Code 24
Code 24 is a diagnostic trouble code related to the Vehicle speed sensor circuit on the Pontiac Fiero.
Code 24 indicates that the ECM is not receiving the expected vehicle speed signal.
Overview
The vehicle speed sensor, often abbreviated as VSS, provides speed information used by the ECM and speedometer-related circuits.
The ECM uses vehicle speed information for functions such as idle control, deceleration behavior, fuel control, and drivability calculations.
Code 24 is set when the ECM detects an unexpected or missing vehicle speed signal under conditions where a speed signal should be present.
Flash pattern
Code 24 is displayed as:
- Two flashes
- Pause
- Four flashes
This represents code 24.
Animated flash pattern:
Meaning
Code 24 usually indicates a problem with the vehicle speed sensor signal or circuit.
Possible causes include:
- Faulty vehicle speed sensor
- VSS wiring problem
- Poor connector contact
- Speedometer circuit problem
- ECM input problem
- Damaged wiring between the VSS, speedometer, and ECM
- Incorrect or incompatible ECM calibration or PROM
- Automatic-transmission park/neutral switch or related circuit problem
- Speed sensor drive or transmission-related issue
- An unintended connection or electrical fault elsewhere on the vehicle-speed signal circuit
Symptoms
Possible symptoms may include:
- Check engine light
- Speedometer problems
- Idle problems when slowing down or stopping
- Stalling during deceleration
- Jerking or surging while coasting in gear
- Poor drivability
- Cruise control problems, if equipped
- Little or no obvious drivability problem
Diagnosis
Basic diagnostic checks include:
- Confirm that Code 24 is present.
- Check whether the speedometer works correctly.
- Inspect the vehicle speed sensor connector.
- Inspect VSS wiring for damage.
- Check for poor connector contact or corrosion.
- Check for other stored trouble codes.
- Test the VSS signal if suitable equipment is available.
- Check ECM and speedometer-related wiring if the sensor appears correct.
- Verify that the ECM and PROM or calibration are correct for the vehicle.
- On automatic-transmission cars, inspect the park/neutral switch and related wiring if indicated by the factory diagnostic procedure.
- If the expected ECM-side voltage is abnormally low, check for another circuit pulling the vehicle-speed signal toward ground.
When following the factory diagnostic procedure, voltage measurements can help distinguish between a failed VSS signal and a problem elsewhere in the circuit. On 1987–1988 2.5L applications, the service procedure calls for approximately 10 volts or more on the ECM side of the relevant instrument-panel connector with the ignition on and the connector disconnected.
A substantially lower voltage can indicate that the signal circuit is being loaded or pulled toward ground rather than simply indicating a failed VSS.
Speedometer relationship
On the Fiero, the vehicle speed signal is related to the speedometer system. A speedometer fault, wiring problem, or signal problem may affect ECM vehicle-speed input.
A functioning speedometer does not necessarily prove that the ECM is receiving a correct vehicle-speed signal. The signal path and associated circuitry should therefore be checked independently when diagnosing Code 24.
Related topics:
1987–1988 2.5L connector C245 issue
An unusual Code 24 condition has been documented on 1987–1988 2.5L Fieros involving connector C245 in the vehicle-speed and cruise-control-related wiring.
On the 2.5L engine, cruise-control functions are handled through the ECM rather than through the separate cruise-control module used with the 2.8L V6. The wiring harness may nevertheless contain the associated connector and wiring.
In one documented 1988 2.5L 5-speed Fiero without factory cruise control, the normal diagnostic voltage at the speedometer connector was only approximately 2.2–2.4 volts instead of the expected battery-level voltage. The VSS, speedometer, ECM, and continuity of the related wiring had already been checked or replaced without correcting Code 24.
The owner eventually traced the problem to connector C245 beneath the center console structure. In the 1988 service manual, the same connector is also identified in some locations as C255. A dark green/white vehicle-speed-related wire passed through this connector to wiring associated with an ECM cruise-control input.
With C245 connected, the vehicle-speed circuit was pulled to a low voltage. Disconnecting C245 restored the expected voltage at the speedometer connector and eliminated Code 24. The owner also reported that a previous jerking sensation while coasting in gear disappeared.
A second owner reported the same condition on another 1988 5-speed coupe in 2026. That vehicle also developed a persistent Code 24 after years of ownership, and the factory diagnostic procedure led to an abnormally low voltage at the speedometer connector. Inspection of the same connector resolved the diagnostic problem.
The reason this condition can apparently appear after years of normal operation is not established. One possibility suggested by owners is that a circuit connected through this otherwise unused wiring can develop a path toward ground and overpower the ECM's pull-up voltage. This explanation has not been confirmed by factory documentation.
For a persistent Code 24 on a 1987–1988 2.5L Fiero, particularly when:
- the speedometer operates,
- the VSS and its wiring test correctly,
- the ECM has already been checked,
- and the ECM-side diagnostic voltage is substantially below the specified value,
inspection of C245 and associated cruise-control wiring may therefore be worthwhile.
Connector C245 is located beneath the center console structure and may require removal of the radio/HVAC console structure for access.
Notes
Code 24 should not automatically be treated as a failed vehicle speed sensor. Wiring, connectors, speedometer circuitry, ECM input, calibration, and any other circuits sharing the vehicle-speed signal should also be considered.
A Code 24 may be especially noticeable if the engine stalls or behaves poorly when the car is slowing down or coming to a stop.
On some cars, a vehicle-speed circuit fault may affect drivability even when the speedometer itself appears to operate normally.
Related threads on Pennock's Fiero Forum
- Code 24 on 88 Base 4 Cylinder — Diagnosis of a persistent Code 24 on a 1988 2.5L 5-speed Fiero, ultimately traced to connector C245 pulling down the vehicle-speed signal. The thread also contains confirmation of a similar fault on a second 1988 5-speed coupe.