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Pelican Technical Article:

Reading Fault Codes

Tom Morr

Time:

15 minutes15 mins

Tab:

$0

Talent:

*

Tools:

Code Scanner

Applicable Models:

Mercedes-Benz R170 (1998-04)

Hot Tip:

The more expensive the scan tool, the more data it generally provides

Performance Gain:

Correcting the fault restores proper powertrain function and might be necessary to pass emissions tests

Complementary Modification:

A bad O2 sensor is the most common trigger; replace if needed

The dreaded Check Engine light, also known as the MIL (malfunction indicator lamp), causes a sinking feeling in many motorists. The immediate assumption is that expensive repairs are imminent.

That may or may not be the case. Plus, a trip to the shop or dealership isn't automatically mandatory. Using a scan tool, Check Engine codes (technically known as DTCs--Diagnostic Trouble Codes) for 1996-newer vehicles can be read at home. Beginning in 1996, passenger vehicles adopted the OBD-II closed-loop engine management system, which uses oxygen sensors before and after the catalytic converter to monitor emissions and adjust fuel management accordingly. Part of the system is an Assembly Line Diagnostic Link (ALDL) port, which uses a somewhat-universal plug for most newer vehicles. To read the fault codes, a scan tool is plugged into this port.

Scan tools run the gamut from affordable ones that only reveal the fault codes up to the multi-thousand-dollar Star Diagnostic System used by Mercedes-Benz dealerships and some independent repair shops. The consumer end of the scale also includes Bluetooth versions that transmit the data to smart phones.

Pelican Parts offers a variety of scan tools, ranging from about $55 to $3,100-plus. Generally, the more expensive the system, the more data it provides. Affordable hand-held scanners normally read and clear fault codes, and they often describe the codes. (Otherwise, the codes must be cross-referenced in a service manual or via web search.) The more expensive systems often include software and a PC interface. These systems (AutoEnginuity is one example) display actual data, including voltages and pressures for the various powertrain, body and chassis sensors.

Before plugging the scan tool into the car's diagnostic port, make sure that the car's battery has at least 11.4 volts with the key off and that the battery cables are clean and secured to the terminals. Plug the scan tool into the data port and turn the ignition on. For other details, refer to the scan tool's owner's manual.

Incidentally, many national-chain auto-parts stores and some independent shops will read Check Engine codes free of charge in case you don't want to buy a scan tool.

The dreaded Check Engine light (arrow) stays illuminated when the car's computer registers a diagnostic trouble code (DTC).
Figure 1

The dreaded Check Engine light (arrow) stays illuminated when the car's computer registers a diagnostic trouble code (DTC). Disconnecting the battery doesn't erase the code.

The scan tool's cable (or a Bluetooth module on wireless systems) plugs into the OBD-II diagnostic port (yellow arrow).
Figure 2

The scan tool's cable (or a Bluetooth module on wireless systems) plugs into the OBD-II diagnostic port (yellow arrow). On the R170 SLK, the port is located on the under-dash panel, behind the hood-release lever (blue arrow). The port is protected by a plastic cover, which pivots away so that the cable can be plugged in.

With the key in the ON position or the car running (refer to the tool's manual), the scanner should show something on its display if the cable is connected properly.
Figure 3

With the key in the ON position or the car running (refer to the tool's manual), the scanner should show something on its display if the cable is connected properly. This budget scan tool reads and clears codes. More expensive models have additional features.

Normally, the scan tool goes through a handshake sequence, where it learns the car's VIN and accesses other information.
Figure 4

Normally, the scan tool goes through a handshake sequence, where it learns the car's VIN and accesses other information.

The main menu outlines the tool's basic features.
Figure 5

The main menu outlines the tool's basic features.

Reading DTCs normally reveals why the Check Engine light is on.
Figure 6

Reading DTCs normally reveals why the Check Engine light is on.

When a fault triggers the car's diagnostic system, the trouble code is stored in the computer until the problem is corrected (as many as 19 trips are necessary for the computer to clear itself after the problem is fixed) or the scanner is used to erase the codes.
Figure 7

When a fault triggers the car's diagnostic system, the trouble code is stored in the computer until the problem is corrected (as many as 19 trips are necessary for the computer to clear itself after the problem is fixed) or the scanner is used to erase the codes.

Thankfully, our project car (a pre-facelift 2000 SLK230 Kompressor) hadn't thrown any codes.
Figure 8

Thankfully, our project car (a pre-facelift 2000 SLK230 Kompressor) hadn't thrown any codes. So, we decided to check Pending Codes--readings that are outside the normal range but not aberrant enough to trigger the Check Engine light.

This scanner has a submenu that references manufacturer-specific codes.
Figure 9

This scanner has a submenu that references manufacturer-specific codes. (Some codes are SAE-standard/universal while others are reserved for components unique to specific makes and models.)

Always record the trouble codes.
Figure 10

Always record the trouble codes. The scan tool stores some data, but certain models erase their stored codes when clearing the car's codes.

If the car appears to be running okay, clear the code, take the car for a drive, and see if the same code re-appears.
Figure 11

If the car appears to be running okay, clear the code, take the car for a drive, and see if the same code re-appears. If not, note the code for future reference and carry on.

Some scan tools have diagnostic and even data-logging capability.
Figure 12

Some scan tools have diagnostic and even data-logging capability. This lower-end unit performs a few basic diagnostics but doesn't provide actual values, such as pressures and voltages.

Many current-generation scan tools define the trouble codes.
Figure 13

Many current-generation scan tools define the trouble codes. However, older scanners and really cheap ones might reveal the code but require you to find its definition in a service manual or via a web search. Codes generally fall in one of three categories: P (powertrain), C (chassis), or B (body).

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Comments and Suggestions:
Greg Comments: Hi Pelecan
My SLK 230 built in 1998 delivered in Australia has no OBD2.It has the multy pin plug under bonet.
I puchased a converter plug to OBD 2 but OBD 2 scanner does not recognize car.
Help?
Many thanks for your articles as they are great.
Regards
Greg
March 10, 2016
  Followup from the Pelican Staff: That may be due to lack of the needed communication circuit for the scan tool. You will need a tool that can communicate on the Mercedes-Benz scan tool. - Nick at Pelican Parts  

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