What does the 'B' mean in this code?

It identifies which suspension control module is being reported. Some vehicles use more than one — a primary module plus a second one at the rear, in a compressor assembly, or handling a specific subsystem — and the SAE naming convention labels them 'A', 'B' and so on. The letter tells you which module went silent, but it does not tell you where that module physically sits; that varies by manufacturer. Look it up for your specific platform before you start pulling trim, because guessing wrong means disassembling the wrong corner of the car.

Is it safe to drive with U0139?

It depends on what your suspension does when it gives up. On a vehicle with adaptive dampers, the dampers default to a fixed setting and the car remains fully controllable — the ride is worse, not the handling, and driving it is fine. On an air-suspension vehicle the answer can be different: if the system has stopped managing height and the vehicle is sitting low or has dropped onto its bump stops, that is not safe at speed and will damage tires and suspension parts. Look at the car. If it is sitting normally and drives normally, get it fixed at your convenience. If it is sitting low or uneven, have it towed.

Does U0139 mean I need new struts or air springs?

No. This code is strictly about a module that stopped talking on the vehicle's network. It says nothing about the mechanical condition of the springs, dampers or compressor. Replacing suspension hardware will not clear it, and a shop that recommends struts on the strength of a U-code alone has not finished diagnosing. That said, the two problems can coexist — a leaking air spring makes the compressor run constantly, and an overworked compressor circuit is one way a module ends up unpowered. Diagnose the communication fault first, then evaluate the mechanical side separately on its own evidence.

Do I need to replace the suspension control module?

Usually not. Most lost-communication codes trace back to power, ground, connector or wiring problems rather than a failed module, and suspension modules are mounted in some of the harshest locations on the vehicle — wheel wells, under trunk floors, on the frame — so corrosion is the leading suspect. Check the fuse, voltage-drop test the ground, inspect the connector for water damage, and measure CAN bus resistance before condemning anything. If the module does turn out to be faulty, budget for programming and a ride-height calibration on top of the part, because most platforms require both.

OBD-II trouble code

U0139: Lost Communication with Suspension Control Module 'B'

Another module on the vehicle network can no longer hear from the second suspension control module. On a vehicle with air springs or adaptive dampers, the suspension falls back to a fixed default — which can mean a harsh ride, a vehicle that will not raise or level itself, or in the worst case a car sitting on its bump stops.

Medium severityNetworkNetwork CommunicationDrivable short-term

Quick facts

System: Network
Category: Network Communication
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $0 – $1,400
DIY difficulty: Intermediate DIY

What does U0139 mean?

Vehicles with electronically controlled suspension — air springs, adaptive or magnetorheological dampers, active anti-roll bars, or automatic load leveling — carry a control module dedicated to that system. It reads ride height sensors, wheel accelerometers, steering angle, vehicle speed and brake input, then continuously adjusts damping and, on air-spring vehicles, the pressure in each corner. Some platforms split the work across more than one module, or place a second module at the rear or as part of a compressor assembly. The 'B' in this code identifies that second suspension control module. U0139 sets when another module on the bus stops receiving its messages for longer than a calibrated interval.

It is worth being precise about what this code claims. It does not say a strut, an air spring, or a compressor has failed. It says a module has gone quiet on the network. Everything downstream of that silence is a consequence: the systems that were waiting for its instructions stop receiving them and revert to whatever default state the manufacturer chose as the safe fallback.

What that fallback feels like varies by system, and it matters. On an adaptive damper vehicle, the dampers usually default to a fixed setting — often the firm end, since a stiff damper is a safer failure than a soft one — so the ride turns harsh and unresponsive but the car remains completely controllable. On an air-suspension vehicle the consequences can be more serious: the system may stop leveling, refuse to raise the vehicle, drop over time as air leaks past unmanaged valves, or in the worst case leave the vehicle sitting low enough to contact the bump stops, which is genuinely unsafe at speed and will damage tires and suspension components. Because that range runs from mild to serious depending on the platform, the honest advice is to identify which type of suspension the vehicle has before deciding how urgently to act.

The causes are the ordinary lost-communication faults. A blown fuse or a bad ground leaves the module unpowered and silent. Suspension modules and their harnesses are frequently mounted low — in a wheel well, under the trunk floor, or on the frame near the compressor — where road salt, water and physical impact take a toll on connectors and wiring far more aggressively than on modules mounted inside the cabin. A weak battery causes intermittent dropouts. And a network problem anywhere on the bus, or another failed module jamming it, can silence a perfectly healthy suspension module.

Common causes

Blown fuse or lost power feed to the suspension control module
Corroded or high-resistance ground connection at the module
Chafed, broken, or salt-corroded CAN bus wiring in the module's harness branch
Corroded or water-damaged module connector — common on modules mounted low or in a wheel well
Water intrusion into a module mounted under the trunk floor or near the compressor
Weak battery or failing alternator producing low-voltage communication dropouts
Another failed module on the same bus disrupting network traffic
Broken twisted pair or a failed bus terminating resistor
Physical damage to the harness from road debris, a curb strike, or suspension work
Aftermarket suspension, air-ride or lowering hardware that removed or bypassed the module
Internally failed suspension control module (verify power, ground and wiring first)

Symptoms

Suspension or ride-control warning message on the dash
Ride quality noticeably harsher or floatier than normal — dampers stuck at a default setting
Air suspension will not raise, lower, or level the vehicle
Vehicle sits low, uneven corner to corner, or sags overnight
Ride height selection buttons do nothing
Check engine light and other warning lights may also be on
Multiple U-codes stored if the network itself is at fault
Symptom appears mainly in cold or wet weather, or after driving on rough roads

Diagnostic steps

1.Scan every module on the vehicle and record all stored and pending codes. U0139 on its own points at the suspension module's circuit; U0139 among a cluster of lost-communication codes points at the bus.
2.Identify which suspension system the vehicle actually has — adaptive dampers, air springs, active anti-roll, or a combination — and locate where module 'B' physically lives on that platform before touching anything.
3.Check the fuses feeding the suspension module and confirm battery voltage at its power pin with the key on.
4.Voltage-drop test the module's ground rather than relying on a continuity check. Suspension modules are often grounded to sheet metal in areas that corrode.
5.Test the battery and charging system. Low system voltage is a leading cause of intermittent module dropouts and is cheap to rule out.
6.Inspect the module connector closely for corrosion, green residue, water staining, and backed-out pins. Modules mounted in a wheel well or under the trunk floor take the worst of the road salt.
7.Follow the harness branch looking for chafing where it passes through the body, crosses suspension components, or runs near the compressor and its heat.
8.Measure resistance across the CAN high and CAN low lines at the diagnostic connector with the key off. Roughly 60 ohms indicates an intact bus with both terminating resistors present.
9.Perform a wiggle test on the module's harness with a scan tool monitoring module status. A momentary loss while flexing the harness locates the break.
10.If the vehicle has air suspension and is sitting low, check for stored air-system codes and inspect for leaks separately — a suspension that will not hold height may have both a communication fault and a mechanical leak.
11.Repair the confirmed fault, clear codes, and verify the module reports present on a full network scan and that ride height and damping control behave normally through a road test.

Repair cost

$0 – $1,400

The low end is zero because a blown fuse, an unseated connector, or a ground that only needs cleaning are real no-parts fixes. Diagnostic time for a network fault typically runs $100-$200. Wiring and connector repair usually lands at $150-$600 depending on where the damage is and how much harness has to be opened; corroded connectors in a wheel well are on the higher side because of access. A replacement suspension control module with the required programming and, on many platforms, a ride-height calibration afterward runs $600-$1,400. Note that this range covers the communication fault only — if the vehicle also has leaking air springs or failed dampers, those are separate repairs with their own costs.

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Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.