Drivable, but ABS/traction usually off — fix soon. On GM vehicles C0191 means the electronic brake control module (EBCM) saw an out-of-range or implausible longitudinal-accelerometer signal — the sensor that reports fore-and-aft acceleration for ABS, traction and stability control. Expect the ABS, traction and StabiliTrak warnings, with those features switched off until the signal reads valid again.
What C0191 means
C0191 is a chassis (C-series) trouble code, and because chassis codes are defined by each carmaker, its meaning is not universal. On General Motors vehicles — the platform where C0191 shows up most — it flags a fault in the longitudinal accelerometer signal circuit. That sensor measures forward-and-backward acceleration and reports it to the electronic brake control module (EBCM), which uses the value for ABS, traction control and StabiliTrak/electronic stability decisions and as a 4WD/AWD grade reference. The accelerometer is fed a 5-volt reference and returns a signal that rests near 2.5 volts when the vehicle is stationary and level, rising or falling as the truck accelerates or brakes. The EBCM stores C0191 when that signal voltage sits outside its expected window, sticks, or disagrees with wheel-speed data. A few code databases mislabel C0191 as a traction-control torque-request fault; on GM the actual subject is the accelerometer signal itself. Once the code sets, the module normally disables traction and stability assistance until a plausible signal returns. Because C-codes vary by make, always confirm the meaning against the specific carmaker's service data before parts-swapping.
Symptoms
- ABS warning light illuminated on the dash
- Traction control and StabiliTrak/ESC lights on, often with a 'Service StabiliTrak' or 'Traction Control Off' message
- Traction and stability control disabled — no automatic wheel-slip or spin intervention
- On 4WD/AWD trucks, loss of the accelerometer grade reference can trigger service-4WD messages or odd transfer-case behavior
- ABS function may drop out, though the base hydraulic brakes still work normally
- No effect on engine starting or running — the vehicle otherwise drives normally
Common causes
- Faulty longitudinal accelerometer — internal sensor failure or signal drift outside the expected bias window
- Damaged, chafed or corroded wiring between the accelerometer and EBCM (5V reference, signal, or low-reference wire)
- Loose or corroded connector at the sensor or at the harness junction (e.g. connector C101 near the fuse block)
- Poor grounds (such as G300/G306) or a bad EBCM ground shifting the reference voltage
- Low or unstable system voltage / charging fault skewing the ~2.5V bias signal
- Internal EBCM fault or software issue (least common; a replacement module must be programmed to the VIN)
Severity & driving advice
Severity: Moderate — The vehicle still drives, but ABS, traction and stability control are usually disabled, cutting your safety margin on wet, icy or loose surfaces.
Can I drive? Drivable, but ABS/traction usually off — fix soon.
Diagnostic approach
- Scan all modules and record freeze-frame data — Read every module, not just the engine, and note whether C0191 is current or history. Capture the longitudinal-accelerometer signal PID and any companion codes (wheel-speed, communication, or low-voltage codes point elsewhere). Clear the code and see if it returns to confirm it is active.
- Check the accelerometer bias and response — With the vehicle stationary and on level ground, the signal should sit near 2.5 volts. During a short drive it should rise under acceleration and fall under braking. A reading stuck at 0V, 5V, or frozen at one value points to a circuit or sensor fault rather than a valid signal.
- Inspect wiring and connectors — The GM sensor is typically a three-wire device: low reference, signal, and a 5-volt reference. Inspect the connector at the accelerometer (often behind the center instrument-panel trim) and intermediate junctions such as C101 by the fuse block. Perform a wiggle test while watching the signal PID for dropouts, and look for chafed, green-corroded, or backed-out terminals.
- Verify reference voltage and grounds — Confirm the ~5-volt reference and a clean low-reference at the sensor connector with the key on. Clean and secure grounds G300/G306 and the EBCM ground, and check charging-system voltage, since a shifted ground or low system voltage can move the 2.5V bias out of range and set the code falsely.
- Replace the failed component and retest — If the reference, grounds and wiring are good but the signal stays implausible, replace the longitudinal accelerometer and re-run the drive cycle. Only condemn the EBCM after wiring and sensor are ruled out; a replacement module must be programmed to the vehicle's VIN to function.
Make & model notes
General Motors (Chevrolet, GMC, Hummer, Buick, Cadillac): This is the platform C0191 belongs to: the longitudinal (4WD/AWD reference) accelerometer signal read by the EBCM. On trucks like the Colorado/Canyon, TrailBlazer and Hummer H2/H3 the sensor lives near the vehicle center behind the IP trim; suspect its connector, the 5V reference/ground, and wiring before replacing the module.
Other manufacturers: C-codes are manufacturer-specific, so C0191 may be unassigned on non-GM platforms or map to a different chassis circuit entirely. Do not assume the GM accelerometer definition — verify the code against the specific carmaker's factory service data before diagnosing.
FAQ
Is C0191 a traction-control torque-request problem?
No. Some code lists label it that way, but on GM vehicles C0191 refers to the longitudinal accelerometer signal circuit — the sensor that measures fore-and-aft acceleration for the EBCM. Traction and stability control get disabled as a consequence, not as the root fault.
Can I keep driving with a C0191 code?
You can drive short-term because the base brakes still work, but ABS, traction control and stability control are typically switched off. That reduces your safety margin in rain, snow or on loose surfaces, so it is best addressed before challenging conditions.
Where is the sensor that causes C0191?
On most GM applications the longitudinal accelerometer sits near the center of the vehicle, commonly behind the center instrument-panel trim, and connects through the harness to the electronic brake control module (EBCM). Check its connector and grounds first.
Do I have to replace the ABS module to fix C0191?
Usually not. The most common fixes are a failed accelerometer, damaged wiring, a corroded connector, or a poor ground. Only replace the EBCM after those are ruled out, and note a new module must be programmed to the vehicle's VIN.