Quick "slow down" light question

Discussion in '348/355' started by ahrddr, Mar 5, 2009.

  1. ahrddr

    ahrddr Rookie

    Aug 1, 2007
    So Cal/N. VA/FL
    Hey guys, got a 'slow down' light on my 96 (5.2 motronic) 355 the other day. cruising down the highway, light came on..5 min. later, light goes out. Headed home anyway so kept going. Normal acceleration from stop sign and light comes on again but this time feels/sounds like passenger side bank of the engine shut down. I continue since I'm like 2 blocks from home and then all of a sudden the shut down side of the engine comes back on (slow down light still on). I go around the block a couple times and the slow down light goes off again. Drive a little more to see if I can get the problem to repeat and sure enough, slow down light and not long after the passenger side of the engine goes out....about 5 min of cruising and the engine comes back on again and another 5 min or so and the light goes out again. Parked it after that-Going to take to mechanic but like to educate myself as much as possible before doing that so that I can make informed decisions and also so I lessen the chance of being taken advantage of. I know it's not much to go on, but are there any "usual suspects" for what I'm describing? Any help would be appreciated. Looking forward to getting back on the road-Thanks in advance.
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  3. 3forty8

    3forty8 F1 Rookie
    Rossa Subscribed Owner

    Apr 25, 2006
    San Diego
    Full Name:
    Sounds like the ECU is shutting down the bank becuase it is getting a signal that the catalytic converter on the same side is overheating. If that is the case it could be due to a faulty temperature sensor or the catalyst material is breaking down and restricting exhaust flow.
  4. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    Full Name:
    Hugh G. Rection
    #3 f355spider, Mar 5, 2009
    Last edited: Mar 5, 2009
    The most likely scenario (been there) is simply a defective catalyst ECU. There are three of them on a 5.2 Motronic car. There are two on the right side (behind the bumper) and one on the left side beneath the left side lateral panel. The upper right side one is for the exhaust bypass valve, and the lower one is for the right side catalyst. If you think the right side is at fault, try this simple diagnostic. Swap the two plugs on the lower right side ECU with the upper right side ECU. Now, if that right side catalyst ECU is truly bad, it will cause you to instead get a "check engine" light for the exhaust bypass valve. Scan for the code, and if indeed that is what comes up, go by a new catalyst ECU from Ricambi America and you are good to go. Ferrari of Seattle assisted me with this diagnostic trick when I was having a very intermittent shut down on my 355...they did a bunch of diagnostics, but could not determine for sure, but suspected the right side catalyst ECU, so I tried the swap and sure enough a month or two later and up came the CE light. In your instance, since it is acting up so much, you should get a CE light in short order. (perhaps three drive cycles)

    If your problem is with the left side, then this will have no effect, and if you experience the same problem, then I would physically swap one of the right side ECU's with the left one.

    Good luck and please report back your findings.

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