Rick's Free Auto Repair Advice

Check engine light on but no code

Why is my check engine light on but no code?

What to do if you have a check engine light on but no code?

I see this question often and my answer is always the same; if you have a check engine light on but no code, you’re using the wrong scan tool. Whenever you have a check engine light or ANY warning light, there’s ALWAYS a trouble code stored. Period. The end.

Why can’t you read the trouble code?

Trouble codes come in many varieties. When OBDII was first set up, a “generic” set of powertrain “P” was established to get common terms used for all car makers. Generic powertrain “P” codes are 5 digit codes starting with P0001 through P0799. Sounds good so far, right?

But car makers complained that they wanted the ability to

image of scan tool with trouble code

You need a high end scan tool to read enhanced trouble codes as well as B, C, and U codes

establish other codes that are manufacturer-specific. In fact, they wanted codes that are not only manufacturer-specific, but model-specific as well. So, the authorities allowed car maker’s to use “enhanced” powertrain trouble codes starting with P1000 thru P3000.

Next, car makers wanted trouble codes that would reference the sub-system in the vehicle, so they established Body “B”, Chassis “C” and digital communications “U” codes.

Most inexpensive scan tools only read generic trouble codes

Since enhanced codes can vary based on year, make and model, scan tool manufacturers must build their tools to accommodate reading and translating the enhanced codes. Very few consumer level code readers and scan tools are built with that level of sophistication. In other words, your scan tool simply can’t read enhanced P, B, C or U codes.

If the check engine light is on, you’ve got a trouble code stored.

The same is true if you have ABS light on, TCS light on, VSC light on, etc. etc.

If your check engine light is on but no code, it’s your scan tool, not the vehicle. You’ve only got two choices at this point; buy a scan tool that can read enhanced as well as B, C and U codes, OR pay a shop to read the codes for you.

©, 2018 Rick Muscoplat



Posted on by Rick Muscoplat

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