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Check-Engine Light: Vacuum Leaks & Oxygen Sensor From "Auto Primer" episode DAPR-102

Leilani Munter is more than just a DIY host. She's also drives race cars.

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In this episode of DIY's Auto Primer, host Leilani Munter discusses your car's check-engine light and deals with some of the things that can cause it to come on. In this third segment of episode 102, Leilani checks for leaks in the vacuum lines and tests an oxygen sensor.

Materials:

Standard automotive tools (socket-wrenches, screwdrivers, nut-drivers, etc.)
Oxygen Sensor
High impedance DC volt-meter
Vise
Propane torch

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	Figure A
	Figure B
	Figure C
	Figure D

Vacuum Leaks

After the dreaded loose gas-cap, one of the most common causes of the check-engine light coming on is a leak in a vacuum line. Here are some things you should know about your car's vacuum system:

• Fuel and control systems depend on vacuum lines for communicating information among components. Leaks in the vacuum system defeat this communication and may cause uneven idle or acceleration, and may also cause the check-engine light to come on.

• Most modern vehicles have a sticker somewhere under the hood with diagrams and information about the vacuum lines and related components. In our case, the sticker is located on the shock tower (figure A).

• The most common of vacuum leaks is a loose oil-dipstick (figure B), so always be sure to check this first if your check-engine light has come on.

• Many vacuum lines (figure C) are made from plastic and, with age, can become brittle and develop cracks or holes. So the next thing to check is the condition of your vacuum lines for obvious leaks, cracks or punctures.

• Check all of the lines for proper connection (figure D), making sure that the connections are snug.

• If you locate a cracked or burned-through vacuum line, you can pick up replacement lines and fittings at a local auto center and replace it yourself.

• If the vacuum leak is not visibly obvious, there are many other factors that could be causing the problem -- including worn seals around the fuel injectors, bad head-gasket or throttle-shaft seal. These types of problems are best handled by a professional mechanic.
  

  Figure E

Another sensor that commonly may come up on a diagnostic scan is the oxygen sensor, (often referred to as an O-2 sensor). The oxygen sensor is positioned in the exhaust pipe near the catalytic converter (figure E). Its purpose is to detect the amount of oxygen in the exhaust, and signal the computer to increase or decrease the air-to-fuel ratio to ensure that the mixture is correct. If the oxygen sensor fails, the computer can no longer sense the air-to-fuel ratio, and it reverts to pre-programmed fuel maps that are less reliable than the real-time data. In this condition, the check-engine light will come on to alert the driver.

	Figure F
	Figure G

On some vehicles, the oxygen sensors are easy to reach and to test. Here are the basic steps.

• Locate the oxygen sensor in the exhaust pipe and inspect it for damage. In our case, the wires were visibly frayed and damaged (figure F).

• Unscrew and remove the sensor. (Some vehicles have bolt-on oxygen sensors. In this case, you'll need to remove the bolts.)

• Carefully place the oxygen sensor in a vise.

• A dc volt-meter can be used to check the sensor. Clamp the negative voltmeter lead to the sensor's metal case.

• Clamp the positive lead to the output wire from the sensor (figure G). Color coding for your oxygen sensor harness is listed in your vehicle's service manual.
  

• Turn the volt meter on, and set it to the two-volt scale (figure H).

• Use a propane or butane torch, set on hig,h to heat the fluted or perforated area of the sensor (figure I). This essentially is replicating the environment that exists inside the exhaust system. It should take from 5 to 20 seconds for the sensor to heat up sufficiently.

  Safety Alert: Be sure to wear safety glasses when using the propane torch.

  Figure H

  Figure I

  
  

  Figure J

• Check the voltmeter. A reading of somewhere between 0.6 and 0.8 should register within 20 seconds (figure J).

• If this reading is not attained, this indicates that the sensor is bad and should be replaced.

• After reaching this reading, remove the flame. The reading should drop to 0.1 volts within 4 seconds.

• To further test the sensor, continue to heat the sensor for 2 minutes and watch for fluctuations in the voltage.

In the segment that follows, Leilani offers tips for communicating effectively with your mechanic.

Note: This is a summary of steps and tips shown in this episode of Auto Primer. These are general guidelines and applicable to most contemporary vehicles. There may be variations in procedures depending on vehicle manufacturer, model, age or condition of vehicle and in the applicable parts, products or kit. Always follow proper safety precautions, and read and follow manufacturer's guidelines, diagrams and safety notices that come with any replacement parts or product that you select. Also, familiarize yourself with your vehicle's owner's manual before doing any work on your vehicle.

Safety Alert: Always wear eye protection when working with power tools or hazardous chemicals.

RESOURCES :

The Complete Idiot's Guide to Trouble-Free Car Care
Author: Dan Ramsey
ISBN: 0028635833
Alpha Books

Auto Repair for Dummies
Author: Deanna Sclar
ISBN: 0764550896

How Cars Work
Author: Tom Newton
ISBN: 0966862309
Publisher: Black Apple Press
To order this title from Amazon, click here.

What's Wrong With My Car?: A Quick and Easy Guide to Most Common Symptoms of Car Trouble
Author: Bob Cerullo
ISBN: 0452269938
Publisher: Plume (1993)
To order this book from Amazon.com, click here.