Troubleshoot Common OBD2 Scanner Connection Problems

Using an OBD2 scan tool to communicate with vehicle's ECU is generally considered pretty straightforward task. However, on some rare occasions certain issues can make connection and communicating to the system troublesome. With error message showing "No-Communication"! 

In this article we will briefly explain some of the reasons and what can be done to troubleshoot any communication error between your OBD2 scanner and your car.

In most cases, 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. Below we explain some of them with more details.

OBD Scan Tool Compatibility

While most OBD code readers should be able to communicate with cars that use the standarised OBDII protocols - or cars with OBD2 compatible protocol to cover the engine/emission management system; a common issue with communication can be that the car you are trying to connect uses a non standarised OBD protocol such as ALDL, MOBD, MUTT or OBD1 etc. These kind of manufafacturer specific protocols require a scan tool with a software that is capable of reading these PIDs. 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 the points 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.

Furthermore, choosing the right diagnostic tool equipment suppliers can help you in any future troubleshooting rand questions you may have. While there are many brands and suppliers in the market, make sure to go for the reputable local supplier who knows the business well and known for their customer support and service.