Check engine light on but no codes stored

Understanding Why the Check Engine Light is On but No Codes Are Stored

The check engine light (CEL) illuminates, it typically means that the vehicle’s onboard diagnostics system (OBD-II) has detected a problem. However, sometimes you can connect your code reader only to find there are no fault codes stored. This situation can be perplexing and frustrating for car owners and technicians alike. In this article, we’ll explore the possible reasons why your check engine light is on but no codes are stored. .

Not all scan tools can read all codes

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

You need a high-end scan tool to read enhanced trouble codes as well as B, C, and U codes
to establish other manufacturer-specific codes.

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.

There’s a third but remote possibility

The fault lies with the vehicle computer or data bus. The ECM is what turns on the check engine light in most vehicles, and it often gets the light request from other models in the vehicle. For example, the transmission control module (TCM) might set a fault code and request a Check Engine light, but when you hook up your scan tool, the ECM can’t retrieve the code from the TCM.

That can be caused by a problem with the module or a problem on the data bus that’s not allowing good digital communication.

This kind of problem is very rare and requires professional help to diagnose the issue. If you’ve tried everything and still find that no codes are stored, you’ll have

©, 2018 Rick Muscoplat