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P1151 and P1131 Fault Codes - 2003 Ford Expedition XLT 4.6L Engine Diagnosis

Model: 2003 Ford Expedition XLT Fault Code: P1131 Posted: 2007-01-25 21:49

The vehicle exhibits rough idle when reaching approximately 165°F engine temperature, despite confirmed good MAP sensor readings, new Bosch O2 sensors, fresh IAC valve and spark plugs, and tested injectors and coils. Using the OTC 4000 diagnostic tool, the only detected differences compared to a 1997 F-150 4.6L are EPC pressure values (4.99 PSI) and EPCV value (.16), which remain stable during engine operation but fluctuate with engine RPM. On the 2003 Expedition XLT, the EPC reading is consistently at 4.99 PSI and EPCV at .16 — no change when engine is running, suggesting a potential EPC malfunction. Additionally, the O2 sensors do not switch properly until reaching 1500 RPM, which may indicate upstream sensor issues or vacuum system leaks affecting sensor performance. Vehicle has accumulated 71,000 miles and has been diagnosed with P1151 (O2 sensor circuit malfunction) and P1131 (exhaust oxygen sensor circuit malfunction). The OTC 4000 monitor is currently being used for real-time monitoring to assess engine behavior during warm-up cycles.

Related fault codes
P1131P1151
Comments (2)
Anonymous 2007-01-25 23:14

Check for vacuum leaks and exhaust system integrity. Inspect the PCV (Positive Crankcase Ventilation) system for damage or blockages. Does the engine run smoother around 1500 RPM? This could indicate a vacuum-related issue affecting sensor readings.

Anonymous 2007-01-28 11:22

After thorough research, I found CULPRET #3W4Z-6767-CA — a 90-degree elbow at the manifold connection that had collapsed. The leak was not easily visible and produced a faint hiss from behind the engine. Identified by removing vacuum hoses one at a time and covering their ends to detect suction loss; the PCV hose showed the weakest signal. After disconnecting the battery positive terminal and resetting the ECU, the vehicle relearned its parameters. Post-repair fuel economy improved: 18 mpg in city, 21 mpg on highway at 75 MPH. Currently experimenting with hydrogen supplementation for long-range driving — previously tested successfully on a 2000 Ford F-150 5.4L, achieving an additional 5 MPG both in city and highway conditions.