1998 Mercury Mystique 2.5L - P1151 O2 Sensor Code Causes Cold Start Flooding and Recurring Fault

Test the injectors on Bank 2 for fuel leakage. A leaking injector can cause a lean condition and trigger P1151, especially in cold starts.

P1151 is typically an upstream O2 sensor fault. Replace the Bank 2 (driver's side) upstream oxygen sensor with a dealer-grade unit—aftermarket sensors often fail to provide accurate signals and may not resolve the issue.

The downstream O2 sensor (aftercat) is an aftermarket unit. Could this be causing a misread in fuel mixture? A faulty downstream sensor might contribute to incorrect feedback, but it typically doesn't trigger P1151 directly—this code points more strongly to the upstream sensor or signal integrity.

I performed a system pressure test after replacing the fuel pressure regulator and removing the intake manifold. No visible fuel leaks were detected along the intake ports, so I believe the injectors are not leaking. The engine runs well once warmed up, suggesting the issue is likely related to sensor feedback or cold-start behavior.

P1151 indicates a lean condition in Bank 2 Sensor 1 (pre-cat). A false lean reading can cause the ECU to add excessive fuel, leading to flooding. First, perform a key-on-engine-off and key-on-running test with a scan tool to check if the O2 sensor signal is stuck lean. Then, introduce propane into the intake manifold for Bank 2—observe if the O2 voltage rises (goes rich). If it does, you have a true lean condition; if not, the sensor may be faulty or reading falsely. Check the white/red O2 signal wire for continuity and integrity. Walmart sells test probes in the craft section—use a DC voltmeter with positive on the wire and negative to battery ground. A properly functioning sensor should switch between 0.1V (rich) and 0.9V (lean). If it stays low, replace the sensor. For true lean conditions, inspect for vacuum or exhaust leaks. Visit an ASE-certified technician specializing in electrical diagnostics if needed.

After further testing, I found no vacuum or exhaust leaks. The upstream O2 sensor on Bank 2 (driver's side) appears faulty based on the slow response during cold starts. Today, while driving, a new code P0156 appeared—indicating a downstream O2 sensor issue (Bank 2 Sensor 2). This suggests multiple sensor failures or ECU misdiagnosis. The car also struggles to start in cold weather—it starts only after prolonged effort and barely runs initially.

P0156 should not directly cause hard starting. I’ve tested the B2S1 with my Innova 3040 scanner: it responds slowly when cold but stabilizes after warm-up. The B2S2 sensor shows a voltage of ~0.3–0.4V at idle and remains stable—indicating no switching behavior, which is abnormal. A non-switching O2 sensor can mislead the ECU into thinking fuel mixture is lean, causing it to add excessive fuel during cold starts. This could explain the flooding. However, I don’t see how a faulty O2 sensor would prevent starting entirely—it might affect performance but not cause complete failure to start.

The P0156 code on Bank 2 Sensor 2 is not typically linked to hard starts. Since you have a scan tool, run the recommended test: monitor fuel trims, O2 sensor voltage, ECT (engine coolant temp), and IAT (intake air temperature) at idle and under load. This will help isolate whether the issue lies in fuel delivery or sensor feedback. If flooring the throttle helps clear excess fuel, it may indicate a rich condition during start-up due to poor mixture control. If not enough fuel is being delivered, adding propane into the intake can test for true lean conditions. If the O2 signal does not respond to propane, the sensor circuit is likely faulty or the wiring is compromised. If the pre-cat sensor fails to respond properly and the circuit checks out, replacement is warranted.

I don’t have any experience with this specific issue—sorry I can't assist further.

The B2S2 P0156 code does not typically cause hard starting. With your scan tool, run the suggested diagnostics: monitor fuel trims and O2 sensor behavior at idle and higher RPMs to pinpoint the problem area. If flooring the throttle helps clear excess fuel, it may indicate a rich condition during start-up due to poor mixture control. If adding fuel via intake doesn’t help, the issue is likely in sensor feedback or wiring. I’ve tried holding the throttle open by unplugging the TPS—this helped slightly, but only when the engine was ready to start. After replacing B2S1, the car started briefly and ran for 25 miles before the CE light came on and P1151 reappeared. I’m now considering that the root cause may be deeper than just a sensor failure.

I replaced the B2S1 O2 sensor, but the car still won’t start after driving 25 miles—P1151 reappeared. I tried starting it this morning and failed completely. Given that the issue persists despite multiple repairs, I’ve decided to take the vehicle to a junkyard. This problem appears beyond repair for my current setup.