Slow down cyl 7-12 is on with buzzing sound

I have a problem with 87 Testarossa and need help to fix it. When I start the car slow down cyl 7-12 is on with buzzing sound. When the car starts I can feel that sometimes only six cylinders are running and very light gas smell for very short time. In the warm up process after maybe 10 minutes light will start to go off and on and eventually will be off. If I shut down and start the car again (when engine is hot) the light will be off or will be on for less than a half a minute and then will be off with everything works perfectly fine. In process to find issue and fix it following was done (one by one) but nothing really helped
New Battery (Interstate MT-34R), swap ignition coils, swap modules, bought new modules just to be sure – no difference, swap ECU’s, replaced spark plugs, check fuel pumps – both are good, checked fuses and relays so not sure what else can be done before car end up at dealership.
There is one other thing that I never had before. Sometimes when I put key and turn to position one speedometer will jump to 100 miles or so and came back to 0.
Regards,
Dean B.

 

ECU’s were swapped as well and no difference at all. Spark is there on both banks and when car warm up everything is fine, starts fine and all light are on and off as should be but when cold light is on until car warm up.

 

Number 1. Maybe this explanation can help little better. The light stays “on” continuously from initial start-up until car warm up. Cold car start fine and run fine for first 10 seconds but then somehow it losses some of the power and yes light 7-12 will stay on all the time. After 10 minutes light will go on and off and when car really warm up light is off. If warm car are shut down and started again no problem. Last night when light was off I drove a car for about 10 miles and light never came again. Car was running strong, sounds perfectly fine and there are not problems at all. So if I start car right now light 7-12 will be continuously on.

 

Problem is fixed and it was the oxygen sensor. I replaced both of them and there in no “slow down cyl 7-12” anymore. The original oxygen sensor is BOSCH # 0258003009 and can be found at O'Reilly ($62), AutoZone ($65) or Napa ($90) under part number 13957. The engine works so nice again with a beautiful sound. For this money I will replace O2 sensors every 1-2 years just to prevent any bigger problem because of them.

 

Both old O2 sensors are the same part number (Bosch 0258003009). Also, if you check O2 sensors at allferrariparts.com they are listed as
125881 - CO2 SENSOR 7 THRU 12 BANK, TESTAROSSA (BOSCH # 0258003009) - $411.25
121515 - LAMBDA SENSOR MOTRONIC 2.5, 328/TESTAROSSA/MONDIAL (BOSCH # 0258003009 - $236.25

 

The normal aging process will eventually cause the oxygen sensor to fail. Over time, oxygen sensors become "lazy" and their output signal deteriorate.
When the air/fuel mixture is rich and there is little O2 in the exhaust, the difference in oxygen levels across the sensing element generates a voltage through the sensor's platinum electrodes: typically 0.8 to 0.9 volts. When the air/fuel mixture is lean and there is more oxygen in the exhaust, the sensor's voltage output drops to 0.1 to 0.3 volts. When the air/fuel mixture is perfectly balanced and combustion is cleanest, the sensor's output voltage is around 0.45 volts.
The oxygen sensor's voltage signal is monitored by the onboard engine management computer to regulate the fuel mixture. When the computer sees a rich signal (high voltage) from the oxygen sensor, it commands the fuel mixture to go lean. When it receives a lean signal (low voltage) from the oxygen sensor, it commands the fuel mixture to go rich.
A dead sensor will prevent the onboard computer from making the necessary air/fuel corrections, causing the air/fuel mixture to run rich in the "open loop" mode of operation, resulting in much higher fuel consumption and emissions.
An additional consequence of any oxygen sensor failure may be damage to the catalytic converter. A rich operating condition causes the converter to run hotter than normal. If the converter gets hot enough, the catalyst substrate inside may actually melt forming a partial or complete blockage. The result can be a drastic drop in highway performance or stalling because of a buildup of backpressure in the exhaust system.
Oxygen sensor performance can be checked by reading the sensor's output voltage to make sure it corresponds with the air/fuel mixture (low when lean, high when rich). The voltage signal can also be displayed as a wave form on an oscilloscope to make sure the signal is changing back and forth from rich to lean and is responding quickly enough to changes in the air/fuel ratio.
Keeping the oxygen sensor in good operating condition will also minimize exhaust emissions, reduce the risk of costly damage to the catalytic converter and ensure peak engine performance (no surging or hesitating).
For these reasons, the oxygen sensor should be considered a "tune-up" replacement item just like spark plugs, especially on older vehicles (those built before the mid-1990s).