How To
John Paul, AAA Northeast’s Car Doctor, hears from a reader whose truck’s AC only works with the blower on high.
The Car Doctor hears from a reader whose truck’s AC only works with the blower on high. AP Photo/David Zalubowski, File
July 8, 2026 | 7:23 PM
4 minutes to read
Q. My 2007 Ford F-150 air conditioner switch only works on high; it will not work on low. The air is cold, but I don’t always want the fan blowing at full force. What is wrong, and what will it cost to fix?
A. The issue could be a faulty switch, wiring, or, most likely, the blower motor resistor. On many vehicles, when the fan only works at high speed, the blower motor resistor is the most common failure. If it is the resistor, replacement usually takes about 30 minutes and the part typically costs between $20 and $65. Lately, some inexpensive online replacement parts have not been the best quality, so try to buy from a reputable supplier or manufacturer.
Q. I have a 2011 Infiniti G25x. It was involved in an accident on the driver-side front. It was not a very major hit, but it affected the radiator support arm, headlamp assembly, radiator, and front bumper. I had to take it to a body shop for repairs and replacement of those parts. The car was driving and running fine before I brought it to the body shop, but after the repairs were completed, I discovered it was running sluggishly, rattling, and hesitating when leaving from a stop. What do you advise?
A. The repairs actually sound fairly extensive. Since the car was operating normally before the accident repairs, something may have been overlooked during reassembly or calibration. Possible causes could include a damaged sensor, loose wiring or ground connection, an air intake issue, or drivetrain damage that was not apparent initially. Normally, I would say to return to the original repair shop, but since they returned the vehicle in this condition, I would get a second opinion on the repair quality and have a third-party shop perform a complete diagnostic evaluation. Depending on what is found, you can then return to the original body shop and ask them to correct any issues related to the original repairs.
Q. My grandson has a 2017 Jeep Compass Trailhawk with less than 40,000 miles. He recently entered the Army and came home, and although we tried to drive it regularly, the battery was always dead and we had to jump-start the car. He had the main battery replaced, and it was fine while he was here. We now have a start/stop message showing on the dashboard. I took it to the dealership and was told it needed the auxiliary start/stop battery replaced, so I replaced the smaller battery. It is back at the dealership now, and I am being told it still needs to be diagnosed to find out what is wrong. He is stationed in Germany and wants to have his car shipped to him. Any thoughts about what is going on?
A. This Jeep uses two batteries: a main 12-volt battery for starting and an auxiliary battery for the automatic start/stop system and accessories when the engine shuts off at a stop. If either battery becomes weak or if the vehicle sits unused for extended periods, the charging and battery monitoring systems can set warning messages and fault codes. The problem could involve the intelligent battery sensor, poor cable connections, charging system issues, software updates, or another fault within the start/stop system. Unfortunately, there is no simple answer at this point. The Jeep will need to be properly diagnosed with factory-level scan equipment by the dealer or a qualified repair shop.
Q. I have a 2013 Toyota Yaris with about 50,000 miles. My check engine light keeps coming on. It’s been doing this for almost a year. I borrowed a car code reader diagnostic tool, and it gave me this reading: CODE: P0138 oxygen sensor circuit high voltage. It was suggested that I add Techron Complete Fuel System Cleaner to a tank of gas and drive the car on the highway for 30 to 60 miles. This worked for a little while, and I was able to get an inspection sticker, but the check engine light eventually came back on. I tried using the fuel system cleaner again, and it worked, but now the light has come back on. A friend told me not to worry about it unless the car started sounding funny. I’m worried that if I don’t have this taken care of, I might do more harm to my car in the long run. Could you give me your opinion on this problem?
A. Driving with the check engine light on for an extended period could reduce fuel economy, increase emissions, and potentially damage the catalytic converter if the engine is running excessively rich. Code P0138 generally indicates a high-voltage condition in the downstream oxygen sensor circuit, often caused by a faulty oxygen sensor, damaged wiring, or less commonly, an engine running condition. Fuel system cleaner may temporarily affect the readings, but it is unlikely to permanently solve the problem. At this point, a technician using a professional scan tool should be able to determine whether the sensor, wiring, or another issue is causing the fault.
Q. Eight years ago, I purchased a 2013 Ford Edge with the 2.0L turbo engine. After putting 700 miles on it, I took a trip. When I came back and checked the oil, very little showed on the dipstick. It was low by about 2 to 3 quarts. The tailpipe extensions were loaded with black soot. However, to this day, the engine still runs fine, although there is still soot on the pipes. What do you think happened?
A. It sounds as though the engine may have experienced higher-than-normal oil consumption during its initial break-in period, possibly because the piston rings had not fully seated yet. Some black soot on the tailpipes is also fairly common on modern direct-injected and turbocharged engines because they tend to run richer under certain operating conditions. Since the vehicle has continued to perform well for many years without apparent engine problems, I would simply continue maintaining it properly and monitor the oil level regularly between oil changes.
John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive business and is an ASE-certified master technician. E-mail your Car Doctor question to [email protected]. Listen to the Car Doctor Podcast on Apple Podcasts or other popular podcast sites.
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