1996 Ford Ranger P0153, P0401, P1151 Codes: Causes, Fixes & Most Inexpensive Solutions

Vehicle: 1996 Ford Ranger - 4.0L V6, 4x4, Automatic Transmission, 126,000 miles Issue Description: The vehicle exhibits poor idle performance and noticeable engine miss at idle. Symptoms improve as the engine warms up but worsen when coming to a stop during driving. The owner reports that these issues are more severe in summer months compared to winter. Diagnostic Codes: - P0153: Oxygen Sensor (O2) malfunction — Exhaust system is recently replaced; vacuum leaks are suspected, though no direct access to the vehicle. - P0401: EGR Flow Insufficient — This code may stem from multiple potential causes. What are the most common and effective starting points for diagnosis? - P1151: Heated Exhaust Oxygen Sensor indicates lean condition at end of test cycle, attempting to correct an over-rich state. This could be linked to either P0153 or P0401, or possibly a vacuum leak — especially related to EGR system components. Request: I’ve had professional diagnostics performed but the complexity of these codes makes it difficult to pinpoint the root cause. I’m seeking community input on which parts or repairs are most likely and cost-effective to address these issues. A thread on a similar 1996 Ranger suggests replacing the thermostat may resolve symptoms — however, I’d like to confirm if this is a viable fix or whether there’s a more direct solution. Goal: Identify the most practical, affordable repair path to resolve all three codes without unnecessary expense.