Rick's Free Auto Repair Advice

Pre-purchase car inpsection

Why get a pre-purchase car inspection?

Pre-purchase car inspection reveals issues you don’t know about and can’t find with just a visual driveway inspection.

Even if you’ve been buying cars your whole life, you can’t check everything about a used car while it’s sitting in someone’s driveway. It needs to be up on a lift and it needs to be checked with a professional-grade scan tool to make sure the seller hasn’t recently erased trouble codes; codes that will pop up again right after you buy the car.

Here’s what’s usually checked during a pre-purchase car inspection

Stationary scan tool review

Check readiness monitors

Any time trouble codes are erased, the vehicle computer must complete a “drive cycle” to test that system and make sure it passes. If that system passes the tests, a “readiness monitor” will show that system is “ready.” However, if the seller recently cleared a trouble code but didn’t conduct the drive cycle, the readiness monitor will show “not ready.” That’s a dead giveaway that the seller has been trying to trick you by clearing the trouble codes.

Check history codes

Many computers keep track of trouble codes that have set in the past. Checking the history codes is a great way to see which codes have been set

Check pending codes

If a sensor is seeing bad data, but the data hasn’t exceeded the threshold to turn on the check engine light or the data has exceeded the threshold but the test requires the data to fail a set number of times before turning on the check engine light, the computer will store the trouble code as a “pending code.” That’s the sign that you’re about to have a check engine light come on.

Check Mode $06 for misfires

Mode $06 is a way to peek behind the curtain to see exactly what data the vehicle computer is seeing. For example, by using Mode $06 you can see the actual number of misfires the computer is tracking for each cylinder, even though the total hasn’t reached the threshold to turn on the check engine light and set a trouble code. Mode $06 is a valuable tool in the hands of a skilled technician

Test drive scan tool diagnostics

Short and long term fuel trim

When a vehicle comes out of the factory, it runs on factory programming. As the engine wears, the computer makes changes to the factory programming to maintain proper operation while meeting emissions standards.

Short and long term fuel trim readings tell the technician the condition of the engine and what changes have been made to factory programming. Short term fuel trim, for example, shows how much extra fuel the computer is adding to factory software to make the engine run properly. Once short term fuel trim exceeds a certain value, the computer figures that it must continue to add that amount all the time and moves that data into the long term fuel trim section.

Short and long term fuel trim data is expressed as a percentage above or below factory programming. So a scan tool review that shows a long term fuel trim of +25% is a sign of a problem since short term fuel trims should generally be less than +10%. What causes such a high fuel trim value? Things like vacuum leaks, clogged fuel injectors, low fuel pump pressure, engine wear.

On the flip side, a negative fuel trim reading means that the engine is receiving too much fuel and the computer is trying to cut back from factory programming. What causes negative fuel trim? Things like leaking fuel injectors or improper fuel pump pressure.

Pre and post O2 sensor operation

The scan tool can also be used to check the operation of the oxygen sensors and the catalytic converter.

Pre-purchase inspection of the AC system can determine the proper performance

The technician installs pressure gauges and operates the AC to determine if high and low side pressures are within normal limits.

While the system is running, the tech checks the outlet temperature at the center duct to determine proper temperature drip.

Belt and belt tensioner inspection

Back in the ’80s and 90’s you could check the drive belt condition by checking for cracks in the ribs. By the late 1990s all car makers had switched away from neoprene belts and moved to EPDM belts that don’t crack. You can inspect an EPDM belt visually for tears and glazing, but you can’t detect wear visually. You must use a wear gauge to determine if the belt is worn. That’s what the tech will use during the pre-purchase inspection. They’ll also check the condition of the automatic belt tensioner to make sure the dampener components are working properly.

Timing belt Condition

The technician will remove a cover from the timing belt system and check for cracks, glazing, missing, or broken belt teeth.

Fluid checks

In addition to checking fluid levels, the technician will check for fluid condition of the coolant, power steering fluid, transmission fluid and brake fluid using test strips and a refractometer.

Brake fluid test strips check for the presence of copper in the fluid, which is an indication of breakdown of the fluid’s anti-corrosion additives. A brake fluid moisture meter detects the presence of moisture in the system.

Coolant test strips detect improper pH balance and a refractometer detects freeze protection and coolant/water concentration


Tires should be checked for even tread depth across the tire and between all tires, tire damage, uneven wear, heat cracks and age

Suspension checks

Shocks and strut condition

A test drive will determine strut and shock condition by checking for nosedive and rear squat during acceleration and vehicle stability in turns. A visual strut and shock inspection will detect leaks and mount condition.

Ball joints, control arm bushings, lateral links, stabilizer bushings and stabilizer bar end links

A visual inspection of all bushings and a test drive can determine the condition of most suspension components and the technician can also measure wear on the vehicle’s ball joints.

Steering checks

The technician will check inner and outer tie rods for wear as well as rack and pinion operation and intermediate link condition.

Leak check

The technician will check transmission and engine seals for evidence of leak, as well as a leaking rack and pinion, power steering pump and lines, transmission cooler lines, radiator and heater hoses, oil pan, valve cover leaks.

Brake check

The technician will remove the wheels to check for the remaining amount of friction material on the brakes and check for proper caliper and drum brake hardware operation. They’ll also check the friction material for signs of uneven wear and check the brake lines for signs of corrosion

Drivetrain components

Check CV axle shaft boots

Check drive shaft U-joints for wear

Chassis inspection

Check for signs of severe rust and previous repairs

Flood inspection

Check for signs of water damage from flood

Body panel inspection

Check for signs of previous body repair


Check all bulbs for proper operation

Test drive to check for proper shifting, wheel balance and tire pull issues, handling in turns, proper engine temperature, stopping ability.

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat


Custom Wordpress Website created by Wizzy Wig Web Design, Minneapolis MN