Under the Hood: Defroster on or off?

Brad Bergholdt, Tribune News Service on

Published in Business News

Q. My husband gives me a hard time for leaving the defroster on after it's no longer needed. Is this really worth hassling someone over?

Melissa R.

A. Staying out of the way on this one! Typical cars and light trucks engage the air conditioning compressor when windshield defrosting is performed as the A/C system's evaporator extracts moisture from the air being blown through it. This is why you'll see water dripping from beneath the car when the A/C is used on a warm/humid day. Having the A/C in use on a cold drizzly morning doesn't seem right, but it's better to blow warm dry air at the windshield when defrosting, as opposed to hot wet air.

Continuing to run the defroster after it's no longer needed isn't a big deal but decreases fuel economy by perhaps 1-2 MPG and increases wear and tear on the A/C compressor, possibly reducing life span slightly. Turning the defroster off when not needed makes sense but there's other things that are a bigger deal in life to worry about!

Q. I'm considering purchasing a scanner to check codes and other things on my family and friend's cars. I'm a bit baffled by some of the terminology that is used when making comparisons. Can you help with terms like graphing, enhanced codes, freeze frame, system readiness, and any others that would be helpful?

Frank L.

A. My hunch is you're considering an OBD-II generic scan tool as opposed to a more functional and expensive enhanced (pro grade) tool costing considerably more. OBD-II scan tools allow a look at the general cause of an illuminated check engine light, providing a starting point for emissions related diagnosis. These tools range from an under $10 ELM-327 device that plugs into your vehicle's data link connector (standardized on all cars built since 1996, 1997 for trucks) and links up with a free or inexpensive app on your mobile device, to a perhaps $200 full-feature tool that has a nice color display and includes some ABS and airbag coverage, wifi connectivity for additional vehicle tips, info, service bulletins, recalls, etc.

A typical OBD-II scan tool displays emissions related diagnostic trouble codes, limited live data related to engine performance/emissions, and can erase codes, provide vehicle ID number, display a freeze frame (a snapshot of driving conditions occurring as a code sets), as well as system readiness (indicates if monitors (tests) have been run of components/systems) which is necessary to pass an emissions test, and possibly test results for various components and systems. Graphing data allows a better look for the cause of an odd or intermittent fault as one can more readily see an oddity in a line graph more so than observing a row of numbers.


Features such as more complex engine and transmission performance data, body, instrumentation, brakes, air conditioning, tire pressure monitoring, immobilizer key updating, and other info and functions requires the use of an enhanced or manufacturer specific scan tool, which can cost a thousand dollars or more.

Highly rated OBD-II scan tools include: BAFX34T5, Autel AL519, Innova 3030, 3040, 3100 series, BlueDriver, CarMD, Actron CP9680, among others.

About The Writer

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at; he cannot make personal replies.

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