C1E2F – Invalid target deceleration requested by ACC (BYD)

C1E2F means your BYD’s Adaptive Cruise Control (ACC) has asked the vehicle to slow down in a way the chassis control system considers invalid. For most drivers, the real-world effect is that ACC may cancel, refuse to engage, or behave inconsistently, and you may get a warning related to cruise/driver assistance. Technically, this is a manufacturer-specific BYD diagnostic trouble code (DTC) used on platforms such as the 2020 BYD Dolphin, and the exact logic behind what is considered “invalid” can vary by BYD model and software level. Diagnosis should follow the scan report’s description: “Invalid target deceleration requested by ACC.”

C1E2F Quick Answer

On BYD vehicles, C1E2F indicates the ACC requested an invalid target deceleration, so the chassis system may reject the request and disable or limit ACC operation until the underlying cause is corrected.

What Does C1E2F Mean?

In simple terms, the car’s ACC asked for braking/slowdown that didn’t make sense or wasn’t allowed at that moment, so the chassis system flagged a fault. In more technical terms (manufacturer-specific to BYD), the chassis domain detected an implausible or out-of-range “target deceleration” request coming from ACC logic over the vehicle’s internal control messaging, and it set DTC C1E2F when that request failed plausibility, permission, or consistency checks.

Theory of Operation

On BYD vehicles equipped with ACC, the ACC function calculates a desired vehicle speed and gap to the vehicle ahead, then outputs a commanded longitudinal control request. That request typically includes a target acceleration/deceleration value and is coordinated with other chassis functions (brake control, stability control, traction control, and sometimes regenerative braking coordination depending on platform design).

The chassis controller(s) responsible for executing the slowdown will only accept ACC deceleration requests that meet internal plausibility rules and current operating permissions. Those permissions can change based on brake system availability, stability events, driver override (brake pedal/accelerator input), sensor/ADAS availability, communication integrity, and internal self-checks. If the ACC request is missing, corrupted, contradictory, or not permitted by current conditions, the chassis system may reject it, disable ACC, and store C1E2F to record that an invalid deceleration request was received.

Symptoms

Drivers usually notice ACC limitations first, followed by warnings or stored faults.

• ACC disabled ACC will not engage, cancels unexpectedly, or drops out during use
• Warning message Driver assistance/cruise control warning displayed (wording varies by BYD cluster/software)
• Unexpected behavior ACC may hesitate to slow down, or may revert to driver-only control more often than normal
• Limited functions Related longitudinal assist features may be unavailable until the next key cycle or until faults clear
• DTCs stored Additional chassis/ADAS/brake-related DTCs may be present alongside C1E2F
• Drive mode changes Some BYD platforms may reduce automation and require more driver input when the fault is active

Common Causes

• ACC request out of valid range: The ACC controller may calculate or transmit a target deceleration value that the chassis/brake control logic considers implausible or outside accepted limits, triggering an invalid request fault.
• CAN communication data corruption: Noise, intermittent connections, or network errors can corrupt the ACC deceleration request message so the receiving module flags it as invalid even though ACC appears to operate.
• Power/ground instability at an involved control module: Low supply voltage, high resistance grounds, or momentary dropouts can cause a module to send malformed or inconsistent deceleration requests.
• Connector fretting or water intrusion: Minor terminal oxidation, poor pin fit, or moisture in connectors for ACC-related modules can create intermittent signal integrity issues leading to invalid decel requests.
• Harness damage in ACC/chassis network paths: Chafing, pinched wiring, or repairs in the harness can introduce intermittent opens/shorts that distort request signals or network messaging.
• Sensor plausibility input issue influencing ACC calculation: Incorrect or inconsistent inputs (for example, vehicle speed/brake feedback signals used for plausibility) can cause ACC to request deceleration that fails receiving-module plausibility checks.
• Software/calibration mismatch after service: If a module was replaced, programmed, or updated improperly, message format/calibration differences can make the ACC target deceleration request appear invalid to the chassis controller.
• Internal fault in ACC or chassis control module: An internal processing or driver fault can create an invalid target deceleration request despite correct wiring and normal-looking external inputs.

Diagnosis Steps

Tools needed: a scan tool with BYD-specific chassis/ADAS coverage (including network scan, live data, and actuation tests where available), a DVOM, a test light or suitable load tool for power/ground checks, and basic back-probing/terminal inspection tools. If available, use a known-good battery support unit during diagnosis to prevent low-voltage artifacts.

1. Confirm DTC C1E2F is present and record freeze frame/environmental data and all stored/pending codes from every module; note whether the fault is current or history and whether ACC was active when it set.
2. Check fuses, relays, and power distribution feeding the ACC and chassis/brake-related modules before probing any ECU pins; also perform a scan-tool network scan to verify the relevant modules appear online and report normally.
3. Verify module power and ground integrity under load: with ignition ON and modules awake, use a load method (test light/load tool) to check for voltage drop or weak grounds at the module power/ground circuits that support ACC/chassis functions.
4. Perform a careful visual inspection of connectors and harness routing for the ACC and chassis/brake control modules: look for backed-out terminals, fretting, corrosion, water intrusion, previous repairs, and harness chafing near brackets, pass-throughs, and sharp edges.
5. Use the scan tool to review live data related to the fault’s definition: monitor ACC “target deceleration request” (if available), brake/vehicle dynamics feedback values used for plausibility, and the receiving module’s status or “request valid/invalid” flag; look for sudden spikes, dropouts, or values that do not change smoothly.
6. If the scan tool supports it, run guided tests/actuation checks for ACC deceleration request and brake intervention (as allowed by the tool and vehicle conditions); verify whether the system reports a reason for rejection (for example, request invalid/blocked) and whether the fault sets immediately during the test.
7. Check network and signal integrity as applicable to BYD platform design: if the request is carried over CAN, inspect for network-related DTCs, and if measurements are required, take them with ignition ON (bias voltage is only present when powered); resolve any obvious network faults before condemning ACC logic.
8. Perform targeted circuit tests based on findings: for any suspicious power/ground, connector, or harness area, conduct continuity and short-to-power/short-to-ground checks with the circuit safely isolated, then retest under vibration/wiggle conditions to reproduce intermittents.
9. If evidence points to a software/configuration issue (for example, after module replacement or update), verify module part number compatibility and configuration status using the scan tool; confirm calibrations match the vehicle application and that no programming-related codes are present.
10. Clear codes and perform a controlled road test (or stationary functional test if required) to confirm whether C1E2F returns; recheck freeze frame comparison data to verify the same conditions no longer trigger an invalid target deceleration request.

Professional tip: Treat C1E2F as a plausibility/validation failure between ACC and the chassis/brake side, not automatically a “bad sensor” or “bad ACC” event. On BYD platforms, intermittent power/ground issues and connector fretting can create request messages that look valid most of the time but fail validation under specific load, vibration, or low-voltage conditions—so duplicate the setting conditions and use wiggle/load testing to confirm.

Possible Fixes

• Restore power/ground integrity: Repair weak power feeds, high-resistance grounds, or poor fuse/relay contacts found during loaded testing to prevent malformed requests.
• Repair harness or connector faults: Clean/repair terminals, address water intrusion, and fix chafed/pinched wiring in the ACC/chassis module circuits or network paths verified to be intermittent.
• Correct network-related issues: Resolve CAN communication faults (wiring, connector, termination-related issues as applicable) that cause corrupted ACC deceleration request messaging.
• Perform correct module configuration/programming: If a mismatch is verified, reconfigure or program the affected module(s) per BYD service procedures so the request format and validation parameters align.
• Replace the confirmed faulty module: Only after power/ground, wiring, connectors, and communication integrity are verified, replace the ACC-related or chassis/brake control module implicated by test results and then complete required setup procedures.

Can I Still Drive With C1E2F?

You can usually still drive a 2020 BYD Dolphin with DTC C1E2F, because this fault is about an invalid target deceleration requested by ACC rather than a direct base-brake hydraulic failure. However, treat it as a safety-relevant chassis/driver-assist issue: ACC may be disabled, may refuse to engage, or may cancel unexpectedly. Do not rely on Adaptive Cruise Control to manage spacing or slow the vehicle. If warning messages appear, braking feel changes, or multiple chassis/ADAS codes are present, keep speeds conservative, increase following distance, and avoid using ACC until the cause is diagnosed and the code no longer returns.

How Serious Is This Code?

C1E2F is often moderate in severity: it may be mostly an inconvenience when it only prevents ACC operation (a driver-assist comfort feature). It becomes more serious when it appears alongside other chassis/ADAS or brake-related faults, or when it coincides with ACC requesting implausible deceleration that forces the system to cancel. In BYD’s control strategy, “invalid request” commonly indicates a plausibility/logic problem between ACC and the braking/vehicle dynamics control path (values out of expected range, missing prerequisites, or conflicting signals). Even if the vehicle drives normally, unexpected ACC disengagement can surprise the driver and should be treated as a safety concern.

Common Misdiagnoses

Technicians often misdiagnose C1E2F by replacing parts that “feel related” to braking (calipers, pads, master cylinder) even though the code definition points to ACC requesting an invalid target deceleration, which is frequently a command plausibility or communication/logic issue. Another common mistake is focusing only on the module that stores the code and ignoring related data from ACC, brake/vehicle dynamics control, and gateway domains; this leads to missed root causes like power/ground drops, connector fretting, or network errors that corrupt or delay the request. Misdiagnosis also happens when freeze-frame and live data are not reviewed: without comparing requested deceleration, ACC status, brake switch state, vehicle speed, and fault counters, a transient event (low voltage, intermittent connector) gets treated as a hard component failure. Avoid wasted spending by confirming whether the deceleration request is missing, out-of-range, or conflicting, and by proving wiring integrity and network health before replacing any control unit.

Most Likely Fix

The most frequently confirmed repair directions for C1E2F on BYD platforms are (1) correcting an intermittent electrical or connection issue affecting the ACC-to-chassis/brake request path (reseating connectors, repairing wiring, addressing corrosion or pin fit, restoring stable power/ground), and (2) resolving a prerequisite/signal plausibility problem that makes the requested deceleration invalid (for example, inconsistent brake switch or speed input seen in live data). These should not be treated as certain until scan-tool data confirms the ACC request becomes valid under the same conditions that set the code and the DTC no longer resets after a road test.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a sensor, a module, or the labor needed to diagnose the fault correctly.

Last updated: March 29, 2026