Answer:   This is difficult because the computer can't detect a problem, your car thinks everything is ok. In reality what is happening is the electrical component inside the MAF sensor becomes contaminated by the impurities in the air (heavies) These containments imbed themselves to electrical component that senses the air flow causing the sensor to be slightly off, but not enough to trigger the MIL (malfunction indicator lamp). Replace the MAF sensor with a new sensor and you're problem will go away, plus your car get better mileage and performance. (Mass Air Flow MAF sensor or Air Intake Sensor AIS, depending on manufacture. It does the same thing with different names).

Question: 2003 Ford Mustang mileage 71,000. Service Engine light on. Car runs rough between 35-40 Miles per hour. “Lopes.” Code scanner says EGR Flow Low. Replaced EGR vacuum regulator and EGR valve with no luck. Lots of black smoke in exhaust when under hard acceleration. Service engine light used to be intermittent but now it's always on. Ford did warranty work on this car at about 20k miles, and removed the intake manifold, and removed "fouling" from the interior of the manifold. Any suggestions?

Answer: Your problem maybe a failing mass air-flow sensor or possibly oxygen sensors.

Question: 2001 Ford F150 mileage: 132,000. I had a long block installed at 104,000 miles. About 5,000 miles ago my check engine light came on, the code indicated it was the oxygen sensor, so I replaced it. The light came on again 1500 miles later, again the code said it was the 02 sensor, so I replaced it. About 1500 later again the same story, so I got a different brand. This one lasted about 2000 miles and again the light is on and the code says it is the oxygen sensor reading lean. What else could the problem be? Could it be giving me a wrong code? The code is 172.

Answer: The oxygen sensor reading lean is probably caused by a malfunction in the Mass Air Flow Sensor

Question: 2003 Ford Taurus mileage: 54,000. I just had a diagnosis done on this car and the Mechanic told me both the Oxygen Sensors were no good...I checked around and it will cost close to $500 to fix it. I talked to another mechanic and he told me you could just take them off, wire brush them and they are as good as new. First, is this true? Second, where are the Oxygen sensors located at on this vehicle?

Answer: If your oxygen sensors are faulty, wire brushing will not fix them. They are located in the exhaust system close to the engine and after the catalytic converter. Replace Oxygen Sensors for about $70.00 each at eAutoParStore.com

Question: 2003 Mitsubishi Eclipse mileage: 53,000. I am replacing the exhaust manifold. Old manifold has a some sort of sensor screwed into it, but the new one does not have a port for the sensor. Can the car function adequately without the sensor? What is the sensor?

Answer: The sensor you are referring to on the exhaust manifold is the oxygen sensor. It is an extremely important part of the computer management system for your engine.

Question: 2003 Chrysler 300M mileage: 72,000. Check engine light is on, idles rough. Had the car in the dealer and they gave me a long laundry list of about $1,200 worth of stuff to fix which included, fan belts, injection service, and water pump. In May the timing belt broke, so we had that replaced, along with the water pump. The code when you do the key thingy, says oxygen sensor. This car had 4 of them, at about $120 each. I picked one of them and replaced it. Unplugged the battery and it went out for about 2 weeks. Now it is back on. My question is this: the car runs fine other than the rough idle, will one of the oxygen sensors being bad cause my problems? Is there any easy way to check them without paying an arm and a leg?

Answer: A bad oxygen sensor will cause an engine to run poor. Scan the computer, it will tell you which of the oxygen sensors has failed. Simply replace the sensor that is bad and the repair is complete, clear trouble codes.

Question: 2004 Dodge Neon, mileage: 34,000. I had the local Chrysler dealer replace the oxygen sensor after the "check engine" light came on for about $250.00. My daughter (the primary driver of the car) states that although the engine seems to run OK, the car now lacks the power it had before the check engine light came on. Is it possible that the replacement O2 sensor is not up to spec, possibly causing the engine to run lean (or rich)?

Answer: The car will actual run slower when the check engine light is on because it is in limp mode. And back to normal when repaired and codes cleared.

Question: 2004 Mazda Miata mileage: 70,000. What are all the problems that a failed oxygen sensor causes if I keep driving my car? Will my car quit on me or anything? What am I in danger of as far as hurting my car?

Answer: Oxygen sensors read the fuel mixture in the exhaust and send the information to the computer. The computer then adjusts the fuel mixture. By not replacing faulty oxygen sensors, you can expect that your check engine light will eventually come on accompanied by erratic idle and poor running. Your car probably will not quit running but there could be damage to the catalytic converter.

Question: 2004 Ford Escape mileage: 36,500. Check engine light is on. Garage suggested several possible problems starting with Oxygen sensor - cost of repair $300.00 My problem is that he does not know if this is the problem, and that I will have to keep on paying for every fault finding repair even if that's not the one! Can you help with diagnosis and the best way forward? PLEASE.

Answer:   We have seen the Mass Air Flow Sensor and oxygen sensors cause this problem. Replace sensors to repair system. Clear codes to recheck system.

Question: 2002 GMC 1500 mileage: 70,000. The service manual for my truck tells me that my truck has 4 oxygen sensors. They are designated: Bank 1 Sensor 1, Bank 1 Sensor 2, Bank 1 Sensor 3, and, Bank 2 Sensor 1. The manual does not clearly indicate the location of each of these sensors. A GMC technician tells me that the term "Bank" refers to a particular side of the engine - Bank 1 refers to the side with cylinder number 1...the driver's side (as the plug wires are numbered 1,3,5,7). You'd think it would be easy to deduce the placement of each oxygen sensor. Here's the problem: Only one oxygen sensor lies beneath Bank 1. The other three are under Bank 2... two in front and one behind the catalytic converter. My information seems to be contradictory. Error code P0153 tells me I need to replace Bank 2 Sensor 1. Can you verify its location? Thanks.

Answer: Your truck could have 3 or 4 sensors, bank one is the side with #1 cylinder, both banks have primary oxygen sensors (before catalytic converter). Depending on exhaust options you may have 1 or 2 sensors behind the catalytic converter (secondary).

Question: 2002 Dodge Dakota mileage: 85,000.  Would a bad O2 sensor (upstream) cause a car to hesitate during acceleration in low gears? What causes these to go bad?

Answer: Yes, it is possible for an oxygen sensor to cause the poor running. Oxygen sensors eventually burn themselves out due to the heat of the exhaust system.

Question:   2003 Toyota Sienna mileage: 61,000.  Dear Bob and Ken: My problem concerns my car, 4-speed manual transmission, most recent oil change was about 1800 miles ago, and no major repairs since purchasing the car at 40000 miles.   I recently made a 1600-mile trip from Virginia to Colorado; I had the oil changed, fluids topped-off, and air filter replaced.   Additionally, I had an emissions inspection done a few weeks before, and the results were excellent.  Approximately half way through the trip and some very bumpy interstate driving in Arkansas and Oklahoma, my "Engine" light came on.  I pulled over immediately to check under the engine, checked the oil level, belts, hoses, etc. and found no noticeable irregularities, except that the overflow container for the radiator fluid was empty.  There was still plenty of fluid in the radiator itself, but I replaced the fluid/water mixture anyway.  I noticed no odd smells or drivability difficulties, so I continued driving.  I stopped overnight and the following morning, I checked my owner’s manual and it said that this light comes on due to a problem with the engine's electrical system.  Being that the roads were rough, I suspected maybe a fuse had been knocked out, so I removed, inspected, then replaced all of the fuses.

 I started the car and the engine light was off.  I continued driving, but after about 6 hours of interstate driving (70-80 mph) the light came on again.   Still, I did not notice any smells, sounds, or drivability problems.  I continued to drive as I was in the middle of nowhere and arrived in Colorado, still with the engine light on.  I spoke with a technician about this before and his recommendation was that the oxygen sensor could be the problem.  I have researched the function and possible problems of the O2 sensor and it seems like if that were the culprit, my exhaust would smell like "rotten eggs" OR produce a black smoke (depending on lean or rich mixture).  Is this correct?  Or could the sensor be malfunctioning WITHOUT a real problem with the mixture?  What type of problems could develop with continued driving?  Could there be another cause to the engine light coming on?   Is there some way to check the sensor myself without having to take to a mechanic?  Could the missing radiator fluid have caused something to go wrong (I have noticed no leaks in the overflow tank since)?  Could the change in altitude cause something?  I apologize for the length of this, but wanted to make sure to get all of the information here.  Thank you in advance for your help!

Answer:   Scan the computer for fault codes. You probably have a faulty oxygen sensor. Oxygen sensors can operate out of range. And can cause the problem you are describing.

Question: 2002 Pontiac Montana mileage: 117,000.  I've recently ( within the last 500 miles) noticed a tendency for the vehicle to stall and hesitate when it's at operating temperature. I also notice the tach jumping between 500 - 1000 rpm when in drive.  The car has a 4 speed automatic and it has been tuned up about 30,000 miles ago.  The coolant sensor has been replaced, but I'm thinking the TPS sensor needs replacing.  During regular driving, it's ok; it's just at stoplights that the stalling and surging occurs.  Am I on the right track with the TPS sensor?

Answer: Scan the computer for fault codes. You probably have a faulty Mass Air Flow Sensor or oxygen sensor.

Question: 2002 Mitsubishi Montero mileage: 10,921. My truck was working fine but due for a major tune up. I took it into SpeeDee tune and they performed the work. When I picked up my truck it drove fine for about 20 minutes. Then when I came to a stop I noticed my truck's idle was very high and fluctuating - from 2500 to 3500 RPMs. The idle should be between 500 and 1500 RPMs ideal is at 750 according to my manual. I took it back to them and they adjusted the throttle. I drove off again and in 20 minutes my idle was at 4500 RPMs. I found a mechanic at 6:30 PM on Saturday, Memorial day weekend, I slipped him $60.00 - he said the distributor was anywhere from 45 to 90 degrees out of position and for the adjustments they made to my throttle they should have been fired. I brought my truck back in today and again they did the timing and adjusted the throttle. They hooked it up to the diagnostic and nothing came up as a problem. Lo and behold, 30 minutes of driving it out of the shop my truck's idle was back to fluctuating between 2500 and 3500 RPMs. Any ides what they could have done during the major tune up to cause this? As the problem was not there before the major tune up? Vacuum hose? Throttle sensor? It has to be something that builds up pressure since it only starts acting up after 20 to 30 minutes of driving. I am baffled I really hope you can help.

Answer:   If a strong fuel injection cleaner was used on the fuel system it may have burned out the Oxygen sensor. Oxygen sensors fail and if it was close to failing at the time a fuel injection cleaner was used, the cleaner would have been enough to push it over the hill. Replacing the Oxygen sensor in this case should remedy your problem.

Question: 2001 Ford F-150 mileage: 105,000. I've cured all of the problems I can - the bad ignition switch. The problem is that the car idles fine when the engine is cold; let it run for a couple of minutes and it searches for an idle erratically, then eventually dies. If you put it into gear after it's warm, it idles ok. I think it's the idle air control valve. What do you think? By the way, there are no codes present at this time.

Answer: Try replacing the primary oxygen sensors (2). It sounds like the idle air control valve is working fine it is just getting a bad signal.

Question: 2003 Mercedes E500 mileage: 56,000. I really, really hope you can answer this one. Let me know if you need more details or if it is something that doesn't have a clear answer. My car periodically increases its idle while at the same time; the antilock brake light comes on. This behavior occurs at 10 or 15-minute intervals at times, or many times the car just idles fast with the ABS light on. Idle is as fast as 20,000 rpms. It has a new alternator and a new battery. I did notice there was no blow-by smoke seen when looking inside the breather with the motor running.

Answer: This problem is difficult because the computer can't detect a problem, your car thinks everything is ok. In reality what is happening is the electrical component inside the MAF sensor becomes contaminated by impurities in the air (heavies) These containments imbed themselves to electrical component that senses the air flow causing the sensor to be slightly off, but not enough to trigger the MIL (malfunction indicator lamp). Replace the MAF sensor with a new sensor and you're problem will go away, plus your car get better mileage and performance. (Mass Air Flow MAF sensor or Air Intake Sensor AIS, depending on manufacture. It does the same thing with different names).

Look up your engine oxygen sensor question in our Car Repair forum section.

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