Troubleshoot Common OBD Scanner Connection Problems

Using an OBDII scan tool to communicate with vehicle ECU is generally considered pretty straightforward task. However, on some rare occasions certain issues can make connection and communicating to the system troublesome. An experienced automotive technician understand that communication issues can be caused by various reasons including: ignition-key position, connector-voltage issues, incorrect communication protocol, hung ECM, missing or bad data, and OEM-updated ECM software that is unknown to the scan tool.

In this article we will briefly explain some of these reasons and what can be done to troubleshoot any communication error between your car and your OBD diagnostic scanner.

Scan Tool Compatibility

While most OBD scanners should be able to access the standarised OBDII protocols which only cover the engine/emission management system; vehicle manufacturers do not have to follow the same protocol for other systems like transmission, body control module, anti-lock braking system etc. Accessing these systems depends on the scan tool used and abillity to diagnose multiple systems on the required car. Even some older vehicles or cars that use older OBD protocols ALDL, MOBD, MUTT or OBD1 will require diagnostic scan tools capable to read manufacturers' specific PIDs from these vehicles. A professional multi-system diagnostic scanner with OBD1 OBD2 compatibility will be required to diagnose wider range of vehicles and systems. 

Ignition Key not in Run position

Most OBDII scan tools require that the ignition key be placed in the run position, or the engine be running. Also, many vehicles take several seconds for all of the modules to finish their boot-up sequences. A good rule is to engage the scan tool after all chimes have finished and all dashboard activity has been completed. The scan tool may not connect to the OBDII system if you do not wait for the complete system boot-up.

No Voltage or Low Voltage on Connector

By specification, every OBDII connector must have 12-v power on pin 16 and ground on pins 4 and 5. The scan tools rely on this power being present. First, the ignition key must be in the run position.

Using a voltmeter set to measure DC voltage, place red/positive lead on pin 16 and the black/negative lean on pin 4 or pin 5. The voltmeter should read 12-vDC (or near to it). If the reading is much below 11 volts, some scan tools have problems connecting. If this is the case, inspect the battery and or charging system first.

If you read no voltage between pins, a blown fuse is most likely the problem. Typically the DLC power is shared with the accessory power on the fuse panel. Using the owner’s manual for reference, locate the fuse panel and check to see if any fuses have been blown. If so, replace the fuse and recheck your power.

ECM Communications Hung Up

Occasionally, the ECM can get into a ‘hung’ mode, in which it functions but does not communicate. The vehicle runs, but the scan tool reports that the communication link cannot be established.

If all other checks have been verified, the ECM may need to be rebooted. To reboot the ECM, disconnect both battery leads and then press on the brake pedal to discharge any capacitors in the system. After the battery has been reconnected, the entire ECM reboots and should start to communicate again. Before disconnecting the battery, consult the vehicle’s owner’s manual on the proper procedures.

Missing Data or Invalid Data

Most scan tools connect to the vehicle and query the ECM for valid PIDs for the vehicle. Some scan tools assume the last vehicle connected is the current vehicle. Thus, the PIDs may not match up. So when evaluating data, some PIDs may show up as not being available. To remedy this situation, scan tools allow you to query for the PIDs. This process may take a minute or more depending on the vehicle. When completed, the PID list should match the vehicle and the data should be accurate. Some scan tools may not automatically scan the PIDs, but they may rely on the vehicle’s VIN to determine what PIDs are valid.

As a general rule, scan for PIDs to make sure that the scan tool is reporting exactly what the ECM is providing. It is well worth the minute of query that it takes to ensure that the scan tool has proper data.


Much like the personal computer industry, vehicles' ECM software can have issues reporting to ECU. Many owners don't realise that in some instances further diagnosis will be required to identify the issue before being able to use the scan tool to read and clear fault codes or perform other service reset functions. While examples above are just examples they are not limited to what can cause this. Using a diagnostic scan tool requires advanced knowledge of vehicle technology and how they work in order to fully utilise the capability of a scan tool you use.