One Bank Rich, and the Oxygen Sensor Is Not Blinking
A 2004 V6 with 130,000 miles sets P0172. Long-term trim is -16 percent on bank 1 and -2 percent on bank 2; short-term is -5 on bank 1. Fuel pressure is 52 psi against a 50 to 57 spec. The bank 1 upstream oxygen sensor sits at 0.78 V and barely moves at 2,500 rpm, while bank 2 switches normally between 0.1 and 0.9 V. Injector resistance is 12.1 to 12.3 ohms on all six. The bank 1 plugs are black and dry; bank 2 plugs look normal. There is no fuel in the pressure regulator vacuum line, and the regulator is shared by both banks.
1. Build the case for and against the two leading suspects: a leaking bank 1 injector and a bank 1 oxygen sensor stuck high.
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Leaking injector (or injectors) on bank 1:
- For: the rich condition is bank-specific, the plugs on that bank are black and dry, and a leaking injector adds fuel the PCM did not command, forcing negative trim.
- Against: injector resistance is normal, but resistance does not detect a mechanically leaking or dribbling nozzle, so that is not a strong argument either way.
Upstream oxygen sensor stuck high on bank 1:
- For: a sensor reporting a false rich signal makes the PCM pull fuel on that bank, producing exactly -16 percent trim and a P0172. The flat 0.78 V trace is suspicious.
- Against: if the mixture were actually correct and only the sensor were lying, the bank 1 plugs should look normal, not black. Black plugs say the cylinder really is rich.
The flat sensor plus black plugs together suggest the sensor is reporting a real rich condition but is also sluggish. The injector is the stronger suspect, but you have not proven it yet.
2. Design a test sequence that separates the sensor from the injector without throwing parts at it.
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- Swap the two upstream sensors side to side. If the rich trim follows the sensor to bank 2, the sensor is at fault. If bank 1 stays rich, the sensor is telling the truth.
- Propane or lean-out test on bank 1. Add propane at the intake and watch the bank 1 sensor. A healthy sensor swings rich immediately; a stuck sensor barely moves. This is a quick functional check that costs nothing.
- Check the injectors for leakage. With the rail pressurized and the engine off, watch for drip at each bank 1 injector, or pull the rail and observe. A leaking injector will wet the port.
- Injector balance or flow test if drip is inconclusive. An injector flowing more than its mates shows up as a larger pressure drop or higher flow.
Order matters: the sensor swap first, because it is free and it either clears the sensor or moves the diagnosis to fuel.
3. Suppose the sensor swap moves the rich condition to bank 2. What is the correct repair, and what should you check before releasing the vehicle?
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Replace the bank 1 upstream sensor, which is now the bank 2 sensor after the swap, and clear adaptives.
Before release:
- Verify the new sensor switches across 0.1 to 0.9 V at 2,500 rpm and that trim returns to within about plus or minus 10 percent on both banks.
- Recheck the plugs on the original bank 1. If they were black from a genuinely rich mixture, they may be fouled enough to misfire after the repair. Clean or replace as needed and note it on the ticket.
- Confirm there is no underlying cause that killed the sensor, such as a coolant leak or silicone contamination from a sealant. A sensor that failed from contamination will fail again.
If trim does not return to normal with a known-good sensor, go back to the fuel side.