the converter, two other things will happen. There are well over 2,000 potential diagnostic fault codes that can be set by the Engine Computer. About half of them refer to something that could have an adverse affect on emissions, and those are the ones that must turn on the Check Engine light. It's plain to see that removing the converter will set that "converter efficiency" code and turn the light on. Now, since the light is already on, how will you ever know if a totally different problem is detected? That problem could be a very minor one that will quickly turn expensive if ignored.

The second problem is the Engine Computer is constantly monitoring dozens of sensor readings and operating conditions. Most of the time the computer compares numerous things to each other to figure out when something is wrong. For example, it knows that when the throttle position sensor is indicating the engine is at idle, it had better not be running at 4,000 rpm. Also, when starting a cold engine, the coolant temperature sensor and the intake air temperature sensor had better be reading the same temperature.

When a problem is detected with a sensor, the computer knows it can't rely on its readings to test anything else it compares to them, so some of those self-tests will be canceled. It's not until the first problem is repaired that those tests will resume. If this goes on long enough, there's a good chance a second or third problem will develop, but if those tests are not taking place, they won't be detected. There won't be any related fault codes set. When you finally take the car to your mechanic, perhaps because now it has a running problem, all he has to go by is the first fault code that is in memory. His estimate for repair will be based on that code and his diagnosis.

Once that repair is completed and he takes the car for a test drive, the self-tests resume, and that's when the second or third problems are detected, and new fault codes are set. Now he has to tell you more diagnosis time and parts are needed. We hate having to do that, and it's a common reason car owners unfairly assume we misdiagnosed the problem the first time.

Without those diagnostic fault codes the computers give us, it would take days to test each circuit until we found the one with the problem, and if it's an intermittent problem that isn't acting up while we're in there testing, we'll never find it. That is what will happen if the catalytic converter is removed. If your engine develops any type of running problem, we are going to have to install a new converter first, then drive the car until a fault code sets, THEN return to the shop and start the new diagnosis.

Why is removing the converter even coming into question?
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Friday, March 13th, 2015 AT 2:52 AM

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