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1997 Ford Probe GT OBD2 Codes P0300, P0304, P0420, P1170, P1173 - Misfire, Catalyst Efficiency, O2 Sensor Issues

Model: 97 Ford Probe GT Fault Code: P0304 Posted: 2012-09-13 12:13

I own a 1997 Ford Probe GT with a 2.5L V6 engine and 5-speed manual transmission. The 'Check Engine' light turned on yesterday and began flashing, indicating a serious issue. After connecting an OBDII scanner, I retrieved the following diagnostic codes: - P0300: Random/Multiple Cylinder Misfire - P0304: Cylinder 4 Misfire Detected - P0420: Catalyst Efficiency Below Threshold - P1170: Rear Oxygen Sensor Voltage Fixed - P1173: Front Oxygen Sensor Inversion The vehicle exhibits slight sputtering, especially at idle—sometimes feeling like it's about to stall—but runs smoothly under normal driving conditions. The onboard monitors show one incomplete monitor (EGR), with all other systems (misfire, fuel, compression, catalyst, O2 sensors, O2 heater) marked as ready. Based on this information, I'm seeking guidance on the most effective troubleshooting steps. I've reviewed various Probe GT forums and suspect potential causes including: a faulty oxygen sensor, worn spark plugs or ignition wires, vacuum leaks, EGR valve issues, dirty MAF sensor, clogged fuel filter, or air filter. However, I need help prioritizing which components to inspect first. What should be my diagnostic order—starting with the most likely root cause and progressing through the most critical systems? Any insights from owners of similar models would be greatly appreciated.

Related fault codes
P0304P0420P1170P1173
Comments (4)
Anonymous 2012-09-13 14:30

A flashing Check Engine Light is a high-priority warning. Focus first on diagnosing the misfire—start with fuel pressure readings and perform a leak-down test. Inspect the spark plug in cylinder #4 specifically. The fuel pressure regulator should not have any fuel in its vacuum line. My hypothesis: the engine is overfueling due to a leak, causing downstream components like oxygen sensors and the catalytic converter to detect excess fuel. Check for wet or fouled injectors and plugs—signs of overfueling may also appear as fuel in the oil. Please confirm if this vehicle has undergone any modifications; I assume it's stock.

Anonymous 2012-09-13 15:12

Could the EGR valve be stuck open at idle? This would cause a rich air-fuel mixture, leading to misfires and abnormal oxygen sensor readings. It might also explain why the EGR monitor is incomplete—this could be a contributing factor in the system's failure to properly regulate emissions.

Anonymous 2012-09-13 15:18

Thanks for the input, Kev2. The vehicle is completely stock with no aftermarket parts or modifications. It hasn't had any major repairs since I replaced the radiator, distributor, rotor/cap, spark plugs, and ignition wires over a year ago. The car has accumulated 164,000 miles and receives regular oil changes. I've taken good care of it for the past 12 years. I'll inspect the spark plugs and fuel pressure regulator later today and update with my findings.

Anonymous 2012-09-13 15:20

Jeff—thanks for mentioning the EGR valve. While I'm not certain about its current state, I’ll investigate it as well since several forum posts suggest that cleaning carbon buildup on the EGR valve has resolved similar symptoms in other 1997 Probe GT owners.