2001 Dodge Caravan O2 sensor reads low voltage now trans no

  • 6 CYL
  • FWD
  • 107,000 MILES
I just replaced the top o2 sensor and the check engine light came back on. We took it and had it hooked to a computer and it said the o2 sensor had low voltage. Now the problem is the check engine light went back off, but the trans is not shifting properly. Iy you put it to the floor it starts off good and when it goes to shift it feels like it slips. If you let off the gas a little it kicks back in and runs great. Also if you go to pass someone it will accelerate to 60mph and then feel like it is stuck, but once again if you let off the gas a little it kicks back in and runs great. Also you can smell it in the cab. Could this all be caused by the o2 sensor reading low voltage?
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have the same problem?
Monday, March 23rd, 2009 AT 6:33 AM

1 Reply

If the sensor is reading low voltage, it's working. It's doing what it's supposed to do which is reading the amount of unburned oxygen in the exhaust. If you have a misfire condition, you might smell the raw fuel in the exhaust, but 02 sensors don't measure fuel, they only see the unburned oxygen. Look for a vacuum leak, pinched off fuel return hose, and defective spark plugs and wires. Normally the engine computer will set a fault code for random cylinder misfire or a specific cylinder misfire. Since it didn't, you might have a sensor issue that results in too much fuel to all cylinders, (think MAP sensor).

With a misfire, the 02 sensor will report unburned oxygen to the engine computer. The computer will try to add fuel to bring it back to the proper mixture, but the extra fuel will go to all cylinders equally. Five cylinders will now have too much fuel, and that one cylinder will still be misfiring and sending unburned oxygen into the exhaust, so you will probably smell that fuel.

With five cylinders running way too rich, don't expect normal performance. The transmission is also controlled with a computer. It controls shifts based on learned information from watching your driving habits. It also updates shift points over time by watching the volume of fluid it takes to apply the clutches. As the clutch plates wear, a tiny amount of slippage occurs during shifts. This happened over many years, so on older cars, you barely noticed until you had to have the transmission rebuilt; then, you would be amazed at how solidly it shifted. With this transmission, as the clutch plates wear and more fluid volume is needed to apply them, the computer learns to apply the next clutch a little sooner before releasing the previous clutch. The result is a nice solid shift until the plates are so worn, the computer can't update any further.

Once the computer can't update any more, slippage occurs during shifts. The problem is you don't have any warning of the impending doom like you did with older cars. One day it shifts fine; the next day it slips. When it slips, the computer defaults the transmission to second gear, and it stays there until you cycle the ignition switch off and back on. If you're running at wide-open-throttle, (which is never a part of a professional's diagnostic procedure), and it does not get stuck in second gear, the transmission is not slipping.

The transmission computer looks at the output speed sensor, throttle position sensor, MAP sensor, engine rpm, and coolant temperature to calculate shift points and harshness. The engine computer uses the same information to adjust fuel metering. Not only are sensor readings considered, the computer also looks at the direction of change and the rate of change, particularly of the throttle position and MAP sensors.

If the battery was dead or disconnected recently, minimum throttle and short and long-term fuel trims were lost from memory. Simply driving the vehicle in a normal manner will relearn this information. Part of the test drive must include coasting down from highway speed for seven seconds without touching the brake or gas pedals.

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Tuesday, March 24th, 2009 AT 6:20 PM

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