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Solving the Mystery of the Check Engine Warning Light PAC002

In this episode Neil and Dave discuss the mysterious check engine warning light that can come on without notice, and stay on for no apparent reason. The vehicle driver may be responsible for the light if the engines oil or coolant are low, if maintenance has been neglected, or if the gas cap is loose. The light can also come on even if the vehicle has been properly maintained. A failed sensors or loose wire connection can cause the dash light to illuminate.

This warning light is the most common symbol to light up on a cars dash. There is a lot of confusion about what the light means. The light can turn on and off depending on the problem. It could come on under different conditions such as: at certain temperatures; after the car has been running a specific time; over bumps; at specific engine or vehicle speed, or due to a number of other conditions.

Every week Neil gets several calls or customer complaints like “my car is running ok but the check engine light is on” or “my car running poorly and light is on”.

When the light is on a computer has recognized a problem and turned the light on. It may be a temporary problem that will go away and the light will go out, or it may stay on.

When a code is read, it is just a description of the symptoms of the failure and is only to lead a mechanic in the direction to find the problem. Only around 60% of the time is the code pointing to the exact problem.

It’s important to not just read the code and if it relates to a sensor, to replace the sensor. That can be a waste of money if the sensor is not bad. The sensor may be affected by some other problem – extra air due to a vacuum leak, some other sensor sending a wrong signal, or a mechanical problem.

When the light is on the vehicles exhaust emission are higher than normal. The vehicle fuel economy may be affected. Also the problem should be addressed soon because it can cause harm to the vehicle.

Reprogramming computers fixes many car problems

Neil also pointed out that some fixes do not require part replacement to be repaired. He mentioned a GMC Envoy with a window problem that had no bad parts. It had a computer software problem that required reprogramming to fix the problem. If a shop or car owner tried to repair it with parts replacements, they would never get it fixed.

Neil described a classic scenario he sees in Palmer’s Automotive frequently. A vehicle with codes P0171 and P0174, which indicate bank 1 and bank 2 are lean. The computer data says it is an Oxygen sensor code. If a customer has the code read and decides to buy oxygen sensors it won’t fix the problem. The vehicle has a fuel delivery problem, a vacuum leak, an air temp sensor problem, or a mass airflow sensor problem.

Another situation that happens almost weekly at Palmer’s Automotive is when a customer has a code read at the parts house and calls or comes by the shop to have the sensor, the code relates to, replaced. If Neil replaced the sensor as requested and it didn’t fix the dash warning light, the customer would be upset. That is the reason Neil never replaces a part without first performing proper diagnostics. He wants his customer to be happy with his service and he knows many wouldn’t be if he didn’t follow proper diagnostic procedures.

Dave asked Neil to talked about his training and the equipment required when performing automotive diagnostics on engine electronics and other computer controlled systems. Neil said he and his staff go to six vehicle specific training courses every year. He said the equipment required to diagnose these systems runs more than $10,000.

Neil made one last point about diagnosing electronic problems. If you have a car with a problem don’t call a shop and ask what it cost to fix it. A shop has no idea of the cost because they haven’t checked the vehicle yet. But, more importantly, the caller should ask or try to determine, “if” the shop can fix the problem. Do they have the tools and years of experience and training necessary to accurately diagnose and repair the cars problem?

Without the tools, training, and experience, diagnosing any warning light complaint can be difficult. Neil is an ASE Master Certified auto technician with over 30 years of experience. He has invested thousands of hours and dollars to become one of the best diagnostic technicians and mechanics in Twin Falls County. If you have a vehicle with a problem call Palmer’s Automotive Repair in Twin Falls, or make an online appointment from our website.

Additional Points You’ll Learn from this Episode about Check Engine Warning Lights:

  • Cars and trucks can have as many as 75 computers. The average car has 20 computers
  • Many of those computers can send a signal to the engine computer indicating there is a problem that can turn on the engine’s warning light
  • An important point presented here is to remind auto owners to not go to a parts house to have a code read and then buy the sensor or part that the code references. Doing so can get very expensive if it doesn’t fix the problem
  • Check Engine Warning light operation
    • When the key is turned on all cars do a system check by lighting all the lights on the dash. The vehicles computers are waking up, talking with the other computers, and checking basic systems
    • When engine starts the check engine light turns off if the engine computer doesn’t see any failures
    • If the light stays on the computer has recognized a problem and leaves the light on and stores the code in the computers memory
    • If you’re driving down the road and the light starts flashing you have a problem that can damage the engine or the exhaust system. It’s recommended to safely stop the car and have it towed to a repair shop for diagnosis.
    • If the light comes on and off there is an intermittent problem that could be a bad part, a loose connection or a bad ground wire.
  • Having a code read and cleared at a parts house or using a “do it yourself” (DIY) scanner to clear the codes, makes it harder for a repair shop to diagnose a check engine light or other electronic problem.
    • So if you plan to take your car to a shop, don’t clear the code. It can delay the repair and may cause the expense to diagnose the problem to be higher.

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