Pinpoint emissions failure

How to avoid time consuming and unnecessary tests for a 1994 Toyota 4Runner failing emissions test for high NOx.

Before you roll your eyes at the 20-year-old vehicle in this case study, remember that Toyota has been using rear O2 sensors and rear fuel trim for many years, so this vehicle works (and fails) just like most of the later OBD II vehicles you’re seeing every day – and the same testing logic applies.

Whether you’re diagnosing a converter efficiency code (i.e., P0420, P0430) or an emissions fault, there are only two steps: verify converter operation and verify everything else. Without verifying “everything else,” you can’t be sure the conditions are correct for converter operation, and you can’t be sure a new converter will light off and last a reasonable time. Hence, we perform a test to let some of the easiest answers direct the diagnostic path. Subsequent tests will be more informative because there was a good reason for them, and we avoid time-consuming, unnecessary tests.

This 4Runner failed an emissions test for high NOx, which could be caused by a number of faults, including: a failed converter, an EGR fault, advanced timing, lean mixture, overheating, excess head machining or carbon deposits, etc.


Lambda test

Lambda is the easiest test because we already have a failed emissions report with the necessary numbers. A Lambda value of 1.00 means the air/fuel ratio is perfect. Higher equals lean and lower equals rich, with a desired range of 0.98 to 1.02. But even if this is a P0420 diagnosis and you don’t already have numbers, a portable 5-gas analyzer is a great place to start.

Most portable gas analyzers, like the emission tester from Automotive Test Solutions and the Kane, have a Lambda calculation built in. In this case, the emissions report does not have Lambda, so we type the numbers into the Lambda calculator found at The results show this engine running a little lean at 1.012. (See Fig. 1). That’s still in the acceptable range, but since a lean mixture adds to NOx, it’s worth looking at. The catalytic converter will “reduce” NOx into N2 and O2, but it can only do so much, and can’t reduce well if there’s already extra O2 going through the converter (like during a lean condition).

Early in the diagnosis, it’s clear that even if there’s a fault causing higher NOx and/or the converter fails, the conditions aren’t optimal for efficient NOx reduction anyway. This means the “fix” might involve finding a combination of causes.


Low-hanging fruit

Next, we could either test for the source of extra O2or for the source of high NOx (i.e., EGR, converter, timing, etc.). We choose simple EGR function and timing tests because they only take a few minutes on this engine. Both are fine, and while there are many other possible causes, they involve more time-consuming tests, so we note potential tests we could have performed and move on.


2-for-1 tests

The next test was chosen because one test connection answers two questions. By watching the upstream and downstream O2 sensors, we test the converter as well as the sensors. For this test, you either graph O2 sensor PIDs on the scan tool or connect a lab scope. On Toyota, PIDs were extremely slow until about 2003, and non-existent before OBD II. But they do provide a handy breakout box (DLC1), so connecting a DVOM or lab scope to the O2 sensors takes only seconds. To make the connections, use blade connectors from a terminal adapter kit. Kits are available from sources like AESwave. Two adapters from the Universal Terminal Driveability Kit (AES No. 16-200) were used in this test (See Fig. 2). For applications without a breakout box, use T-pins or back probing tools to connect your test lead directly to the sensor circuit, as shown in Fig. 3 (page 30).


Oxygen sensor test

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