OBD2 Troubleshooting Guide

If your OBD2 scanner won't turn on or won't read OBD (on-board diagnostic) codes, the cause is usually the car, not the scanner. Check these in order: the fuse that powers the OBD socket (often shared with the 12-volt socket or cigarette lighter), the ignition in the "run" position, about 12 volts between pin 16 and pins 4/5 of the socket, damaged or dirty socket terminals, and finally the scanner's own plug, cable and fuse.

Work through those before you give up and drive the car to the shop.

TL;DR

• Most common cause of a dead scanner: a blown OBD socket fuse, often shared with the 12-volt socket or cigarette lighter.
• Swapping a blown fuse takes about five minutes; the typical fuse is 15 to 20 amp, but always fit the rating in your manual.
• The socket must show about 12 volts between pin 16 (power) and pin 4 or 5 (ground); below 11 volts some scanners won't connect.
• Ignition must be in "run" (or engine running) and all modules finished booting before you scan.
• If the ECM hangs, disconnecting the battery reboots it; check your owner's manual first.
• Last, check the scanner itself: bent pins, worn cable or its internal fuse.

If your scanner works and you want to use it to enable features, read Mercedes OBD activation for CarPlay and Android Auto.

Table Of Content

1. What is OBD (On-Board Diagnostics)?
2. How do you troubleshoot an OBD scanner that won't turn on?
3. How do you check if the OBD scanner itself is damaged?
4. How do you fix OBD scanner connection issues?
5. What is Mercedes OBD and what can it do for you?
6. Conclusion

1. What Is OBD (On-Board Diagnostics)?

Cars have had a built-in troubleshooting system called OBD since the mid-1990s. A computer in your car monitors sensors for engine temperature, exhaust gas mixture and other values.

The computer regularly checks that these sensor values are within the manufacturer's safe range. If one goes outside that range, the computer stores it as an error code, and may turn on the check engine light. A modern car has hundreds of possible error codes, each pointing to a different problem. As fueleconomy.gov puts it, the check engine light (malfunction indicator lamp) means something is wrong with your engine, even if the car seems to drive normally.

A mechanic, or a capable do-it-yourselfer, can read these codes to assess the engine's health.

You do this by connecting a scan tool to the diagnostic port on your vehicle (its location is shown in your owner's manual) and downloading the codes.

Then look up the codes on a website like OBD-Codes.com to see what they mean. Most car parts chain stores will scan your codes for free. For Mercedes-specific codes, see the Mercedes P13DF fault guide and Mercedes ECU 128 error.

2. How Do You Troubleshoot an OBD Scanner That Won't Turn On?

Plugging in your scanner and finding it dead is worrying, because some of these tools are expensive. Don't panic: this is common, and it's rarely the scanner itself.

A blown fuse on the OBD socket power supply circuit is the most typical reason a scanner won't switch on. If the check engine light does not illuminate with the ignition on, that fuse has most likely blown. Check the fuse box and replace the blown fuse.

The sections below cover the most common causes, how to diagnose them and how to fix them.

2.1 Blown OBD Socket Fuse

Plugging in the scanner and finding it won't power up is a common problem. The DLC (Data Link Connector), also called the OBD port or socket, supplies power to your scan tool.

If your tool has no power, the OBD socket is probably not getting electricity. A check engine light that stays off with the ignition on points to the same cause.

The most typical reason the socket has lost power is a blown fuse. Replacing it takes about five minutes, and the OBD socket works again.

2.2 How Do You Locate The OBD Fuse?

The OBD fuse usually blows because the circuit is overloaded. It is often shared with the cigarette lighter, the 12-volt socket or other consumers, so plugging in a broken accessory, such as a phone charger, or a high-power device, such as a fan, can blow it.

So if your 12-volt socket or cigarette lighter is also not working, you have probably found the problem.

Most cars have more than one fuse box; three is common: one under the hood, one in the trunk and one behind the steering wheel.

The OBD fuse may be labeled OBD II, DLC, Accessory port, 12-volt power point or cigarette lighter on the fuse cover, and is most likely inside the cabin.

The owner's manual is the fastest way to find it. A good manual lists the fuse location, type, color, rating and what it powers under the heading "fuses". Mercedes-Benz USA owners can download theirs from the official owner's manuals page.

2.3 Examining The OBD Fuse

You can test a fuse in three ways:

• With a test light.
• With a voltmeter to measure the voltage.
• By removing the fuse and checking the conductive strip.

Pulling the fuse is the quickest method. Most fuse boxes use blade-type fuses, which are easy to remove but small and hard to hold. A puller tool is often clipped to the back of the fuse box cover.

Hook the tool over the fuse and pull it out. Hold the fuse up to the light to see the strip; replace the fuse if the strip is broken.

Never assume the fuse you removed is the correct size; always check the rating in the owner's manual. Often it is not, and a fuse rated below spec will just blow again.

A fuse rated higher than recommended can damage components or, in the worst case, cause an electrical fire. The most typical size is 15 to 20 amp, but use the manufacturer's rating.

2.4 Open DLC Power Or Ground

If the fuse is fine, a damaged OBD socket is the next most likely cause of no power. It is very common in older cars: the more the port is used, the more the delicate terminals wear or lose tension.

If the DLC power terminal doesn't make solid contact, the tool gets no power.

2.5 Examining Damage To The OBD Socket

Two checks will show whether the socket is damaged.

First, inspect the port and its terminals with a flashlight. Check whether terminals 4 and 16 look larger (spread) compared with the rest.

Terminals 4 and 16 are chassis ground and power. Your scanner needs both to turn on, so they are the ones to look at first.

In a workshop, a test probe can be dragged across the terminals to check their tension. If tension is low, a fine metal tool can tighten the terminal legs. Unless you can clearly see a spread terminal, don't push anything into the front of the DLC.

The second check needs a test light or voltmeter.

Test both the power and ground paths. If you get no reading on the voltmeter or test light, test each one separately to find out whether power or ground is missing.

It's best to back-probe the DLC terminals, though that can be difficult. Otherwise, probe gently with a paperclip or similar.

If either path has a fault, look under the dash for damaged wiring. The area right beneath the dash is the most vulnerable, because things can knock into the wiring there.

3. How Do You Check If the OBD Scanner Itself Is Damaged?

Scanners are tough and can last for years, but they can still be damaged. A worn or damaged scanner cable is the most likely cause of a communication failure on the scanner side.

3.1 Examining The Scanner's Plug

Check the pins on the scanner plug: they should be straight and properly positioned. A bent pin can't make contact with the vehicle's port. The good news is that the cable is usually universal and replaceable.

If the cable looks fine, try the scanner on another (donor) vehicle. If it fails there too, the scanner is the problem.

3.2 Examining The Scanner's Fuse

All scanners are fused, most often with a glass (bottle-type) fuse. Getting to it is not as simple as checking a car fuse; you may need to open the scanner. Ask the manufacturer for the procedure.

3.3 Port Are Clogged With Debris

Another reason for no reading is a scan port full of dust after years of not being used. Don't use a spray cleaner or anything that can get the port wet, but wiping it with a soft cloth or blowing compressed air across it can clear whatever is blocking your scanner.

If one of these fixes works and you can now read the stored codes, you can take the car to a shop or your own garage for the repair. If not, a trip to the store for a new cable or scanner may be the answer.

4. How Do You Fix OBD Scanner Connection Issues?

This section covers the causes of communication problems between your OBD scanner and your car, and what you can do about them.

Communication problems can come from the ignition key position, low connector voltage, the wrong communication protocol, a hung ECM, missing or invalid data, or OEM-updated ECM software the scan tool doesn't know about. The most common ones are below.

4.1 Compatibility With OBD Scan Tools

Most OBD code readers can talk to cars that use the standard OBDII protocols, or an OBD2-compatible protocol for the engine and emissions system. A common problem is a car that uses a non-standard protocol such as ALDL, MOBD, MUTT or OBD1.

These manufacturer-specific protocols need a scan tool and software that can read their PIDs. To diagnose a wider range of cars and systems, you need a professional multi-system scanner with OBD1 and OBD2 support.

4.2 The Ignition Key Is Not In The "Run" Position

Most OBDII scan tools need the ignition in the run position or the engine running. Many cars also need several seconds for all modules to finish booting.

A good rule is to start the scan only after the chimes have finished and the dashboard has completed its checks. If you don't wait for the whole system to load, the scan tool may not connect.

4.3 Low Or No Voltage On The Connector

Every OBDII connector must supply 12-volt power on pin 16 and ground on pins 4 and 5. Scan tools depend on this power. Start by turning the ignition to the run position.

Set a voltmeter to DC volts and put the red (positive) lead on pin 16 and the black (negative) lead on pin 4 or pin 5. It should read 12 volts DC or close to it. Some scan tools struggle to connect if the reading is much lower than 11 volts; if so, check the battery and charging system first.

If there is no voltage between the pins, a blown fuse is the most likely cause. The DLC power is usually shared with the fuse panel's accessory power. Find the fuse panel using the owner's manual, check for blown fuses, replace any you find and check the power again.

4.4 A Hang In The ECM Communication

The ECM can occasionally "hang": it keeps working but stops communicating. The car runs, but the scan tool says the communication link is unavailable.

If every other check passes, the ECM may need a reboot. Disconnect both battery leads, then press the brake pedal to drain any capacitors in the system. When the battery is reconnected the ECM reboots and should communicate again. Check the owner's manual before disconnecting the battery.

4.5 A Missing Or An Invalid Data

Most scan tools connect to the car and ask the ECM which PIDs are valid. Some tools assume the car they connect to is the same as the last car they saw, so the PIDs may not match and some data may show as unavailable.

To fix this, tell the scan tool to query for PIDs. Depending on the car this can take a minute or more. When it finishes, the PID list should match the car and the data should be correct. Some scan tools don't scan PIDs automatically and use the car's VIN to decide which PIDs are valid instead.

As a rule, query the PIDs so the scan tool shows the same information as the ECM. It's worth the minute it takes.

5. What Is Mercedes OBD And What Can It Do For You?

A Mercedes-Benz OBD diagnostic tool reads error codes from the fault memory of the car's computer systems. The fault memory stores codes for many different systems and areas of the car.

The codes point to faults in the area concerned. Generic codes apply to all makes of car; manufacturer-specific codes are unique to each manufacturer.

A tool built for Mercedes-Benz reads both the generic and the Mercedes-specific codes, which gives a fuller picture of the car's health. The same OBD port is also used for jobs like lowering the air suspension.

6. Conclusion

For many drivers OBD seems like black magic: the workshop plugs in a serious-looking gadget, reads some cryptic codes, tells you what's wrong and clears the codes.

It is not that mysterious. With your own Mercedes OBD2 scanner you can read and analyze the codes yourself, check the health of your car's systems, and even check for mileage manipulation.