Bad G.C. Oxygen Sensing
Q:I replaced an exhaust manifold in an acquaintance's 4.0-liter 1997 Jeep Grand Cherokee. Since then, the Jeep's been running rough and the Check Engine light is on. I scanned the vehicle and found a fault for the MAP sensor and an oxygen sensor. He's since taken his Jeep to the dealer for diagnosis and was told that because the MAP sensor had the fuel trim too rich it ruined the catalytic converter and seized the O2 sensors. Can the MAP sensor cause the cat and the O2 sensors to fail?
A:Nothing's absolute, but it's almost a given that when a vehicle enters a repair shop running fine and leaves running poorly, the technician involved accidentally caused the problem. He may have cracked a spark plug, knocked off a vacuum hose, etc. MAP stands for manifold absolute pressure; the MAP sensor reads the engine's level of intake-manifold vacuum. Manifold vacuum is typically high at idle and decreases as engine speed increases. The oxygen sensor is mounted within the exhaust system and measures oxygen content in the exhaust gases. This information helps the Powertrain Control Module (engine's master computer) determine how rich or lean the air/fuel mixture is, so it can make the appropriate adjustments to fuel delivery. With that said, a vacuum leak or a misfire will have an effect on both engine vacuum and exhaust oxygen content, while causing the engine to run poorly. These conditions can often set trouble codes referring to particular sensors, even though the sensors themselves haven't failed. A MAP sensor causing a rich condition, which could damage the catalytic converter and an oxygen sensor, would certainly be considered atypical. There may be a difference in oxygen-sensor readings before and after the catalytic converter indicating a problem with the converter, but you want to be absolutely sure. Catalytic converters aren't cheap. Double-check everything you touched during the exhaust-manifold replacement. It may even be worth a second opinion from another Jeep technician.
RAM Van Hardware Problem
Q:While driving my 2001 Dodge Ram Van 1500 in the rain, I went through a puddle and the Check Engine light came on. At an emissions-test facility, code P0601, Internal Control Module Memory Check Sum Error, came up. The van runs fine, starts fine, and gets the same gas mileage as before the light came on. I then took it to a repair shop, where a technician cleared the computer and the Check Engine light came back on. He didn't reflash the computer--he just cleared the light and said the van needs a new PCM. I must get it fixed to pass the emissions test or the state will cancel my driver's license and revoke my license plate (isn't Illinois great?).
A:While I'm not up to date on Illinois emission inspection regulations, I know every vehicle in the U.S. needs a valid inspection certificate. The technician's recommendation to replace the PCM was correct. Engine code P0601 indicates an internal PCM failure--that's hardware, not software. A reflash, or reinstallation of software, isn't going to fix the problem--it's like reinstalling Windows on a PC to repair symptoms from a cracked motherboard. This is one of those "no ifs, ands, or buts" trouble codes. The PCM on the 1500 series van is mounted inside the engine compartment, against the firewall, near the wiper motor. Why there? Good question. A lot of other vehicles' PCMs are located under the dash in the passenger compartment, where they're better protected from heat, dirt, and moisture. Control units located under the hood utilize a more insulated and weather-resistant housing, but that wasn't enough in this case. It seems crashing that puddle sent a good-size wave right where you didn't want it to go and contaminated the PCM. Retail price on the part can range from $200 to $600 depending on the specific application and where you buy it. The labor cost shouldn't be too bad for installation and reprogramming.
Trailblazer Blower Motor
Q:I have a 4WD 2004 TrailBlazer EXT LS with the inline-six. All the vents work except the ones on the dashboard pointed at the passengers. I read somewhere that the blower-motor resistor might be the problem, so I replaced it. It still worked the same--the air is cold, but doesn't come out of those vents. I'll check the fuses, but have no idea which one does what.
A:The blower-motor resistor assembly turns the blower motor on or off, and changes blower motor speeds. It has nothing to do with directing air to the appropriate vents. There is a series of electronic actuators (motors) and doors which handle air delivery. A fuse that supplies voltage to these actuators is fuse #39, marked HVAC 1, located in the rear fuse block beneath the second-row seat. However, if turning the defrost vents on and off and controlling air temperature work, don't bother going under the seat--fuse #39 must okay because it also powers the defrost and temperature-control actuators and doors. What we're zeroing in on is the mode actuator. When the mode actuator is turned on it positions the mode door to direct air out the front panel vents. When it's turned off, airflow is directed to the lower vents for heat. In your situation the mode door may be stuck in the heat position, the actuator itself may be no good, or it's just not receiving the appropriate electronic control signals, if any. This is where it gets technical and requires special tools and experience to efficiently diagnose the problem. The HVAC (heat, ventilation, and air-conditioning system) is controlled by the HVAC control module. Like other control modules we know and love, this one stores trouble codes in its memory when a failure is detected, and supplies other valuable diagnostic information to technicians using the appropriate computer scan tool. Bottom line: The problem involves the mode door, and in all likelihood it's going to take more than a fuse to fix it.