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Here is the story we hear over and over. A 2002-2005 Explorer or Mountaineer starts flaring on the 2-3 or 3-4 shift, sets a gear ratio code, and the owner or a general repair shop puts in a solenoid pack. It shifts better for a week or a month. Then the flare comes back. So a valve body goes in. Same result.
On the 5R55W and 5R55S, a big share of those repeat failures are not electrical and not the valve body at all. They are the intermediate and overdrive servo bores in the aluminum case. Once a bore wears, the servo leaks apply pressure, the band cannot clamp its drum firmly, and no new solenoid will seal a hole that got bigger.
Symptoms That Point at the Servo Bores
- 2-3 or 3-4 flare — engine RPM jumps between gears before the next ratio grabs. Usually worse when hot.
- Harsh or delayed shifts that change with temperature.
- Slipping in a specific gear rather than everywhere.
- P0733, P0734 or P0735 — incorrect gear ratio in 3rd, 4th or 5th. These tell you the ratio was wrong, not why.
- Repeat failure after a new solenoid pack or valve body. This is the biggest tell. If good parts did not stick, suspect the case.
None of these symptoms alone proves bore wear. A sticking shift solenoid, a worn valve body bore, a burned band or a low fluid level can produce the same flare. That is exactly why you test before you buy.
Why the Bores Wear
The 5R55W/S uses bands and servos for several of its ratios, and those servos cycle on nearly every shift. The case is aluminum; the servo piston and its seals are working against that soft bore thousands of times a year. Add fluid that has gone past its life and picks up friction material and metal fines, and those particles act like lapping compound. Over high miles the bore goes out of round and oversized, the piston seal loses its grip, and pressure bypasses the piston instead of pushing it.
Heat makes it worse: hot, thin fluid leaks through a worn bore far faster than cold fluid. That is why the classic complaint is “fine in the morning, flares after 20 minutes on the highway.”
Our shop-floor opinion: a lot of the band wear blamed on these units is a result of servo leakage, not the cause. A band that cannot be clamped hard slips, cooks, and sheds material into the fluid, which wears the bore more.
How to Diagnose It Before Buying Parts
1. Check Fluid Level and Condition
Set the level correctly at operating temperature first. Then look at it. Clean and red means you likely caught it early. Brown, burnt-smelling fluid with heavy friction debris on the magnet shifts the math toward a rebuild.
2. Scan for Codes and Watch the Data
Log commanded gear versus actual ratio and look at when the ratio error shows up. A flare that appears only on specific shifts and worsens with fluid temperature fits a hydraulic leak. Any solenoid circuit code sends you to the pack or harness first.
3. Air Check the Servos
With the pan and valve body off, apply low-pressure regulated shop air to the servo apply passages in the case. A healthy servo applies the band with a firm thump and holds with minimal hiss. A worn bore hisses loudly, may apply weakly, or lets air escape around the piston. Many rebuilders prefer a vacuum test on the servo bores, which gives a more repeatable pass or fail than listening to air, and repair kit makers sell fixtures for exactly that.
4. Rule Out the Solenoid Pack and Valve Body
While the valve body is off, inspect the separator plate and gaskets, check for valves sticking in their bores, and look at the solenoid pack connector for burnt or loose terminals. If the servos test tight and the valve body is worn, you have your answer. If the servos leak, fixing only the valve body is money wasted.
Your Fix Options Compared
| Fix | What It Solves | Rough DIY Cost | Rough Shop Cost |
|---|---|---|---|
| Solenoid pack only | Electrical and solenoid faults. Does nothing for a worn bore. | A few hundred dollars | Several hundred dollars installed |
| Servo bore ream + oversize piston kit | Restores a sealing bore for the intermediate and/or overdrive servo. | Kit and tooling, typically a few hundred dollars | Commonly high hundreds to low four figures with valve body labor |
| Case sleeve repair | Bores too far gone to ream oversize. Usually bench work. | Rarely a DIY job | Varies widely; often done during a rebuild |
| Transgo shift kit + valve body work | Corrects known hydraulic weaknesses. Supports a repaired bore; does not replace it. | Kit plus your time | Usually quoted with valve body labor |
| Full rebuild | Bores, bands, clutches, seals, and everything else worn. | Master kit plus hard parts | Often several thousand dollars |
Costs vary a lot by region, shop, and what else is found inside, so treat those as ballpark ranges, not quotes.
Servo Bore Reaming Kits
Aftermarket repair kits, the kind Sonnax is known for, use a guided reamer to machine the worn bore to a clean, true, larger size, then install a matching oversize piston and seals. The guide is the whole point; freehand honing makes it worse. Follow the kit instructions to the letter and keep every aluminum chip out of the case.
Transgo Shift Kit
We like the Transgo 5R55W/5R55S shift kit on these units because it addresses hydraulic problems the factory calibration left behind. But be straight with yourself: a shift kit on a leaking bore is a patch that buys time. On a repaired case it is a real upgrade.
Valve Body and Solenoid Pack
If testing shows the valve body is worn too, a replacement 5R55W valve body goes in after the bores are fixed. If a solenoid circuit code or a failed solenoid test says the pack is bad, use a 5R55W/5R55S solenoid block pack.
When to Walk Away and Rebuild or Replace
- Repair the bores if fluid is reasonably clean, the vehicle is otherwise worth keeping, and the flare is recent.
- Rebuild if the pan is full of friction material, bands are glazed or burned, or it has already had a solenoid pack and valve body that failed to fix it. At that point the unit has been slipping long enough to damage more than the bores. Start with a 5R55W master rebuild kit, or an overhaul kit if you only need gaskets and seals.
- Replace with a quality reman unit if you do not have a bench, tooling, or time, or if the case is past sleeving.
- Walk away from the vehicle if it is a rusty, high-mile SUV where a rebuild costs more than the truck is worth.
What we do not recommend: a third solenoid pack, a bottle of additive, or a used junkyard transmission that has the same aging case in it. Buying parts for a 5R55W? You can also shop CoreTransmissionParts.com for hard parts and kits.
Need Parts? Ships Same-Day
Once the servos check out or the bores are repaired, get the rest of it right. We stock 5R55W rebuild kits, valve bodies, solenoid packs, and Transgo shift kits.
Shop Gearbox Insider →FAQ
Can a shift kit fix 5R55W servo bore wear?
No. A shift kit raises and recalibrates pressure, which can hide a mild leak for a while, but it does not seal a worn bore. On a case with real bore wear, a shift kit is a band-aid. Pair it with a bore repair or do not expect it to last.
Can the servo bores be repaired with the transmission in the vehicle?
Some servo bore repair kits are designed to be used with the pan and valve body off and the transmission still in the vehicle, while others are meant for a case on the bench. Read the instructions for the specific kit before you buy it, and be honest about how well you can keep chips out of a case you cannot flush.
Will a new solenoid pack fix P0733, P0734 or P0735 on a 5R55W?
Only if the solenoid pack is actually the problem. Gear ratio codes mean the computer saw the wrong input to output speed relationship. If the servo bores are leaking, the band cannot hold and the code comes back after a new solenoid pack or valve body. Test the servos before buying parts.
Is it worth fixing a 5R55W with worn servo bores?
It depends on the rest of the unit and the vehicle. If the fluid is clean, the bands are not burned, and the truck is otherwise solid, a bore repair plus valve body work is worth doing. If the fluid is black with friction material or the unit has already been cooked, rebuild or replace it.