Check Engine Light On? How We Diagnose the Real Problem

Crossroad Garage service team · Ellensburg, WA

Few things are as annoying as a check engine light that pops on during your drive. It might be something minor, or it might be the first sign of a problem that gets expensive if you ignore it. The only way to know is to find out what the vehicle is reporting and why.

Steady light or flashing light?

• A steady check engine light means the engine computer has found a fault. The vehicle is usually safe to drive to a shop, but have it checked soon.
• A flashing check engine light usually means a severe misfire. Continuing to drive can overheat and damage the catalytic converter and other parts. Reduce speed, avoid heavy loads and get it looked at as soon as possible.

If the light comes on along with overheating, low oil pressure, a big loss of power or strange noises, pull over safely and call for help.

What a trouble code tells you, and what it does not

When the light comes on, the computer stores a diagnostic trouble code. A code describes what the computer saw, such as a sensor reading out of range or a misfire on a cylinder. It does not always say which part has failed. For example, a code pointing to a lean condition could come from a vacuum leak, a weak fuel pump, a dirty sensor or a clogged filter. Replacing parts based on a code alone can mean spending money without fixing the problem.

How we diagnose it

1. Talk with you. When did it start? Does it happen cold or warm, at idle or under load, on the highway or towing up a grade? Those details matter.
2. Scan the system. We read stored and pending codes and the freeze-frame data that shows conditions when the fault happened.
3. Look at live data. Sensor readings, fuel trims, temperatures and pressures tell us how the engine is actually running.
4. Test, do not guess. Depending on the code, that can mean checking wiring and connectors, testing sensors, smoke-testing for vacuum leaks, checking fuel pressure or doing a road test.
5. Explain what we found. We go over the cause and the repair options before any work is approved.

Common causes we see

• Loose or faulty gas caps.
• Ignition problems such as worn spark plugs or failing coils on gas engines.
• Oxygen sensors and other sensors that fail or get contaminated.
• Vacuum and intake leaks.
• Fuel delivery problems, including weak pumps, clogged filters and injector issues. See fuel systems.
• Wiring and connector damage from corrosion, heat or rodents.
• On diesels, problems with sensors, fuel pressure, turbochargers and charge-air leaks. See diesel engine builds and repair.

Cold weather and Central Washington driving

Ellensburg winters can bring out weak batteries, marginal sensors and wiring problems, and a low battery can set codes of its own. Dust from gravel roads can clog air filters. Corrosion from winter roads can attack connectors. When we diagnose a light, we consider how and where the vehicle is driven.

Other warning lights

The check engine light is only one of several. ABS, traction control, airbag, battery and temperature lights each point to a different system. We can scan these too and explain what they mean.

When to come in

Call us right away for a flashing check engine light, or a light that comes with overheating, low oil pressure or a big loss of power. For a steady light with the vehicle running normally, schedule a diagnostic visit soon. Do not just clear the code and hope; the problem usually comes back.

See all of our auto and diesel services, diesel and performance fuel systems, diesel engine builds and repair, or browse more tech tips on the Crossroad Garage blog.