Using an OBD scanner is a common step for car owners when the check engine light illuminates or when experiencing performance issues. One frequent question that arises is: should your car be running when using an OBD scanner? Let’s explore this question and delve into a real-world scenario to understand the nuances of OBD scanner usage.
Generally, it is recommended to have your car turned to the “ON” position, with the engine off, when initially connecting and using an OBD scanner. This allows the scanner to draw power from the car’s battery and establish communication with the car’s computer system to read diagnostic trouble codes (DTCs). However, there are situations where you might need to have the engine running while using the scanner, particularly for live data monitoring.
To illustrate this, consider a case with a 2000 Nissan Altima exhibiting stalling issues during acceleration. The owner noticed the “Service Engine Soon” light and initially had a friend use an OTC scanner. Interestingly, after this initial scan attempt, the car, which was previously starting but stalling, refused to start at all. No error codes were retrievable at this point.
After overnight rest, the car surprisingly started again. A temporary fix was found in replacing a chewed vacuum hose, suspected to be the cause of stalling and the “Service Engine Soon” light. While the light remained, the car ran. When the friend returned with the OTC scanner for a second attempt to read codes, the act of connecting the scanner once more caused the car to stall and again refuse to restart. This time, however, the car was running when the scanner was connected.
The scanner was eventually able to retrieve codes: P0100, P0505, P0325, P1490, P0446, and P0464. This situation highlights a less common but important issue: sometimes, the act of connecting an OBD scanner can interfere with the car’s electronics, potentially due to voltage fluctuations or communication glitches. In this case, it even prevented the car from starting temporarily after it was running.
The solution in this scenario was a hard reset of the car’s Engine Control Module (ECM). This involves disconnecting both battery terminals and briefly connecting them together (while disconnected from the battery) to discharge capacitors and reset the ECM. This procedure allowed the Nissan Altima to start again.
This experience underscores a few key takeaways when using an OBD scanner:
- Start with the engine off, ignition ON: For initial code reading, this is generally the safest approach.
- Live data requires a running engine: If you need to monitor sensor readings in real-time, the engine must be running.
- Scanners can sometimes cause unexpected issues: While rare, scanner connection can sometimes disrupt the car’s electrical system.
- ECM resets can resolve scanner-related issues: If you suspect the scanner caused a problem, a hard ECM reset might be necessary.
- Vacuum leaks are common culprits: As demonstrated, simple issues like vacuum leaks can cause stalling and trigger check engine lights.
In conclusion, while typically safe, be mindful when using an OBD scanner. Start with the engine off for code reading, but understand that live data requires the engine to be running. And if you encounter unexpected issues after scanner use, consider an ECM reset as a potential solution. Always prioritize safety and consult your vehicle’s repair manual for specific recommendations.
