Tech: Troubleshooting OBDII Engine Computer Systems Short Cuts

| Toyota 4Runner | Toyota Tech | Toyota Section |

By: Dan Eddleman - November, 2003

Check Engine Light

1800 miles from home and the Check Engine light comes on. What are you going to do? Ignore it and hope it's not anything that will either strand you, or cause damage if you keep driving? Leave the remote area you invested so much time and money to get to and waste precious vacation time to drive 100 miles to the nearest town that might have a Toyota dealer? Risk some unknown mechanic introducing additional problems into your vehicle while trying the fix the original problem? I faced this dilemma on a recent 6000 mile trip out west, but fortunately, happened to have the right tool to solve the dilemma. The tool is the BR-3 OBDII Interface Adapter and accompanying Windows or DOS application software that runs on a Personal Computer.

First, a little bit of information about the engine management computer systems used on late model vehicles. Since 1996, federal law has required manufacturers to equip their vehicles with On Board Diagnostic generation II or OBDII engine management computer systems. The mandate came about due to several concerns. First, the proliferation of non-standard, proprietary engine management computer systems would result extreme difficulty for anyone, other than the original manufacturer dealerships, to be able to troubleshoot and make repairs. Second, with the increasing complexity of these closed loop systems, it could become very difficult to diagnose and repair problems causing out of compliance emissions. Therefore the primary objective of the OBDII standards were for the systems to be able to self diagnose failures, and standardize the overall design and error codes produced so anyone with the proper training, documentation and equipment would be able repair one of these systems.

OBD-II Test Interface

That's the good news about OBDII. The not so good news is that when the check engine light comes on, all you know is there is some type of problem unless you happen to have the specially designed equipment needed to read out the diagnostic and system performance information. For a long time this special equipment was too expensive for the average home mechanic. On the older systems, error codes could be read by inserting a jumper and reading error codes from a flashing light on the instrument panel. But this method would be woefully inadequate for the huge volume of data that can be retrieved from an OBDII system. All OBDII equipped vehicles have a system test interface as shown to the right located under the driver's side instrument panel.

One word of caution. Just being able to read the OBDII trouble codes and diagnostic data won't get you very far in diagnosing a problem. You need to assess your troubleshooting skills, and if you decide to proceed, invest in the Factory Service Manuals or "FSM" for your vehicle. Then spend the time needed to learn to troubleshoot the problem in accordance with the procedures contained in the FSM. Toyota documentation can be ordered from the contact information at the end of this article.

BR-3 OBDII Interface Hardware
BR-3 Interface Adapter

As with most high-tech systems, over time the prices drop and become more affordable as exemplified by the BR-3 unit covered in this review. The BR-3 Interface Adapter currently sells for $88.00 which is a very cost effective tool. This unit is an interface converter, allowing data from the vehicle OBDII interface to be communicated to a personal computer via the standard RS-232 serial interface. This means you will need a laptop computer if you want to use this tool on the road. The BR-3 supports all three of the commonly used OBDII interface electrical standards which are the VPW (Variable Pulse Width Modulation) used primarily on General Motors vehicles, PWM (Pulse Width Modulation) used by Ford and, the International Standard or ISO 9141 used by the Chrysler motor vehicles, European and Asian manufacturers, including later model Toyotas. It also supports ISO 14230 or the "Keyword Protocol 2000" which some vehicles manufactured and sold in Europe are using. This standard is similar to the ISO 9141 standard.

The VPW interface operates at 10.4kbits/sec, the PWM at 42.6kbits/sec and the ISO 9141 at 10.4kbits/sec. OBDII systems not only store data for later retrieval, but can provide operational data real time over this interface.

BR-3 OBDII Windows and DOS Application Program
BR-3 Application Startup

To use the BR-3 interface you will also need to download and install either the Windows or DOS version of the BR-3 application program on your personal computer. These applications are available from BR-3 OBDII Automotive Diagnostics Home Page. Since this software can run in simulator mode, you don't have to have the hardware to see exactly what it does. I highly recommend anyone interested in the BR-3 to first download the software and give it a try in simulator mode. When the program is started, the first screen to come up will be to select the protocol to be used to communicate with the vehicle, or to set the program to operate in simulator mode.

For 1998 and later model Toyotas, the IS0 9141/14230 protocol should be selected. For many manufacturers there was a transition period in switching their vehicles OBDII. This was the case with Toyota. Thanks to a forum member, I am told that the 1996 and 1997 4Runner ECU's use the GM VPW protocol. The earlier models use a totally different ECU that is not interchangeable with the newer models.

Also, as seen on this screen the application supports data recording for later playback of collected data. I have found this useful for example in road testing. You should not attempt to drive and look at the data at the same time. With the record/playback feature, you don't have to, simply turn on record mode, perform your road testing, and then analyze the data off line with the playback feature.

The following feature description is summarized from the BR-3 application help file. Since the Society of Automotive Engineers (SAE) J1979 standard is one of the key documents for OBDII, you will find references to various "modes" defined by this standard.

SAE J1979 defines 9 modes. Each mode is intended for specific functions as follows:

BR-3, Studying the FSM, and Reading 4x4Wire Forum Posts, Saves the Day

During our 6000 mile trip out west, the Check Engine or as defined in the Toyota service documentation, the "Malfunction Indicator Light" or "MIL" started coming on after driving on some extremely dusty forest trails near Custer, South Dakota. After collecting the freeze frame data from the system and monitoring the O2 sensor output, I reset the MIL as I wanted to see if the problem was a one time occurrence, or a reoccurring problem and if so, under what driving conditions.

With the BR-3, the MIL is reset by clicking on the "Clear Diagnostic Data" button shown in the Mode 3 and 4 photos below. With this button, there's no need to pull the EFI fuse or disconnect the battery to clear the trouble code. Be sure to examine all recorded data such as the onboard diagnostic test and freeze frame data before clearing the trouble code.

In trying to determine the cause of the problem, I was mentally stepping through the FSM diagnostic procedure for the P0171 trouble code that I had previously read, and recalling some valuable posts on the 4x4wire forum about this problem. To read these posts, click on the "Short Cuts" window at the top of this article, select "Trail Talk BBS", then on the next screen, click on the keyword "Search", and then fill in the search screen as shown below. Other items listed in the FSM troubleshooting procedure such as leaks in the air induction system, exhaust, and engine temperatures were checked and found to be OK. Although I had thought about it, I did forget to bring the FSM on the trip so, the listing of the standard trouble codes are contained in the BR-3 Help screens came in handy.

Forest trail northwest of Custer, South Dakota P0171 Troublecode Information on Forum Using the Graphical Monitor to view the O2 Sensor Signal BR-3 Help Screen Standard Trouble Code Listing

Even though the P0171 error doesn't point to the oxygen sensor, I was concerned about its operation, the use of 15% ethanol fuel, and possible effects from the higher altitude, all of which coincided with the initial failure. As it turned out, none of these were the problem. The photo above shows the laptop computer and the BR-3 attached to the 4Runner. Here, the real time data from the oxygen sensor was being viewed using the graphical monitor feature to observe the characteristic sine wave swing that should be present. Close observation of the swing values, as compared to acceptable waveforms recalled from reading the FSM confirmed the O2 sensor was not the problem. The next day while pulling up a long, steep hill in 3rd gear the light came on again. And this became the pattern. At this point, I hooked up the BR-3 and the laptop computer to monitor the fuel trim (or mixture adjustment values) in real time over the next several days as the wife drove. In several brief full throttle runs up hill, the trim levels stayed quite low which indicated there wasn't a fuel delivery problem, for example, clogged fuel injectors or low fuel pump pressure. The BR-3 does provide for a fuel pressure reading, but apparently Toyota doesn't have a fuel pressure sensor and can't provide this information.

The first photo below shows the main screen indicating that the MIL or Check Engine Light is on and, a few of the engine parameters that were set to display are visible. The second photo again shows the main screen, but with additional data values selected and the MIL light had been turned off using the Mode 4 screen shown further below. The third photo shows the freeze frame data collected when the P0171 trouble code was set. The P0171 trouble code means that the engine management system is having to over compensate to keep the engine from running lean. After observing the fuel trim values over time, it became apparent that the trim values were going beyond specification only during brief, intermittent periods on up hill climbs, typically with the engine at 1/2 to 2/3 throttle with engine load in the 40-50% range. So, I decided to just take it easy on the hills and continue on our road trip as planned. I did reset the MIL light at the end of each day that it came on, so I could tell by the light coming on again whether I was taking it easy enough on hills.

Mode 1 Main Screen indicating one trouble code stored Mode 1 Main Screen with the Toyota applicable data values enabled Mode 2 Freeze Frame Data showing conditions when Trouble Code was set. Mode 3 Diagnose Trouble Codes showing a P0171, running lean code

Shown below are some additional screens and a brief description of their purpose. The Mode 4 screen is for clearing out the trouble codes and additional information as previously noted. The second screen shows additional data about the performance of the oxygen sensors. The third screen shows manufacturer specific test data, which can be accessed with the BR-3, but you need to equip yourself with the Factory Service Manual or "FSM" to be able to interpret these values. Most manufacturers have specific tests and error codes beyond those mandated by the OBDII standard. The last photo shows the on board monitor tests that run continuously and the other periodic tests performed in in the OBDII system. Many states have adopted the practice of reading the status of these onboard tests as the emission pass/fail test via the OBDII system test interface. This is a much more accurate test of the vehicle emissions system than the tail pipe sniffer test.

Mode 4 Clear Diagnostic Data (example photo from simulator mode) Mode 5 Oxygen Sensors Test Data Mode 5 Additional Manufacturer Specific Data Mode 6 & 7 Continuous and Non Continuously Monitored On Board Tests

If I had not had the BR-3 with me so I could investigate the severity on the spot, the above problem would have put a real wrinkle in our road trip plans. After returning home at the end of the trip, I re-read the 4x4wire forum posts on this problem and read a number of Internet articles found by searching on "hot wire mass air flow sensor", the type of mass air flow sensor or "MAF" used in the 3rd generation 4Runner 3.4L V6 engine. I cleaned the MAF as recommended in a number of posts on the 4x4wire forums, and P0171 is now history.

This article has the details on how to clean the MAF hot wire sensor used on third generation 3.4L V6 Toyotas. Click on "Supplemental Maintenance" to go direct to the procedure.


After receiving and studying the Factory Service Manuals for my 2000 4Runner, I became aware of the enormous amount of vehicle performance and condition information contained in the OBDII system. These are complex, but understandable systems. After waiting close to 3 years for an affordable OBD-II scanner to come on the market, I am very pleased with the BR-3 Interface Adapter. I highly recommend anyone contemplating working on, or just wanting to learn more about their vehicle, to order and study the Factory Service Manual, particularly the OBDII diagnostic service procedures for their vehicle. There are detailed explanations of every trouble code and a diagnostic flow chart for each trouble code. After the experience with the Check Engine Light on our trip out west, the BR-3 and my laptop computer are must have tools for any long trips.

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