In the “Idle Clatter” column in the Turbo Diesel Register, Issue 89, we briefly discussed the dashboard check engine light (CEL) and what it means. As a TDR member you can download the diagnostic trouble codes (DTCs) that turned on the check engine light using a code reader/scan tool and interpret what they mean. You then can know where to look to fix the problem. A good mid-priced scan tool is therefore the next recommended item to go into your tool kit.
The scan tool is plugged into the OBD2 (on-board diagnostic, series two) connector which is usually found under the bottom of the dashboard near the steering column. This 16 pin rectangle shaped connector can not only give you trouble codes, but if your scan tool is capable, it can give you a running data stream of powertrain operation while driving. It can also be used to erase the code(s) and turn off the check engine light. To use a scan tool, plug it in and turn on the ignition key without starting the engine. Codes will be displayed in a few moments. When finished getting the codes, turn off the key before unplugging the scan tool. It’s that simple!
Read on to learn more about some added available scan tool functions. Computers, sensors, and DTCs don’t have to be the mystery that many owners think they are.
In 1988, federal regulations were introduced with the goal of further reducing vehicle exhaust emissions. This required computers to monitor sensors and then output commands to modify fuel/air ratios. California’s CARB and the federal EPA then required on-board computer diagnostic routines that monitored and tested emissions related sensors to ensure their performance didn’t deteriorate and the engine stayed within emissions limits. If your state has vehicle emissions testing, you can use a scan tool with testing capability to ensure your vehicle will pass before going for the mandated test. However, in most states if the check engine light is not illuminated you’ll pass the emissions “test.” (Oops, did some unscrupulous person remove the CEL bulb in the used car you purchased?) Nowadays the emissions test mostly serves to generate tax revenue.
The first Dodge/Cummins trucks contained a very basic computer called a powertrain control module (PCM) that mainly controlled battery charging, some of the dash gauges, cruise control, and automatic transmission operation through inputs from a few sensors, and then the PCM issued output commands. There was no electronic engine control module (ECM) until the 1998.5 model with the 24-valve engine where the computer controlled a mechanical fuel injection pump’s operation to further lower exhaust emissions. Today’s computer controlled vehicles monitor and control all engine operating parameters, automatic transmission operation, and monitor and operate various body and electrical functions, too. As fellow TDR writer John Holmes says, “We’re driving a computer on wheels these days.”
Many members feel that they can no longer work on their trucks because it has a “computer.” Well, you can, given some knowledge and the right tools. We’ll attempt to increase your understanding of how new trucks operate and how you can troubleshoot computer reported problems without getting too technical about it all.
About Diagnostic Trouble Codes
Some DTCs are known in the trade as “nuisance” codes. To help you understand the difference between nuisance codes and something more serious, we should explore code classifications. For instance, if a problem is detected with a sensor but is not repeated during the next drive cycle or next several drive cycles, the CEL can be turned off by the computer. The code will still be stored in the computer for future retrieval. If a malfunction continues during another drive cycle, the light stays on. Sometimes, simply shutting down your ride and restarting it after a minute or two will turn off the light. That was a nuisance DTC, a temporary malfunction.
DTCs consist of a letter and then several numbers. The format is common to all vehicle manufacturers, hence the term “generic” for many DTCs. Other DTCs are manufacturer specific. DTC code lists can run into more than a hundred possible codes on the latest models of cars and trucks.
The first digit is a letter: “B” for body sensors; “C” for chassis sensors; “P” for powertrain sensors; “U” for network problems.
The second digit is a number: “0” (zero) for generic codes common to all automotive computerized emission control systems; “1” for manufacturer specific (non-generic); “2” generic; “3” includes both generic and manufacturer specific codes.
The third digit identifies the system where the problem is detected/located: “1” fuel and air metering; “2” fuel and air metering in fuel injection circuits; “3” ignition system or misfire; “4” auxiliary emission control system; “5” vehicle speed control and idle control system; “6” computer output circuits; “7” transmission; “8” transmission.
The fourth and fifth digits identify what section of the system is malfunctioning. That’s clear as mud, right?
Here’s an example: If your scan tool readout tells you that code P0201 has been set, we can interpret the code as “P” for powertrain (engine or transmission), “0” for generic, “2” for fuel and air metering in the injection circuit, and “01” to mean there is a malfunction in the number one cylinder’s fuel injector circuit. Many mid-priced scan tools will give you the code and also give the code definition in plain text. You can then troubleshoot the wiring between the engine computer and the number one fuel injector or the fuel injector’s solenoid to fix the problem. If you’re not going to do the repair yourself, don’t erase the code as the tech who does the repair will need the additional data stored in the computer along with the code as an additional diagnostic aid.
This would be called a serious problem that needs to be repaired as quickly as possible as it not only affects engine power and performance, but also affects exhaust system equipment and sensors.
Editor’s note: Jim says it is serious, but if you ask two or three more people, you’ll get two or three different answers. I say if the truck’s still running right—keep driving. Heck, mine’s got a P022E right now. Everyone’s threshold for alarm is different. Really, keep driving ‘till you can find some black electrical tape to put over the light.
An example of a nuisance DTC would be # P0381, “wait to start lamp inoperative.” You could continue to drive your truck, but the lamp may not come on. It should be fixed, but not today unless you have trouble starting the engine when it is cold. Using the scan tool, you can erase this code from the computer and see if it comes back the next time you start your truck. It may have been a temporary or random malfunction that won’t be repeated.
Any malfunction that is exhaust emissions related will turn on the CEL right away. If the problem is temporary, the computer will turn off the light after two or three drive cycles of similar operating conditions if there was no fault found. Most of the better scan tools display the most important DTC first, followed by the less important ones in descending order.
Finally, a flashing CEL as opposed to a steady CEL can further help differentiate how serious the problem is.
How are Sensors Part of the System?
There are all sorts of sensors that monitor brake system operation, automatic transmission operation, and body electrical functions. Inputs from these sensors will modify the computer’s operation of the affected system. By monitoring many electrical circuits, the computer can even tell you if a brake light is out or if the speed of the engine fan isn’t what it should be. In today’s vehicles, many driver inputs, such as turning on the air conditioner, don’t simply result in completing an electrical circuit to the air conditioner compressor clutch. Instead, the computer takes over and commands the compressor to operate as needed while also issuing engine fan commands and monitoring refrigerant pressure to make sure you get cold air from the dash registers. A related cabin temperature sensor makes sure you are comfortable. There may be other computers in any vehicle that monitor and operate other functions, and these sub-computers all communicate with the main computer. All of these computers will run self-diagnostic routines at engine startup, and the engine control computer will often run self- test routines almost continuously on exhaust emissions related sensors. The computer is therefore capable of doing thousands of tasks per minute. It does get complicated, doesn’t it?
The sensors in your vehicle operate on small voltages, ranging from near zero to plus five volts. Many trouble codes are set because of “rationality.” Rationality means that the computer has determined that the reading from a particular sensor can’t be right because other associated sensors are reporting more normal values. The “rationality” problem is often caused by a corroded connector or broken sensor wire that alters voltage values that are reported back to the computer. The sensor itself is okay.
Need an example? The engine air intake temperature sensor tells the computer that the air temperature is 86°, while the transmission temperature sensor says the fluid temperature is below zero. There is a “rationality” error, so the computer turns on the CEL. Using the scan tool while doing a test drive can determine in which circuit the problem lies and you diagnose only one circuit.
Time for a Scan Tool
As has been said many times in this magazine, a technician often pulls a scan tool out of his/her toolbox before using any other of the more traditional mechanic’s tools because it has become such an important diagnostic aid. You can use one too and they are readily available at your local auto parts store or on the internet. Scan tools for shade tree use are relatively inexpensive. The cheapest ones are about $20 US and they’ll tell you very little beyond a code number. Plan to spend about $100 US and get much more information and several more functions. Top end models for professional techs can cost thousands. Make sure any scan tool you buy is compatible with CAN/BUS systems, the latest protocol for downloading DTCs from the OBD2 connector usually found under the dashboard. Also buy a scan tool that is upgradeable by using a USB cable to hook it to your computer. This tool should work on almost any car or truck, gasoline or diesel, made in the last 15-20 years so you will find that it has many uses if you troubleshoot more than one vehicle. Got a new motorcycle, snow mobile, or jet ski? There is probably an engine computer and an OBD2 connector on it, also. Got a big road tractor? The same tool works for it, too. Scan tools in the recommended price range can also download emissions related engine operating conditions that were present at the time the fault was registered as well as test all federally required emissions related sensors, an important feature if your state has emissions testing.
Using a scan tool to check that all emissions related sensors are reporting properly can help ensure that your vehicle will pass an emissions test where it is mandated by state law. The scan tool will tell you that all emission related sensors are working properly but only a sniffer inserted in a vehicle’s tailpipe can determine that your vehicle is emissions legal, so don’t be misled.
Using the scan tool while driving the vehicle will give you continuous operating data for most computer inputs and outputs while you isolate the problem or condition that set the DTC. Take a buddy along to either drive or record the data so that you don’t have a wreck while staring at a small computer screen instead of the road ahead.
If your scan tool shows a DTC code without an explanation, it is probably a non-generic code that is manufacturer specific or is not in the code list contained in your tool’s internal database. There are many internet sites you can check that will give you an explanation for this type of code, and there is probably an internet reference site associated with the maker of your scan tool that can also help.
For Ram trucks and many other Chrysler vehicles, a list of codes is available at the TDR website. Search for Ram Diagnostic Trouble Codes. Some codes are diesel engine specific while others will pertain to any Chrysler sponsored vehicle.
If your scan tool is upgradeable, you can connect it to your computer and get updates of DTC lists from your tool maker’s website. These updates are often free, so update your tool’s internal database soon after you buy it and at least yearly thereafter. Vehicle manufacturers are continually adding new DTC codes, so updating your tool is important, particularly if you work on more than one vehicle.
When troubleshooting, use the “kiss” method: Keep It Simple, Stupid. You may find that a DTC was set because a wiring connector to a sensor or a switch came apart, so the computer simply lost that sensor’s signal. Sometimes an electrical connection has become corroded and needs to be cleaned because the signal was intermittent or out of normal ranges. And sometimes you may need to replace the sensor, but check the simple stuff first. If you have a service manual for the vehicle you’re working on, there are detailed troubleshooting charts that are useful. But it all starts with the questions of “what’s the trouble code?”, and “what does it mean?”
Hopefully this material has helped to take some of the mystery out of the dreaded check engine light/ DTC stuff that inhabits the modern computerized world of today’s vehicles. Happy scanning!
If you don’t mind spending a little more money, an alternative is the Edge Insight monitor, color touch screen (CTS) that is sold by Geno’s Garage. It can do more than just scan for trouble codes and has a much more legible screen that can be seen easily while driving. Check it out.
This article originally appeared in the Turbo Diesel Register.
Issue 91 – February/March/April 2016
TDR Writer: Jim Anderson