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1997 Ford Taurus 3.0L Misfire and Stumble at Idle/Load - P0420 Catalyst Efficiency Code

Model: 97 Taurus 3.0 Fault Code: P0420 Posted: 2006-10-19 17:49

My 1997 Ford Taurus 3.0L engine exhibits misfires and stumbles specifically at idle and under load (around 1-2,000 RPM). Highway driving above 2,000 RPM is smooth with no issues. I've already replaced the fuel filter, checked and replaced spark plugs and wires, and flushed the transmission fluid with a new filter — initially suspecting a transmission problem. The PCV system functions normally, and disconnecting the EGR vacuum line had no effect. The only diagnostic code set is P0420: Catalyst Efficiency Below Threshold (Bank 1). The engine originally swallowed water, and the current engine block has approximately 20K miles. I'm unsure about the brand of spark plugs or wires used; I plan to verify these tomorrow. I noticed a cyclical behavior — the engine runs smoothly for a while, then stumbles, catches, and returns to smooth operation. This pattern is consistent. A failing fuel injector or compression issue would typically cause continuous misfires and power loss, not this intermittent cycling. I suspect an electrical sensor might be triggering the cycle without setting a visible DTC.

Related fault codes

P0420

Comments (5)

Anonymous 2006-10-19 20:22

What brand of spark plugs and wires were used? Are any diagnostic trouble codes (DTCs) present besides P0420? Consider checking fuel injectors for proper operation, ensuring adequate spark, and verifying engine compression.

Anonymous 2006-10-19 22:08

Only DTC recorded is P0420 — Catalyst Efficiency Below Threshold for Bank 1. The original engine was submerged in water, and this replacement unit has about 20K miles. I don't know the exact brands of plugs or wires used; I'll check them tomorrow. The symptoms began with a lurching and bucking sensation, which led me to believe a motor mount had failed — acting like a bad transmission. The vehicle was then passed to me for repair. The engine runs smoothly at times, then stumbles and catches before returning to normal. A failing injector or compression issue would typically cause persistent misfires and power loss. This behavior is cyclical and consistent. I suspect an electrical sensor may be triggering the cycle without setting a visible DTC.

Anonymous 2006-10-20 09:14

Possible causes: dirty Mass Air Flow (MAF) sensor or vacuum leak. If the engine previously ingested water, it could have damaged the catalytic converter.

Anonymous 2006-10-20 14:27

The MAF sensor is clean and sparkly; no audible vacuum leaks are detected. Intake vacuum holds at 17" at idle and rises to 20" at higher RPMs. The engine can spin up to the RPM limiter (5,000+ RPM) with a downshift. I observed a vacuum line connected to the EGR at idle — disconnecting it or the EGR electrical switch had no effect. Disconnecting the Idle Air Control (IAC) causes stalling and flooding. Disconnecting transmission connectors results in loss of overdrive, but the engine still stumbles at 1500 RPM under load (e.g., slight uphill). Fuel economy has dropped to around 20 MPG on highways — possibly indicating a failing catalytic converter or other exhaust system issue. Note: How can I diagnose a bad catalytic converter? While not certain, the drop in fuel efficiency suggests it may be clogged or malfunctioning.

Anonymous 2006-10-20 14:40

Use a vacuum gauge connected to the intake manifold. When you snap the throttle, if any positive pressure develops above 2 psi, this could indicate an exhaust restriction. Scanning both upstream and downstream oxygen sensors can help identify converter issues. Additionally, comparing exhaust gas temperatures with a heat gun can confirm whether the catalytic converter is functioning properly.