U01D5 – Illegal count received from Rear Corner Radar (RCR)

Last updated: March 30, 2026

U01D5 means your vehicle received a bad or nonsensical “count” value from the Rear Corner Radar (RCR). You may notice disabled blind spot monitoring, rear cross-traffic alert problems, or warning messages. According to factory diagnostic data, this code indicates “Illegal count received from Rear Corner Radar (RCR).” In plain terms, another module cannot trust what the radar reported. That problem often comes from a network message integrity issue, not a proven bad radar. Treat U01D5 as a direction to test power, ground, wiring, connectors, and network health before any parts decisions.

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U01D5 Quick Answer

U01D5 points to invalid data coming from the Rear Corner Radar (RCR) over the vehicle network. Check RCR power/ground and the network wiring/connector integrity first, then confirm the radar appears consistently on a module scan.

What Does U01D5 Mean?

U01D5 code meaning: “Illegal count received from Rear Corner Radar (RCR).” A receiving module expected a valid counter value in the RCR message, but saw a value that did not make sense. In real use, the vehicle may limit or shut off features that depend on that radar, such as blind spot monitoring or rear cross-traffic alert. The code does not prove the RCR failed. It only proves the receiving module rejected the data.

Technically, the receiving controller monitors message content and sequence, not just message presence. Many ADAS messages include a rolling counter or “alive” count to detect corruption, repeats, or out-of-order frames. U01D5 sets when the count violates the receiver’s rules for that message. That points you toward a network integrity problem, a module reset, poor power/ground, or a software/configuration mismatch that corrupts message logic.

Theory of Operation

Under normal conditions, the Rear Corner Radar (RCR) measures targets near the rear corners. It then publishes object and status data on the vehicle network. The receiving modules use that data for blind spot monitoring, lane change assist, and rear cross-traffic alert, depending on the vehicle. Those modules expect messages at a steady rate and with valid integrity fields. The integrity fields often include a rolling counter and a checksum or similar validation.

U01D5 sets when the receiver gets a message but rejects its count value. A corrupted frame can cause that, and so can a radar that resets mid-stream. Low voltage at the RCR can also restart it, which breaks the counter sequence. Network issues can do the same thing. High resistance at a splice, water in a connector, or a shield/ground problem can distort data enough to fail integrity checks without a complete loss of communication.

Symptoms

U01D5 symptoms usually show up as ADAS warnings and inconsistent radar-related functions.

• Scan tool: RCR appears on the module list but shows intermittent communication, frequent network DTCs, or repeated “present/absent” behavior during a network scan
• Blind spot warning: blind spot monitoring disabled, unavailable, or warning lamp stays on
• Rear cross-traffic alert: feature disabled or alerts become inconsistent or false
• Driver information message: “Radar blocked/unavailable” or “Driver assist unavailable” message that comes and goes
• Intermittent operation: features work after key cycle, then drop out after bumps, rain, or temperature change
• Multiple network codes: additional U-codes stored in ADAS, gateway, or body modules that reference radar data validity

Common Causes

• Rear Corner Radar (RCR) power supply drop: Low module voltage during crank, key-on, or heavy electrical load corrupts message data and triggers an illegal count.
• High-resistance RCR ground (voltage-drop issue): Corrosion or a loose ground point lets the module “brown out” under load, which distorts network messaging without fully losing communication.
• Water intrusion at the RCR connector: Moisture wicks into terminals and changes impedance, which can scramble CAN/LIN signal quality and cause an out-of-sequence counter.
• CAN bus wiring fault near the rear bumper harness: A chafed, pinched, or stretched harness can create intermittent opens or shorts that produce malformed frames or dropped increments.
• Poor terminal fit or backed-out pin: Terminal spread or a partially latched connector creates momentary disconnects that look like “bad data” rather than a clean U-code for no communication.
• Shared network splice pack or junction corrosion: Corroded splices add resistance and reflections on the bus, which can corrupt message counters from the RCR while other modules still talk.
• Aftermarket device or trailer wiring interference: Added wiring near the rear body harness can introduce shorts, poor grounds, or induced noise that disrupts radar network messages.
• Rear Corner Radar internal fault (less common): An internal processor or transceiver issue can generate incorrect counter values even when power, ground, and bus integrity test good.
• Module software/configuration mismatch (rare): Incorrect programming, variant coding, or a failed update can produce messages with unexpected counter behavior for the receiving module.

Diagnosis Steps

Tools: a scan tool that can run a full network scan and read radar-related data, a quality DVOM, and current wiring diagrams for the vehicle. Use back-probing pins, a load tool or headlamp for ground testing, and connector inspection tools. An oscilloscope helps on CAN faults, but you can still verify power, ground, and bias voltages with a meter.

1. Confirm U01D5 and record freeze-frame data, pending codes, and confirmed/stored codes. Focus on ignition state, vehicle speed, battery voltage, and any other network or ADAS codes present when U01D5 set.
2. Run a full network scan and verify whether the Rear Corner Radar (RCR) appears in the module list. If the scan tool cannot see the RCR at all, treat this as a power/ground or hard network fault first.
3. Check fuses, relays, and power distribution feeding the RCR and its network circuits before you touch the module connector. Confirm the fuse type and rating match the diagram, and look for signs of heat at fuse terminals.
4. Use freeze frame to guide your reproduction plan, then capture a scan tool snapshot during a road test if the fault acts intermittent. Freeze frame shows conditions when the code set, while a snapshot captures live data when you provoke the problem.
5. Perform a targeted visual inspection at the rear bumper harness area. Look for impact damage, pinched loom, trailer wiring taps, and water tracks leading into the RCR connector or splice packs.
6. Verify RCR power and ground under load with voltage-drop testing. With the circuit operating, measure power feed drop from battery positive to the RCR B+ pin, then measure ground drop from the RCR ground pin to battery negative; keep ground drop under 0.1V with the module powered.
7. With ignition ON, check communication line bias voltages at the RCR connector and at a convenient network access point. Do not use ignition-OFF readings as a reference, because network bias only appears when modules power up.
8. Isolate an intermittent harness problem by performing a controlled wiggle test while monitoring scan tool data and DTC status. Move the harness near the connector, along the bumper beam, and at any splice/junction points.
9. Inspect the RCR connector terminals closely for corrosion, pushed-back pins, terminal spread, and poor retention. Perform a gentle drag test with the correct terminal tool if you have it, and correct any fit issues before reassembly.
10. If power/ground and the bus test good, clear codes and perform a verification drive under similar conditions to the freeze frame. If U01D5 returns as a confirmed/stored code on subsequent trips, follow OEM pinpoint tests for message counter validity and consider software/configuration checks before suspecting the radar module.

Professional tip: Illegal-count faults often come from “almost bad” power or ground. A clean continuity check can still pass. Load the circuit and voltage-drop it at the module connector, then repeat after heating, spraying water near the connector, or adding electrical load to expose the failure.

Possible Fixes

• Repair corroded or loose power and ground connections to the Rear Corner Radar, then verify with voltage-drop under load.
• Repair chafed, pinched, or stretched CAN wiring in the rear body/bumper harness and restore proper routing and retention.
• Clean, dry, and re-pin the RCR connector as needed, including terminal replacement for corrosion or poor pin fit.
• Remove or correct aftermarket wiring issues near the rear harness, especially trailer wiring and poor ground taps.
• Perform required module programming or configuration corrections when OEM procedures confirm a software/variant mismatch.
• Replace the Rear Corner Radar only after you prove power, ground, connector integrity, and network health.

Can I Still Drive With U01D5?

You can usually drive with a U01D5 code, but you should treat it as an ADAS reliability problem. The Rear Corner Radar supports features like blind spot monitoring and rear cross-traffic alert on many vehicles. When the network receives an “illegal count” from the RCR, the vehicle may disable those features or show warnings. Drive normally, but do not trust lane-change warnings or cross-traffic alerts until you fix U01D5. If the dash shows multiple network faults, shifting issues, or stability warnings, park it and diagnose the network first. A weak battery or water intrusion can turn a “driveable” fault into a no-start.

How Serious Is This Code?

U01D5 ranges from an inconvenience to a safety concern, depending on what the radar feeds. On many platforms, the car keeps running fine, but the driver loses blind spot monitoring, rear cross-traffic alert, and related assistance features. That loss raises risk during lane changes and backing maneuvers. If the RCR message stream corrupts the network, other modules can log additional U-codes, and some vehicles may enter reduced-function modes. Any time you replace a radar module or disturb its mounting, plan for calibration or initialization before you rely on ADAS features. A mis-aimed or uncalibrated radar can create false alerts or missed detections.

Common Misdiagnoses

Parts get replaced too early with U01D5 because the wording points at the radar. The most common mistake involves skipping network basics and ordering an RCR module. An “illegal count” often traces to unstable power, a weak ground, or high resistance at the connector that corrupts data, not a dead sensor. Another frequent miss involves water intrusion behind the bumper or in the quarter panel harness, where corrosion changes CAN signal quality. Technicians also misread this as a “no communication” code and stop when the scan tool can still talk to the module. Always confirm power, ground, and bus integrity under load before any module decision.

Most Likely Fix

The most common U01D5 repair path starts with restoring clean power, ground, and network connections to the Rear Corner Radar. Fixes often include cleaning and tightening the RCR connector, repairing corroded terminals, and correcting harness damage near the rear bumper or quarter panel. If voltage-drop testing shows a weak ground or supply, repair that circuit first and recheck for illegal message counts. When tests prove the wiring and network are healthy, then consider an RCR module fault or a software update, followed by the required calibration or initialization procedure for that vehicle.

Repair Costs

Network and communication fault repairs vary by root cause — wiring/connectors are often the source, but module-level repairs or replacements can be significantly more expensive.