Hello! Back in December the code P0138 was stored. Following online advice, the simplest course of action was to change the air filter. I did and the code went away. The trim fuel values continued to hover around 8%~12%, a $28.00 P10 HUD dash display can show that live data stream. Reading over plenty of stuff over the past months and as others have pointed out, the upstream sensor can be the one actually at fault. For the Prius C the DENSO part number is 8946752080. This link convinced (among other items) me enough to buy and replace the upstream sensor, this one gives the gist of how the fuel trim values are calculated. Both sites are in agreement: The Upstream sensor is used to calibrate the fuel management system. A marginal upstream sensor is congruent enough for the ECM to still use and the air/fuel mixture will be off by a bit as a result. Usually everybody points out to a slow sensor. Another failure mode has to do with giving lean voltage values when the fuel mixture is actually a bit rich. At this point these assumptions where made: No vacuum leaks, wiring, injectors, fuel pump, planet alignment in the solar system, all were and are OK. After buying the sensor, a 66750B crow foot, and replacing it a few days ago I can see how the trim values are moving down to 0%. Most of the time the short term fuel trim % are negative and the long term % are now around 5%. Two items that where not expected: I have a loose heat shield that had stared to make noise about a year or so, at idle speed when parked, if the engine was running for a minute, I would rattle for that minute. If the battery was being charged, the sound was stronger. Now, the rattle only happens for an instant while the engine spins up. The rest of the time is hard to notice. In errands, I usually drive a stretch that allows me to set cruise control to 45Mph. I have always noticed that in that particular highway the MPG bar hovers to 75 and it stays there for 5 something minutes. The P10 showed how engine temp went from 88-89C to 92 to 94C, maybe 95C on a hot Texas day. Now, after changing the upstream sensor, temperature stays below 92C. The P10 is a fairly new addition, so I do not have anything to compare from 12 years ago, when the car was new. I never paid attention to the trim values until now. I do believe they can give an early warning of something not right very early. The P10 display was added about 3 years ago when I learned that the rotor in the engine water pump can fail as early as 30K miles. On purpose I did not disconnect the battery, and there is no need to remove anything. A torque that looks like a 3/8 socket extension, the O2 tool, etc, can maneuver in that space with patience. 20 minutes? The engine does not burn oil, I had used Mobil, then Red Line, now I am using Valvoline Restore & Protect. The oil filter is a B1432. Before I forget: A rich mixture slowly kills the cat, another piece of information that nudged me into replacing the upstream sensor. I hope this helps.
