Denny Mandeville, AAM is the owner of Canyon Automotive in Sedona and the Verde Valley Chapter President he can be reached at 928-282-4424

                Sometimes knowing (or thinking we know) too much causes us more problems than what we want, or need, to have in diagnosing, and repairing, drivability concerns. For example, P0171 is, normally, bank one lean- right?  A fairly common DTC with a relatively limited set of possibilities and with, equally,  reasonably straight forward diagnostics- right?   Except, as I found out, if it is a Lexus (and possibly other Toyota’s) 3.0 engine.

                The ’99 Lexus RX 300 came in with the MIL on, DTC P0171 set in memory and active. I performed the usual check for air leaks in the intake system, checked for vacuum leaks using propane while monitoring fuel trim/O2 PID’s, and, finally, graphed left and right side O2’s at 1700 RPM. Nothing out of the ordinary showed up, so I told the customer about the common reasons for P0171 and the negative results found during my testing. That left fuel delivery issues, so a fuel injection and plenum service was sold with the precaution (based upon mileage) that one O2 sensor may be slow enough in response to set this DTC. Swell, customer agreed with fuel service, turning light off and seeing what the response would be. Two weeks later, she called and said to order the O2 sensors. Sensors (plural), because we want to keep them the same age for similar response. After replacement I test drove, monitored, and graphed, the sensor values.  After two drives there were no pending codes, but. I wasn’t too sure I was happy because the pre and post graphs really weren’t all that different- but, oh well- I had done a pretty good diagnostic (I thought).

                  Customer called back the next day- you guessed it, the MIL was on. Once again I ran the testing for P0171 with same results. Now, I normally clean MAF during fuel service, and this was no exception- the MAF had been inspected and cleaned. But a dirty MAF doesn’t affect just one bank- does it? So, at this point I had better go see what the flow chart showed for diagnosing P0171-and here is what I found; Lexus calls P0171 SYSTEM lean- not bank ONE lean. And checking for pattern failures in Identifix I find directions for me to check MAF- not at idle, but at WOT. I had run into Toyota MAF problems before, but had forgotten all about this- shame on me. Anyway- 3.0 Toyota engines have a pattern failure of out-of-range MAF sensors- but they have to be checked at WOT acceleration. ECM needs to see greater than 85% flow. Less than that will set the engine lean code. Luckily for me the customer didn’t care about the mis-diagnosis because I had performed a careful diagnosis and had graphed everything- she was just grateful the last diagnosis fixed the car with minimal additional costs. Am I going to run to the book for every DTC to see whether the OE description fits the normal call outs? Nah, but at least I have been forewarned not every manufacturer plays the same game rules (as if I didn’t already know this).