Symptom
Transmission warning light illuminated
A gearbox or cog symbol, sometimes an exclamation mark inside a gear, lit separately from the engine management lamp. It means the transmission control module has stored a fault and, on most vehicles, has already taken protective action — which is why it so often arrives together with harsh changes, a gearbox locked in one gear, or a refusal to select drive. The lamp being separate matters for diagnosis: it says the fault was reported by the transmission's own module rather than the engine's, which narrows where to look before anybody connects a scan tool.
Also described as: transmission light, gearbox light, gearbox warning, cog light, gear symbol light, transmission fault light, transmission warning, چراغ گیربکس روشن شد, چراغ گیربکس, هشدار جعبه دنده, luz caja cambios, testigo transmision, luz transmision encendida, لمبة الجير, علامة ناقل الحركة, إشارة عطل الجير
What this usually means
The lamp being separate from the engine management light is diagnostic in itself: it says the fault was reported by the transmission's own module rather than the engine's, which narrows where to look before anybody connects a scan tool. Read transmission codes, not powertrain ones — a generic reader that only sees engine codes will report a clean car.
On most vehicles the module has already taken protective action by the time the lamp appears, which is why it so often arrives together with harsh changes, a gearbox locked in one gear, or a refusal to select drive. A transmission holding a single ratio with the engine revving freely is a fail-safe, not a mechanical failure — Japanese references describe exactly that, a five-speed restricted to third, and it presents convincingly as an engine complaint.
Fluid is the first thing to establish, and it is the one most often skipped on modern vehicles because many are described as sealed for life. They are not sealed against heat: fluid degrades, and degraded fluid changes the friction behaviour the whole control strategy is calibrated around. Dark or burnt-smelling fluid is a finding, and on several transmissions it is the entire fault.
Beyond fluid, the population splits into three: solenoids and the valve body, which control which clutches apply; the speed and range sensors, which tell the module what is happening; and the wiring through the case connector, which is a known leak path — transmission fluid wicks up the harness and into the module, so a soaked connector is worth inspecting before a control unit is condemned.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Lamp with the gearbox locked in one ratio, engine revving freely | A fail-safe — the module has selected a safe ratio to protect the transmission | Read transmission codes, not engine ones. Note which gear it holds, which narrows the solenoid or circuit involved. | Diagnose the named fault. Fitting engine sensors for this is a common and expensive misdirection. |
| Fluid dark or smelling burnt | Degraded fluid — the friction behaviour the control strategy assumes has changed | Inspect against a fresh sample. Check the vehicle's actual service position rather than accepting 'sealed for life' at face value. | Service to the manufacturer's exact specification. The wrong fluid causes faults rather than curing them. |
| Harsh or delayed changes alongside the lamp | Solenoid or valve body — pressure is not being applied as commanded | Read codes and compare commanded against actual gear and line pressure live. A specific gear failing repeatedly names the circuit. | Renew the solenoid or service the valve body. Re-learn the adaptation afterwards where the vehicle requires it. |
| Fluid visible at the case connector or the harness, module faults stored | Fluid wicking up the harness into the connector or the module — a known path | Inspect the case connector and the harness above it. Fluid inside a connector is unmistakable once looked at. | Renew the connector seal and the affected harness section. Replacing the module without this repeats the failure. |
| Lamp with implausible speed readings or no drive at all | Input or output speed sensors — the module cannot see what the transmission is doing | Compare input and output speed live against road speed. A sensor reading zero while the car moves is the finding. | Renew the sensor or repair its wiring. Check the reluctor where the vehicle uses one. |
| Lamp came on with several other unrelated warnings | Low system voltage rather than a transmission fault — the module misbehaves like any other | Test battery and charging before diagnosing the transmission. Clear and re-scan after repair. | Repair the supply first. Only what returns after clearing is a genuine transmission fault. |
-
Lamp with the gearbox locked in one ratio, engine revving freely
- Likely cause
- A fail-safe — the module has selected a safe ratio to protect the transmission
- How to confirm it
- Read transmission codes, not engine ones. Note which gear it holds, which narrows the solenoid or circuit involved.
- What fixes it
- Diagnose the named fault. Fitting engine sensors for this is a common and expensive misdirection.
-
Fluid dark or smelling burnt
- Likely cause
- Degraded fluid — the friction behaviour the control strategy assumes has changed
- How to confirm it
- Inspect against a fresh sample. Check the vehicle's actual service position rather than accepting 'sealed for life' at face value.
- What fixes it
- Service to the manufacturer's exact specification. The wrong fluid causes faults rather than curing them.
-
Harsh or delayed changes alongside the lamp
- Likely cause
- Solenoid or valve body — pressure is not being applied as commanded
- How to confirm it
- Read codes and compare commanded against actual gear and line pressure live. A specific gear failing repeatedly names the circuit.
- What fixes it
- Renew the solenoid or service the valve body. Re-learn the adaptation afterwards where the vehicle requires it.
-
Fluid visible at the case connector or the harness, module faults stored
- Likely cause
- Fluid wicking up the harness into the connector or the module — a known path
- How to confirm it
- Inspect the case connector and the harness above it. Fluid inside a connector is unmistakable once looked at.
- What fixes it
- Renew the connector seal and the affected harness section. Replacing the module without this repeats the failure.
-
Lamp with implausible speed readings or no drive at all
- Likely cause
- Input or output speed sensors — the module cannot see what the transmission is doing
- How to confirm it
- Compare input and output speed live against road speed. A sensor reading zero while the car moves is the finding.
- What fixes it
- Renew the sensor or repair its wiring. Check the reluctor where the vehicle uses one.
-
Lamp came on with several other unrelated warnings
- Likely cause
- Low system voltage rather than a transmission fault — the module misbehaves like any other
- How to confirm it
- Test battery and charging before diagnosing the transmission. Clear and re-scan after repair.
- What fixes it
- Repair the supply first. Only what returns after clearing is a genuine transmission fault.
Codes that produce this symptom (5)
Listed with the codes that most typically present this way first — the likeliest place to start.
- P0700 Transmission Control System (Malfunction Indicator Lamp Request) Typical of this code Moderate
- P0800 Transfer Case Control System (Malfunction Indicator Lamp Request) Typical of this code Moderate
- P0802 Transmission Control System Malfunction Indicator Lamp Request Circuit/Open Typical of this code Moderate
- P0898 Transmission Control System Malfunction Indicator Lamp Request Circuit Low Typical of this code Moderate
- P0899 Transmission Control System Malfunction Indicator Lamp Request Circuit High Typical of this code Moderate
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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