MB Medic Mercedes-Benz Repair Guides, Service, How To DIY
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What Happened
Recently, I had a check engine light for the O2 sensor on a Mercedes-Benz. The car also sounded like it had an exhaust leak.
After getting the car up on the lift, I discovered that the catalytic converter was separated. I received a couple of quotes, one for an aftermarket part that cost $1,400. The other option was to cut and fit a universal catalytic converter for $300 to $400.
Before spending $$$, I decided to remove the whole catalytic converter. The ceramic substrate was still intact, so I thought, why not fix the existing one? Welding the old catalytic converter fixed the exhaust leak and the O2 code.
Here is a picture of the catalytic converter split in half.
Is the Converter Worth Repairing?
The ceramic structure was still intact, which is what made this repair worth doing. If the ceramic substrate is cracked, melted, or missing, there is no point in trying to weld the shell back together. The check engine light will stay on, and the car will fail an emissions inspection. Do not remove, bypass, or disable the catalytic converter. Any repair must leave the emissions system fully functional and comply with federal and state requirements.
Here is the converter after welding. This was only my second time welding, so don’t be too harsh on me.
After sealing the exhaust leak, I cleared the fault code and completed a drive cycle to confirm that the code did not return.
Check the Sensor Data Before You Replace Anything
A split converter is an exhaust leak, and a leak ahead of the upstream oxygen sensor lets outside air in, so the sensor reads lean and the engine computer adds fuel to compensate. That is why a leak like this sets an oxygen sensor code even when the sensor itself is fine. Before replacing an oxygen sensor or the converter, use the UCAN-II Pro to read the exact fault code and compare the upstream and downstream oxygen sensor live data. An upstream sensor that switches normally while the fuel trims sit high points to a leak; a downstream sensor that switches as fast as the upstream one points to a converter that has stopped working. After the repair, clear the code and drive a full drive cycle, as I did here.
For where each sensor sits, see the Mercedes oxygen sensor locations; to replace one, see how to replace the oxygen sensor; and for the lean codes a leak can set, see Mercedes codes P0170 to P0173.
Before you buy parts, scan every control module, not just the engine. It shows the root fault behind the warning lights and tells you whether the problem is still active. See how to do a full health check.
