P0171 code

Any indication of possible damage to the 02 sensors during the job?

There's no smell of exhaust in the engine compartment? As in you're certain the flange gasket and mounting bolts between exhaust header and catalytic converter are functioning normally?

Also, I always like to start addressing exhaust problems by first making sure Throttle body and MAF sensor was recently cleaned, spark plugs are ok and 12 battery is disconnected so all engine settings are reset.

 

There is no one sensor that can cause the P0171. Several sensors are read and from this data, the ECM injects the optimum fuel into each cylinder. It can only successfully do this if the data is reliable.

I would inspect the whole airflow system from the MAF onward looking for any leaks that could be letting unmetered air in.

Other possible areas to look at:

• Air induction system - The injector has a blockage
• Mass air flow meter - The engine coolant temperature sensor
• Fuel pressure
• Gas leakage in the exhaust system
• Open or short in A/F sensor (bank 1 sensor 1) circuit
• A/F sensor (bank 1 sensor 1)
• A/F sensor heater (bank 1 sensor 1)
• EFI M relay (integration relay)
• PCV valve and hose - PCV hose connection
• ECM

This should all be in the diagnostic list you mentioned.

 

Ok, those are some great questions. So the S1 upstream Air Fuel sensor is part of the fuel control feedback loop. It measures the A/F ratio of the exhaust gases and the ECM uses that to make adjustments.

The S1 A/F sensor has a heater and sensing cell. The ECM circuitry maintains a 300mV difference across that cell. The mixture of exhaust gas going past the sensor changes how much current is needed to keep 300mV. Normal range (at steady throttle) is usually less than +0.1 to -0.1 milliamps. Zero mA is 14.7:1 AFR (lambda of 1.0 or stoichiometric), negative current is "rich", positive current is "lean". Goes up to 2 mA on fuel cut. It can measure AFR at least 12:1 to 18:1.

You can read the mA values if you use a scantool with a decent generic or global OBD2 function.

Toyota engineers decided to display that current signal as a voltage. 3.3V is stoich, under is "rich", over is "lean", fuel cut is 5.0V. Normal steady throttle closed loop operation is "around" 3.3V, fluctuating by 100-200 mV or so.

The S2 sensor is a "narrow band" Oxygen (O2) sensor. The sensing cell produces a voltage, near 1.0V at or below AFR of 14.6:1 (rich), near 0.0V at or below AFR of 14.8:1 (lean). AFAIK, the Prius only uses the S2 O2 for monitoring catalyst function - some other cars use it to tweak fuel control.

The ECM has two mechanisms for adjustment.

Short Term Fuel Trim is designed to keep the S1 sensor at the programmed target value (whatever that may be for the current operating conditions). If AFR is too high (lean), then STFT makes positive correction that adds fuel to base injection ontime. If AFR is too low (rich), STFT makes a negative correction that removes fuel from base injection on time. The "%" of fuel trim is the percentage amount of whatever the injector on time in milliseconds is for that particular moment. STFT is (purely) a response to the S1 sensor signal.

Long Term Fuel Trim is designed to keep STFT near 0. If "average" STFT goes to +10%, then over time LTFT will increase to +10%, and STFT will decrease back to 0. LTFT values are stored in ECM memory, with different "fuel trim cells" for different combinations of RPM and load. Those values are cleared if you remove 12V power from the ECM.

You add STFT to LTFT to get "total" fuel trim. +5%ST plus +7%LT equals +12% total. +3% plus -3% equals 0% total. If total trim goes above +33%, the ECM sets a P0171. If total FT goes below -30%, the ECM sets P0172.

So you want to start looking at ECM data for clues. Get the ICE (engine) up to operating temperature , 180°F. What does S1, S2, STFT and LTFT show at idle, high idle, cruise above 40mph? (you have some of that already).

We want to see if S1 reads normally (around 3.3V but moving some), and if S2 also reads normally (around 0.7V -ish for a healthy cat). This can tell if the ECM has "good" fuel control and if S1 or S2 is lying. A bad sensor or (even tiny) exhaust leaks near them can cause problems.

Next is look at LTFT under different conditions. A vacuum leak has a big affect at idle, but very little effect at 65mph. So LTFT would be high +% at idle, and closer to 0% at 65. High LTFT under all conditions points towards something affecting it all the time (think MAF, biased S1, restricted injectors. low fuel pressure is possible, but is almost always a pump problem that gets worse under heavier RPM and load).

An "easy" way to check MAF function is to operate the car under different conditions as noted in this thread (from my "known good" 06) and compare. https://priuschat.com/index.php?posts/3395501

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