The check engine light is on but no codes show up when you scan?

The check engine light is on but no codes show up when you scan?

The check engine light (CEL) is on, but when you use a scan tool to check for stored codes, nothing shows up.

Frustrating, right?
This scenario isn’t necessarily common, but it does baffle many amateur mechanics. Here’s what to do if the check engine light is on but no error codes are found. How the Check Engine Light Usually Comes On
The Check Engine Light usually comes on when the vehicle’s ECM/PCM detects feedback from one or more sensors that are out of range.

The vehicle’s Intelligent Control Module is programmed at the factory to a preset range for each value and is continuously monitored. Any value recorded outside of this limit is considered an outlier and an error associated with this out-of-range criteria is logged.

Essentially, the Check Engine Light simply serves as a notification that a diagnostic trouble code has been actively recorded. This trouble code can be retrieved using an OBD-II diagnostic tool and can be used to point a technician or amateur mechanic in the right direction when attempting to diagnose and repair the problem at hand. The first case occurs when the vehicle is started for the first time and performs its first self-diagnostic tests. If any of these self-diagnostic tests fail, an active error will usually be logged.

Error codes can be logged at any time while the vehicle is running. This occurs whenever out-of-range feedback is logged by any of the vehicle’s various sensors.

Possible Causes of a CEL Illuminating Without a Code

There are several possible reasons why your vehicle’s Check Engine Light may be illuminated despite no faults being logged. Knowing these possibilities can help you figure out the best way to proceed.

#1 – Poor OBD-II Port Connection

If your vehicle’s Check Engine Light is on but there are no codes, there may be a connection issue with your vehicle’s OBD-II port. We recommend checking the tightness of the harness pins immediately.

#2 – Blown OBD-II Fuse

It’s also possible that the fuse that powers your vehicle’s OBD-II port/communications network has blown. However, most manufacturers power the ignition system and fuel system on the same circuit, so in most cases your vehicle will not be able to start and run in this situation.

#3 – Faulty diagnostic tool

Like any electronic device, diagnostic tools can malfunction too. If this is the case, it is very likely that you will not be able to retrieve the faulty code that turned on your vehicle’s check engine light.

#4 – Improperly programmed diagnostic tool

Another reason why you may not be able to see the trouble code that turned on your vehicle’s check engine light could be that the diagnostic tool you are using does not have the software capabilities to retrieve the faulty fault.

This often happens when trying to read manufacturer-specific errors using a cheap generic diagnostic tool.

#5 – ECM/PCM failure

Although rare, it is possible for one or more channels in your vehicle’s engine control module to fail, causing communication errors. Most types of vehicle control modules are expensive replacement parts, not even including the cost of reprogramming, so this type of problem must be diagnosed carefully.

DIY Diagnosis
If you are trying to diagnose a problem that does not show a seemingly unobtainable error code, the first obvious step is to purchase or rent another diagnostic tool. As mentioned above, it is very likely that the brand or quality of the diagnostic tool you are using is the direct cause of the problem.

The next option when attempting this type of DIY repair is to use a digital multimeter and a vehicle-specific wiring diagram to verify that all expected OBD-II port feeds are present. This test will tell you right away whether a connection problem with this port is the cause, or if a module error is suspected.

Advanced Diagnosis
If you suspect that your vehicle’s ECM/PCM may be the culprit behind your inability to read and display trouble codes, it’s best to contact a professional. This type of module is very expensive to replace and often requires programming upon installation, creating an additional financial burden. Therefore, misdiagnosis must be avoided at all costs.

A professional technician will perform “pinout” tests of each communication circuit in the ECM/PCM, following the manufacturer’s recommendations. Dealership technicians also usually have brand-specific diagnostic tools that can flag even the most subtle of uncommon faults. Should you continue driving until the problem is fixed?If the check engine light comes on but you don’t get a valid trouble code, you should be aware of the possible consequences of trying to drive the vehicle.

Without a code, there is very little information that can be gleaned about what exactly is not working. Furthermore, if there is an issue with the ECM/PCM, further outcomes can be quite unpredictable. When considering whether you should continue driving in this situation, it is good to consider whether other symptoms are present.

If the vehicle’s own handling characteristics are questionable, or the engine is idling or otherwise behaving erratically, it’s probably best to park the affected vehicle until a solution can be found. This is especially true if the check engine light is flashing, which usually indicates a slightly larger problem.