Yup. You already have the first two answers, at least as far as which circuit to look at. The manifold absolute pressure (MAP) sensor measures intake manifold vacuum which is an indicator of load on the engine. More load equals the need for more fuel. This sensor has the biggest say in how much fuel enters the engine. It also reads barometric pressure before the engine is started. That affects how much air is drawn into the engine so the Engine Computer adjusts how much fuel it delivers accordingly.
The three voltages should be measured on the three wires to this sensor. The suply voltage must be very near 5.0 volts. The ground wire will typically have 0.02 volts. When the engine is not running, the signal voltage should be around 4.2 to 4.5 volts. When the engine is running, the signal voltage will drop to somewhere around 1.0 to 1.5 volts. The fault code indicates the signal voltage is too low compared to the operating characteristics of the engine. That voltage should be low during coasting, for example. If it remains low during acceleration, it suggests it's reading a high manifold vacuum which doesn't agree with the acceleration. The MAP sensor can read low vacuum when it shouldn't due to a loose or cracked vacuum hose going to it, but to incorrectly read high vacuum, the sensor itself would have to be at fault.
That fault code could be interpretted a different way. Normally "low input" means low signal voltage, but if by "input" they mean manifold vacuum, which is the input to the sensor, that COULD be caused by a cracked vacuum hose. The place to start is by measuring the signal voltages under various conditions. If no other problem can be found, replace the MAP sensor.
The air temperature sensor high is most often caused by the sensor being unplugged. You will measure 5.0 volts on the wire when it is unplugged. It must read considerably lower when the wire is reconnected. As air temperature goes up, the signal voltage on the sensor will go down. It must also read the same temperature as the engine coolant temperature sensor after the engine has cooled down for around six hours or more. All you can do with this sensor is to verify the voltage is between 0.5 and 4.5 volts at all times. 2 to 3 volts is typical at 70 degrees. To determine if the sensor is accurate, you will need to have your mechanic connect his hand-held computer, called a scanner, that can read live sensor data, and read the exact temperature the sensor is reporting. It will not be off by a few degrees. If the sensor is defective, it will read some wild value such as -20 degrees when it's really 50 degrees.
Either of these two codes can be set by disconnecting things while the ignition switch is turned on. They will erase by themselves after about 50 engine starts if the problems don't come back. You can erase them yourself by disconnecting the battery for a minute or two. If the codes come back right away, suspect a sensor OR wiring problem. If the codes take a while to come back, just suspect the sensors. If the codes do not come back, suspect they were caused by a temporary thing such as the mechanic pulling off a connector to insect the pins for corrosion. In that case, there may not even be a problem.
The misfire code can also be set intentionally but only by performing other tests. It generally takes a while to set it too. One quick test isn't sufficient. The question then is when did the codes set? If the Check Engine light turned on while you were driving, that is probably when it set the code. Finding the cause is a little more involved than the first two codes. As I mentioned previously, missing spark or missing fuel will cause the misfire. The code itself doesn't tell which one is missing. Lack of compression can also cause a misfire, but that would usually be there all the time and you would definitely feel it. I'm going on the assumption you don't feel the misfire while you're driving. That's the advantage of the computer detecting it. It sees things long before you feel them.
When a misfire occurs very infrequently, it can be real hard to diagnose. A lot of people just start replacing parts, but that adds a whole bunch more variables to worry about. It's better to determine the exact cause, then put the new part in and verify the problem is gone. Some potential causes of a single cylinder misfire include the spark plug, the fuel injector, and the coil for engines with individual coil-on-plug systems. With coil-on-plug, it is fairly easy to switch two coils and two spark plugs. If the fault code comes back but for a different cylinder, suspect those parts. If the code is for the same cylinder, suspect the injector. Injectors are a little more involved to switch, but the same principle applies.
Holler back if / when you try some of these things.
Thursday, April 22nd, 2010 AT 3:29 PM