The factory CD gives a one paragraph description on FOM. I have some additional questions.

If a car goes into FOM, I assume that it can only be reset after the ignition is turned off and restarted and the computer goes through it self checks/tests?

If a car has a marginal/defective O2 sensor, would FOM only occur after the car reached operating temps since the O2 sensor has no effect unit the car is hot? Or does the computer rely on slight voltage from the heated sensor to check that it is functioning?

Tinker

I wonder what kind of car it is.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

Sorry,

1987 190e. But anything with the CIS KE system has the fixed operating mode.

Tinker

The 1987 190E 2.3 does have a FOM, however it doesn't the ability that later cars do have. IF while running a sensor gives bad info the computer wil go back to a known good reading until the sensor starts working again OR if the car is switched off. THEN you will have problems if the sensor still isn't working.

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M.B. Doc,

Soooo. If a sensor is bad at cranking time, the car goes into FOM and one would expect to find a fixed duty cycle at the #3 pin at the X11 plug?

Tinker

No! If the sensor is bad before starting the computer doesn't have a reference to make that adjustment! ie; coolant temp sensor, when cold high resistance(open circuit) means a cold engine so the computer will try to enrichen the mixture even if the engine is HOT.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

M.B. Doc,

So what causes a fixed duty cycle at the X11 plug? Is it the computer indicating there is a problem and giving a hint where to look?

As always, thanks for the education.

Tinker