Got an ML350 in the shop with the infamous check engine light because of catalytic converter fault code. All four (upstream and downstream) O2 sensors are swinging to beat the band. On healthy ones I've watched the downstream signal is steady. Is the fact that all four swinging a sure thing that the cats are bad?

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Jim

If the rears are oscillating then the catalysts are bad. The million dollar question is why did two catalysts fail?

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

It is a used car that they guy just purchased. Showing around 220k miles. I may pull the plugs and see what they look like. As you said, something is going on.

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Jim

Finally got this one back in (for a brake job) and the check engine light is no longer on. I'll put the SDS on it later and see what the outlet o2 sensors are doing. Wonder what would make it come on with catalytic converter issues and then clear up?

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Jim

The downstream sensors are steady now.

After hooking up the SDS I found a couple of "f" fault codes indicating "The efficiency of the right cat converter is insufficient". Same fault code with the left one too. During driveability test the vehicle seems to use a lot of fuel. The complaint is that after the vehicle is up to temp every now and then it will stall in traffic. The vehicle is showing 240k miles.

I will say after looking at some live data that the M16/6 - throttle valve actuator signal 1 and 2 are NOT within spec.

Is there a definitive way to determine if the cats are shot other than observing the downstream O2? The CEL is not on.

Any thoughts/comments are appreciated.

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Jim