Auto Repair Help

by Mark Davidson

This website is designed help you learn about and diagnose your vehicle: car, truck, van, or Sport Utility Vehicle (SUV). This site contains a wealth of information that will help you learn and understand how your vehicle operates. Although your vehicle functions as a whole, it is really numerous systems working together to accomplish its goal.

Successful diagnosis is the result of an organized diagnostic approach. There is no magic wand available to wave over the hood that will instantly pinpoint the problem. OBD-II trouble codes offer only a clue as to what may be causing a particular problem with your car. An OBD-II trouble code, once extracted from your vehicle’s onboard computer, should be looked up in a chart and used in conjuction with other data such as symptom observables, history of the problem in question, TSB’s (Technical Service Bulletins), etc in order to reach an accurate diagnosis. A single symptom can be caused by numerous components on the vehicle. An accurate diagnosis should isolate the failed component. Sometimes however, it is more cost effective to replace the most suspect part and then spend the time or money on further diagnosis. Only you will be able to determine at what point this is.

Special tools are often required to work on late model vehicles. Often these tools can be rented or purchased for a nominal fee (in relation to having the work done). You should consult your local parts store for information regarding the availability of special tools or test equipment. When special tools or equipment are used, follow the tool manufacturers instructions for proper usage.

Servicing, diagnosing, and learning about your vehicle can be a rewarding challenge. Take the time to understand your vehicle by reading the associated articles for the system being serviced. It is money well spent to invest in the official manufacturer’s manual which covers your particular make and model vehicle. This manual was written by the engineers which actually designed your vehicle, thus it is the most authoritative reference for your vehicle. Such a manual may cost well over $100.00 but it will help lead to accurate diagnosis of almost anything which can go wrong with your vehicle. Contact your nearest dealer parts department to purchase a manufacturer’s manual which covers your make and model car.

Once you gain a working knowledge of how your vehicle operates, you are in a much better position to diagnose it. As you can see from the sub-topics of each section, there are many components that comprise each system of the vehicle. Each component accomplishes a specific goal. When a component fails, it fails to accomplish its goal and results in degraded vehicle performance and a symptom. There are less symptoms than components. In other words, many different components can fail and cause the same symptom. A leaking wheel cylinder, leaking caliper, or defective master cylinder can all manifest themselves with the same symptom, a low brake pedal or a brake warning lamp that remains on.

Visual inspection is the most effective diagnostic tool (along with OBD-II trouble codes). Once a symptom is selected, the diagnostic section usually prompts you to perform a visual inspection. It will also instruct you on specifically what to look for. Take time to look over each item carefully.

Read over the symptom diagnostic section carefully. Once you have determined what system you are diagnosing, you should familiarize yourself with the system by reading over each components theory and operation. You can access this information by clicking on the appropriate vehicle section or by searching this website for the component name. Do all this prior to opening the hood! Often by reading the theory and operation of each component, you can isolate the cause much more quickly, enabling you to efficient auto repair procedures.

(Mark gave up on sports when the Browns left Cleveland and now spends his Sundays working under a shade tree in the back yard tuning his son’s soap box derby car.)

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12 Comments on "General Auto Diagnostics"

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3 years 1 month ago


William Hall
2 years 11 months ago

1)Will a cam position sensor affect the shifting in the transmission of a 2006 Nissian Altima.

2) Is this something I can replace my self.

1 year 7 months ago

Yes, a cam sensor relays important information to the PCM (power train control module). This helps it shift at the right time instead of over reving on an automatic transmission. For example, the camshaft turns once for every two revolutions of the crankshaft. This with the crank positions sensor lets the computer (PCM) tell at what stage of operation the valve train is in, and on some systems an accurate tachometer reading.

2 years 10 months ago

Please I have Hyundai Elantra 2004 model GT and sometimes I don’t get quick start . I also have problem when acceratting it does not pick as it should pick , sometimes press’s full accelerator and the car would not move and suddenly it will pick automatically and start to move on top spread, I have taken to the mechanic and still have the problem after some days, I have change the cold/ water starter sensor , TPS I have change , change crank position sensor , change completely fuel pump and system , bought new plugs and plug cable , but I still have the problem where sometimes you accelerate and the car does not move at the rate of you pressing the acceratter . Sometimes the fire comes down and up and when in neutral gear or park the fire comes cone but when you put at drive it comes to normal . Kindly help me

1 year 10 months ago

I have a vw jetta 2010. It was running perfectly, I run over a piece of metal that damaged the nut from the oil tank, I end up with almost no oil. The car was fixed it started right. Next day when I tried the car the engine started shaking when in park was better when I shiffted to D or R shaked more, I drove the car and it stopped. It did not sarted again. I tried today and the same thing happenned with the shaking . I call a mechanic and he mentioned that it might be the distributor sparkplug component, is this possible ?

1 year 7 months ago

On your 2010 Jetta, it uses coil packs or a coil over plug (COP) design, and not a distributor ignition system. You should check the codes on the cars computer. A miss fire is either P0300(random miss fire) P030x (misfire on x cylinder). Volkswagen , however may have factory specific codes. You can also preform a cylinder power balance test. You should avoid removing COP designs as that can dammage some during the power balance test. It is only safe to do so when they are not being energized. Also, the stumbling , if not ignition related could be a vaccum leak/egr, variety of things. However my first comment seems more likely .

1 year 5 months ago

Knowing your vehicle and its system is really helpful to diagnose any problems that may arise and troubleshooting becomes easier.

1 year 3 months ago

have 1991 dodge 150 ,5.2 eng. failed high speed emissions, 1st test-high speed co7.16 high speed hc 516
2nd test high speed co 7.39high speed hc 223 .
already chged. egr & pcv cleaned and gaped sparkplugs and checked distributor new air filter.whats next ? oxygen sensor ?what else may be cause ?

4 months 7 days ago

My daughter’s 2000 vw jetta is surely giving her some problem. It is an automatic so when she describes that her car does not shift when she is driving. I assume she means it over reving on an automatic transmission. What can the issue be? Please help.

2 months 18 days ago

Mark, there seems to be a lot that you can learn from general automotive diagnostics. I like the tip about taking the time to understand your vehicle by reading articles related to the system being serviced. I’ve heard that you’re less likely to be scammed or cut short if you have an idea of what’s being repaired.