Distronic warning on the dash of your A-Class, GLC, C-Class or E-Class, often paired with 'Active Brake Assist function limited'? They share the same radar behind the front star badge, so when one's flagging, the other usually is too. Common after a bumper repair, a parking knock, or front-end work where the badge had to come off. Here's what's happening and what calibration costs.
You can drive the car. Distronic adaptive cruise, Active Brake Assist and the Pre-Safe pre-collision functions stay offline until the radar's recalibrated. Normal brakes, steering and conventional cruise still work. Leave more following distance and treat the assists as not there until it's sorted.
Mercedes mounts the forward radar behind the three-pointed star badge on the front grille, and the multifunction camera at the top of the windscreen. Distronic (adaptive cruise) and Active Brake Assist (AEB) both read from the same pair. Here's what we see.
When the warning text reads 'Distronic fault' or 'Active Brake Assist function limited', they're usually flagging the same root cause. The radar behind the front star sits in a single housing that feeds both the cruise function and the emergency braking function. One misalignment, several warnings.
On almost every Mercedes with Distronic, the radar is positioned directly behind the three-pointed star on the front grille. Bumper repair, grille replacement, even a replacement badge can disturb the radar's aim. The badge area is a common kerb-strike contact point too.
The Mercedes quirk that catches some workshops out. The radar can't be calibrated without removing the front badge first. Most fast-fit chains aren't set up for that. An accredited workshop removes and refits the star as part of the calibration job.
Mercedes 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 for newer Mercedes.
Mercedes Distronic on most models uses the multifunction camera at the top of the windscreen alongside the radar. A new windscreen moves the camera, and the camera-radar pair gets out of sync. Distronic can flag a fault even when the radar itself is fine, because the camera reference is now wrong.
Commercial Mercedes vans (Sprinter, EQV) sit the radar at a higher front-end position than passenger cars. Urban delivery routes with frequent kerb contact and parking sensor strikes shift the radar more often than on an A-Class or C-Class. Same calibration job, more common trigger profile.
If the trigger was a real impact, the radar housing or the 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 other warnings are showing alongside Distronic (Active Brake Assist, Pre-Safe, Active Lane Keeping often flag together), and look along the front bumper for an uneven gap that signals a previous repair. If Distronic is still showing, the radar needs recalibrating, and the front star has to come off first.
On a Mercedes that means a static procedure: the workshop removes the front star, sets a Mercedes-spec radar target board at the correct standoff distance, connects via OBD with XENTRY (or a verified J2534 pass-through on pre-MY2018 cars), and runs the alignment routine. The badge goes back on as part of the job.
It's a fixed £349 through our network: scan, star removal, calibration, badge refit, certificate. The dealer route for the same procedure typically runs £600 to £900 because Mercedes XENTRY subscription costs are passed onto the labour rate. Same job, same OEM-spec result, fixed price through us. 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.
Distronic is Mercedes' adaptive cruise control, the system that maintains a set distance to traffic ahead. Active Brake Assist is the automatic emergency braking system, which intervenes if a collision is imminent.
They read from the same forward radar and (on most models) the same multifunction camera. When the radar's aim is off, both systems flag the loss of confidence at the same moment. One calibration brings them both back.
Radar calibration is a fixed £349 across our accredited network, the same wherever you are in the UK. That covers the diagnostic scan, the front star removal, the calibration, the badge refit, and a calibration certificate. The Mercedes dealer route for the same procedure typically runs £600 to £900 because XENTRY subscription costs are passed onto the labour rate. Same job, fixed price.
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.
Because the radar sits directly behind the three-pointed star on the front grille on almost every Distronic-equipped Mercedes. The calibration target board has to be positioned directly in front of the radar without the badge in the way, and the radar itself can't be accessed for connector checks while the star is in place. The badge comes off, the calibration runs, the badge goes back on. Part of the standard job through our network.
The rear-quarter radar that runs Blind Spot Assist, a separate sensor with its own calibration job.
The cross-make guide for the Distronic/Active Brake Assist family of warnings on other manufacturers.