Blind Spot Assist warning on your A-Class, C-Class, GLC or E-Class, often with Active Lane Change Assist dropping out at the same time? They share the two rear-quarter radars on either side of the car. After rear bumper work or a side-impact, those radars need recalibrating. Here's what's happening and what calibration costs.
You can drive the car. Blind Spot Assist, Active Lane Change Assist and the related side-monitoring functions stay offline until the rear radars are recalibrated. Standard mirror checks still work, so leave more room when changing lanes until it's sorted.
Mercedes mounts two short-range radars in the rear quarter panels, one each side. Blind Spot Assist reads from both, along with Active Lane Change Assist and Cross-Traffic Alert. Here's what we see disturbing them.
The trigger we see most. The two rear-quarter radars sit just inside the bumper corners, behind the bodywork. A rear-end knock, a refitted bumper, or a new rear bumper cover from a body shop disturbs the radar mounting. Blind Spot Assist will flag one or both sides within a few drives.
On Mercedes models with Active Lane Change Assist (the system that automatically changes lanes when you indicate on the motorway), the rear radars feed both Blind Spot Assist and the lane-change function. When one radar's off, both systems flag the loss of confidence. A single calibration brings them back.
Cross-Traffic Alert (the warning when reversing out of a parking space) reads from the same rear radars. If you've also seen the cross-traffic warning flag a fault, that's the same root cause. One calibration covers Blind Spot Assist, Cross-Traffic Alert and Active Lane Change Assist together.
Fitting a towbar, hardwiring a reversing camera, or other rear-end electrical work can disturb the radar wiring or shift the bumper trim where the radars mount. The fault may not appear straight after the work, sometimes a few drives later when the system fully self-checks.
Mercedes blind-spot radar calibration runs through XENTRY, the manufacturer's diagnostic system. From around MY2018 onward, J2534 pass-through tooling doesn't cover the full procedure, and aftermarket scan tools are increasingly locked out. Our network includes workshops with current XENTRY access.
Replacement rear suspension components, or a significant ride-height change, can shift the angle the rear radars read at. If the warning started after suspension work rather than rear-end repair, that's the likely cause. The calibration restores the radar aim to the new geometry.
If a side-impact disturbed the radar, the housing or its bracket may be cracked, not just shifted out of aim. Calibration won't hold on damaged hardware. A diagnostic scan tells you which case you're in before any work is booked.
Worth checking first: confirm whether the warning is for one side or both (Blind Spot Assist can flag left, right or both), and note any recent rear-end work or towbar fitment. If Blind Spot Assist is still showing, the affected radar needs recalibrating.
On a Mercedes that means a static procedure: the workshop sets manufacturer-spec target boards behind the car at the correct distances, connects via OBD with XENTRY (or a verified J2534 pass-through on pre-MY2018 cars), and runs the alignment routine for each rear-quarter radar. Both sides are typically calibrated in the same visit even if only one's flagging.
It's a fixed £349 through our network: scan, calibration, certificate. The dealer route for the same procedure typically runs £600 to £900 because Mercedes XENTRY subscription costs are passed onto the labour rate. The full procedure is in our ADAS calibration guide.
Pricing is fixed across our network. Same price wherever you are in the UK. Your tier is set by what work has been done, not by your postcode or your car's make.
All prices include the diagnostic scan, the calibration procedure (static, dynamic, or both as required), a post-calibration check, and a calibration certificate. No charge for diagnostic if you decide not to proceed.
Send your registration and a line on what's happened. We'll come back with the fixed price, the nearest accredited workshop, and the soonest available slot.
Because the impact or work disturbed one radar more than the other. The radar on the side closer to a parking knock, a kerb strike, or a towbar fitting takes more of the disturbance. The other side often stays in tolerance.
Our network calibrates both sides in the same visit either way. The cost is the same whether one or both need work, and it catches a borderline radar before it starts flagging on its own.
Radar calibration is a fixed £349 across our accredited network, the same wherever you are in the UK. That covers the diagnostic scan, both rear-quarter radar calibrations, and a calibration certificate. The dealer route for the same procedure typically runs £600 to £900.
Not always, but it's a common trigger we see. A towbar fitting that disturbs the rear bumper trim, the radar wiring, or the rear-end electrical loom can shift the radars' mounting or break their coding to the vehicle. If the warning appeared right after a towbar was fitted, that's almost certainly the cause, and the calibration with a coding refresh brings it back.
No. Any workshop with current XENTRY access (or a verified J2534 pass-through for pre-MY2018 cars) and a calibration bay can do it. Our network includes workshops set up for XENTRY across the Mercedes range. The dealer will charge their own labour rate for the same procedure. The certificate we issue is accepted by insurers and sits on the car's service history.
The front radar that runs DISTRONIC and Active Brake Assist. A separate sensor with its own calibration.
After a real-world collision, multiple modules typically need clearing alongside the radar calibrations.