Moving along the timeline a little further, right around frame 1426, the front O2 drastically changes back again to a more normal pattern. Then shortly after, the rear O2 reacts and starts reporting a less lean condition. On further inspection of the frames, I could see how the trace dips straight down to an almost completely lean level, holds there nearly steady for a period, and then it works its way back to a normal bias-rich 700 millivolts.
The evidence was the check engine light and code P0171 being set because the long term fuel trim is temporarily spiking. The fuel trim is spiking because the O2 sensors are temporarily reporting a lean condition. The O2 sensors appear to be functioning properly. Except for a very slight bump I felt when the check engine light came on, the engine seems to be running fine through this entire drive cycle.
This tells me, it must be something other than an engine component causing the code to set. This rules out the obvious -- vacuum leaks, injector sticking, and fuel system issues. We have to consider what else on this vehicle can cause the O2 sensors to temporarily report an extremely lean condition.
Clearly, excessive amounts of oxygen will cause the O2 sensors to produce a low voltage and report to the computer the system is running lean. But if not from the engine, where could this additional oxygen be coming from?
Through my research, I discovered that this vehicle incorporates a Secondary AIR injection process. The AIR pump sends fresh pressurized air into the exhaust system during cold warm-up. This was designed to cut down catalyst warm-up time. There is also a solenoid control/shut off valve, which regulates the air flow from the pump to the exhaust system. When the catalytic convertor reaches operating temperature the solenoid closes and blocks off air flow to the exhaust system. But, the pump keeps running for a short period of time after the solenoid closes. The solenoid also incorporates a pressure sensor. After it closes, the pump should build up pressure to insure hosing integrity and the pressure sensor sends the information to the computer.
Also, remember, the fuel system is continually shortening the injector on time during the warm-up process. When the computer commands the solenoid to close it expects an immediate drop in air flow across the O2 sensor. When this action does not occur, it reasons the engine must be running lean. There is our evidence and the culprit in this case.
The guilty party can only be an intermittently sticking AIR solenoid control/shut off valve! I replaced the valve and the owner has driven the vehicle for a few months with no re-occurrence of the check engine light. Another case solved!
Mark Hicks is an ASE Master and L1 certified. He is an independent shop owner of 15 years, and he has spent 18 years working for Wells Vehicle Electronics as a technical services manager.
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