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1999 Mercury Grand Marquis P0420 and P0430: Lean Codes, Jerking, Stalling - Diagnosis & Fixes

Model: 99 MGM Fault Code: P0420 Posted: 2017-04-19 17:22

My 1999 Mercury Grand Marquis has been experiencing persistent drivability issues despite having P0420 (Catalyst System Efficiency Below Threshold - Bank 1) and P0430 (Catalyst System Efficiency Below Threshold - Bank 2) codes from the start. Initially, I accepted these as normal for older vehicles with aging catalytic converters. However, about a month ago, the car began exhibiting severe jerking and stalling—especially when accelerating uphill or shifting into reverse. When placed in reverse, it immediately stalls unless I apply just enough throttle to keep it running. Even then, the vehicle lurches violently against my brakes as if trying to stall. After disconnecting both catalytic converters and their exhaust piping (to isolate the system), the car still stalls under load and produces a loud diesel-like rattle. The issue persists even with the cats removed, which suggests the problem is not related to the catalysts themselves. I've already replaced the Mass Air Flow (MAF) sensor and believe my Throttle Position Sensor (TPS) and Idle Air Control (IAC) are functioning properly. My OBD2 scanner now shows only P0171 and P0174—System Too Lean Bank 1 and Bank 2—which aligns with the original lean condition. The Haynes manual suggests a possible issue with heated oxygen sensors (HO2S) that are switching slowly or not at all. However, I’ve tested for vacuum leaks using brake cleaner spray and didn’t observe any immediate surges during idle—making this seem less likely. I’m now considering whether the symptoms point to transmission issues, such as a failing torque converter, especially since the car runs smoother in neutral but produces an unusually loud roar. The transmission fluid appears slightly darker than normal, though it doesn't smell burnt. The problem seems worse when cold and improves slightly after warming up, which may indicate thermal-related sensor behavior or fuel system response. I’m seeking advice on how to diagnose this without a $3000 OBD2 scanner. Are lean codes typically caused by vacuum leaks, low fuel pressure, EGR issues, or faulty sensors? Can transmission problems be ruled out based on the symptoms described? Any practical, cost-effective troubleshooting steps would be greatly appreciated.

Related fault codes

P0420P0430

Comments (4)

Anonymous 2017-04-19 17:50

1999 Mercury Grand Marquis - Learn something new every day. Lean codes typically indicate a fuel trim issue. Start by checking for vacuum leaks—use the old-school carburetor spray test (available on YouTube tutorials). A simple, inexpensive method to locate hidden leaks. Also consider a fuel pressure check; many auto shops offer rental fuel pressure gauges at low cost. If you're seeing P0171 and P0174, these are both 'System Too Lean' codes across both banks—common in older vehicles. However, the absence of catalytic converter efficiency codes after removing the cats suggests the issue may not be related to catalysts themselves. Focus on fuel delivery or sensor accuracy. Note: A vacuum leak can cause lean conditions and erratic idle behavior, even if it's not immediately obvious during testing.

Anonymous 2017-04-20 09:48

Just checked my scanner today—after removing the second bank of catalytic converters and exhaust. Now only seeing P0171 and P0174: System Too Lean Bank 1 & Bank 2. This is interesting because my Haynes manual points to slow or failed heated oxygen sensor (HO2S) response as a potential cause. I’ve sprayed brake cleaner around the engine bay during idle and didn’t see any surges—so if it’s a vacuum leak, it's likely subtle or intermittent. The jerking and stalling are worse when cold and improve slightly after warming up. This could point to thermal sensor behavior or fuel system responsiveness under load. I’m now considering whether this is truly a transmission issue—I just don’t have access to tools that can read transmission fault codes.

Anonymous 2017-04-20 10:28

Yes, code descriptions vary by manufacturer—always important to ask for full details. Lean conditions are very common in older vehicles like the Mercury Grand Marquis. Begin with vacuum leak checks and fuel pressure testing. A faulty EGR valve or sensor can also cause lean readings. Note: Ford-based systems often store transmission fault codes in the P0700 range—many basic OBD2 scanners will only show a generic 'P0700' message, not specific trans issues. With no catalytic converters or exhaust piping present, fuel trim readings are skewed. This makes it harder to interpret actual engine performance. Have you tried revving the engine in neutral or park? Does the behavior differ when hot vs. cold? Also—have you checked transmission fluid level and condition? Reference: https://www.obd-codes.com/p0171 P0174 is identical to P0171 but for Bank 2. If the vehicle runs smoothly in neutral, that may reduce suspicion of a torque converter issue—but it doesn’t rule it out entirely.

Anonymous 2017-04-20 10:49

Revving in Park or Neutral produces a loud, aggressive roar—my mechanic says this will have all women within a five-block radius chasing after me. (Ha-ha!) Idle in neutral is smooth and sits around 800–900 RPM according to live data. Transmission fluid appears slightly darker than normal but doesn’t smell burnt—this is expected given the car’s 120,000-mile history and purchase from a used lot for $500. I suspect a transmission flush may be needed. The jerking and stalling during stops improve slightly when warm, though acceleration from a stop still causes hesitation and lurching. On the positive side, removing the catalytic converters has eliminated P0420 and P0430 codes—so at least that part of the issue is now resolved. This suggests the root cause may lie in fuel delivery, sensor response, or transmission health—but I need more data to confirm.