Ford V10 RV Engine: Fix Lean Bank Codes P0171/P0174 and Transmission Leak Diagnosis

Bottom Line Up Front

both of your 2004 E450 V10 issues are manageable but need attention. The oxygen sensor replacement on your Ford V10 is moderately difficult - expect 2-4 hours and $200-400 in parts for all four sensors. Since your shop's "fix" failed after 50 miles, you likely need new O2 sensors rather than just cleaning. The red fluid leak near your transmission/torque converter connection is concerning and shouldn't be ignored, even though it's intermittent. Ford E450s are known for transmission cooler line leaks and torque converter seal issues, especially when reversing frequently for parking maneuvers like you're doing.

Root Causes

Your "lean on both banks" code commonly includes failing oxygen sensors after 100,000 miles, which is right on schedule for your 2004 V10. The Ford V10 has four O2 sensors - two upstream (pre-cat) and two downstream (post-cat). When these fail, they can't properly measure the air/fuel mixture, causing the engine to run lean and triggering the check engine light. The fact that your shop's repair only lasted 50 miles suggests they may have cleaned the sensors or reset codes without actually replacing the failing sensors.

The transmission fluid leak you observed likely stems from one of three common E450 weak points: the transmission cooler lines where they connect near the radiator, the torque converter seal, or the transmission pan gasket. The red fluid appearing specifically during backing maneuvers points to increased pressure and heat in reverse gear, which can cause seals to weep. Ford E450s are notorious for transmission cooler line failures, especially the rubber portions that deteriorate over time.

Before You Start

WARNING: Always allow the engine to cool before working on exhaust components to avoid severe burns. Use proper jack stands when working under the vehicle - never rely on jacks alone.

For the oxygen sensors, you'll need a digital multimeter, oxygen sensor socket (22mm for Ford V10), anti-seize compound, and possibly a small propane torch if the sensors are seized. Purchase OEM Motorcraft oxygen sensors (correct part for your model - verify with dealer) rather than cheap aftermarket ones - expect to pay $60-80 each. Have the engine warm when removing sensors as they come out easier.

For the transmission leak investigation, clean the entire area around the transmission bell housing and cooler lines with degreaser so you can identify exactly where any new fluid originates. Check your transmission fluid level and color - it should be bright red and at the proper level on the dipstick when warm. Place cardboard under the suspected leak area after cleaning to catch any future drips and pinpoint the exact location.

The Repair Process

Before replacing oxygen sensors, first check for vacuum leaks around intake manifold gaskets, vacuum hoses, and clean or test the MAF sensor, as these are more common causes of P0171/P0174 codes. If no vacuum leaks are found and MAF sensor tests good, then proceed with oxygen sensor replacement. The upstream sensors are located in the exhaust manifolds and are more accessible, while the downstream sensors are in the exhaust pipes after the catalytic converters and require getting under the vehicle. Remove the electrical connectors first, then use the oxygen sensor socket to remove each sensor. Apply a thin coat of non-conductive anti-seize compound specifically designed for oxygen sensors to the threads of the new sensors and torque to 30 ft-lbs. Clear the codes with your scanner after installation and take a test drive.

For the transmission leak, first inspect the transmission cooler lines running from the transmission to the radiator - these are the most common failure point on E450s. Look for wet spots, corrosion, or swelling in the rubber sections. If the leak is from the torque converter area, you'll see fluid accumulating on the lower transmission case. A torque converter seal replacement requires transmission removal, but cooler line repairs are much more straightforward and typically cost $100-200 in parts.

If you find the cooler lines are leaking, you can replace just the rubber portions with transmission cooler hose and hose clamps as a temporary fix, but full line replacement is recommended. The metal lines can be purchased from Ford for about $150-200 for the complete set. When installing new cooler lines, ensure proper routing away from heat sources and sharp edges.

Professional Help Needed

While oxygen sensor replacement is within most DIY capabilities, consider professional help if the sensors are severely seized or if you're not comfortable working under the vehicle safely. A good shop should charge 2-3 hours labor plus parts for all four sensors. Make sure they use OEM Motorcraft sensors and provide you with the specific fault codes before and after repair.

For the transmission leak, if it's confirmed to be the torque converter seal or internal transmission issue, this requires professional repair as the transmission must be removed from the engine. However, if it's cooler line related, this is a manageable DIY repair. Given that you're parking this motorhome regularly and the issue appeared during backing maneuvers, don't ignore even small leaks - transmission repairs are expensive, but catching cooler line issues early can save thousands in transmission damage from overheating.