The Smart Roadster throttle actuator P2042 fault code indicates the electronic throttle body actuator motor circuit is out of range or has failed. The most common cause is a worn or seized actuator motor inside the throttle body assembly. The fix is either cleaning and freeing the actuator or replacing the throttle body unit entirely.
The throttle body actuator is one of those components that most Smart Roadster owners never think about — until it fails. When it does, the car may refuse to rev cleanly, drop into limp mode, or cut power so aggressively that driving becomes genuinely hazardous. Given that the 698cc Mitsubishi-derived engine relies on precise throttle control to manage boost and fuelling across a narrow rev band, a faulty throttle actuator causes problems that ripple through almost every aspect of engine behaviour. This article explains exactly what the actuator does, why fault code P2042 appears, how to test the component yourself, and what your realistic repair options are.
What the Smart Roadster Throttle Actuator Actually Does
The Smart Roadster uses an electronic throttle control (ETC) system, often called drive-by-wire. There is no mechanical cable connecting the accelerator pedal to the throttle plate. Instead, the accelerator pedal position sensor sends a voltage signal to the Bosch MEG 1.1 ECU, which then commands the throttle body actuator — a small DC electric motor with a gear reduction drive — to open or close the butterfly valve to the precise angle required.
This system allows the ECU to modulate throttle opening independently of pedal input, which is essential for traction control, idle speed regulation, and importantly, turbo boost management. The throttle body sits on the intake side of the engine, and the ECU monitors two position sensors (TPS1 and TPS2, which operate in opposition) to confirm the butterfly is where it is commanded to be. If the two sensor readings disagree, or if the actuator motor draws too much current or too little, the ECU raises a fault and typically forces the engine into a restricted power mode.
Understanding this interaction is crucial: the smart roadster throttle actuator P2042 code is not always caused by the actuator motor alone. The position sensors, wiring harness, and even ECU supply voltage all play a role.
Smart Roadster Throttle Actuator P2042: What the Code Means
P2042 in the Smart Roadster context translates as Throttle Actuator Control Motor Circuit Range/Performance. The Bosch MEG 1.1 ECU monitors the H-bridge driver circuit that controls the bidirectional DC motor. P2042 is triggered when:
- The motor current draw is outside the expected window (suggesting mechanical seizure or a short)
- The actual throttle position does not match the commanded position within a set time
- The two TPS signals diverge beyond their acceptable tolerance
- Supply voltage to the throttle body module falls below threshold during actuation
You may also see companion codes alongside P2042, most commonly P2101 (Throttle Actuator Control Motor Circuit Range/Performance — a close variant), P2111 (Throttle Actuator Control System — Stuck Open), or P2112 (Stuck Closed). The presence of P2111 or P2112 alongside P2042 is a strong indicator of a mechanically seized actuator rather than an electrical fault. A lone P2042 with no companion codes is more likely to be a wiring or sensor issue.
Note that if your scan tool is also showing MAP sensor-related codes at the same time, the two faults may share a common cause such as a failing supply or ground circuit. Our detailed guide on diagnosing the MAP sensor and its associated wiring is worth reading before you assume the throttle body is at fault in isolation.
Common Causes of Throttle Actuator Failure on the Smart Roadster
Carbon Build-Up on the Butterfly Valve
The Smart Roadster’s engine routes blow-by gases through the intake. Over time, oily carbon deposits coat the inside of the throttle body and the edges of the butterfly plate. A light deposit increases friction on the spindle bearings, making the actuator motor work harder. A heavy deposit can physically obstruct the plate from closing fully — which is exactly the scenario that generates a P2112 (stuck closed) or P2111 (stuck open) companion code.
Actuator Motor Brush Wear
The DC motor inside the throttle body uses carbon brushes that wear over time. On a car that is now up to 20 years old, brush wear is increasingly common. The symptom is an intermittent P2042 that comes and goes, often clearing itself after a key cycle, before becoming permanent.
Wiring Harness Deterioration
The throttle body wiring loom on the 452 runs close to the engine and is subject to heat cycling. Cracked insulation, corroded pin terminals in the throttle body connector, and high-resistance joints are all capable of generating a P2042 without any mechanical fault in the throttle body itself. Always inspect the connector before condemning the unit.
Low Battery or Charging System Fault
The H-bridge motor driver in the throttle body requires adequate supply voltage. A weak battery or failing alternator can pull the supply voltage low during cranking or at idle, causing the ECU to lose control of the actuator and log P2042. If your car is also showing charging-related symptoms, review our guide on recognising and replacing a failing Smart Roadster alternator before spending money on a throttle body.
How to Test the Smart Roadster Throttle Actuator
Step 1 — Visual and Connector Inspection
Disconnect the throttle body connector (a 6-pin Bosch Econoseal connector on the standard 60kW car). Inspect every pin for corrosion, pushed-back terminals, or cracked pin boots. Clean with electrical contact cleaner. Manually rotate the throttle butterfly — it should move smoothly with light spring resistance and return to rest cleanly. Any grinding, roughness, or sticking is a red flag.
Step 2 — Resistance Test of the Motor
With the connector removed, measure resistance across the motor terminals (pins vary by connector, check your wiring diagram). A healthy motor typically reads 1–5 ohms. An open circuit reading (OL) confirms a broken brush or broken winding. A reading below 0.5 ohms suggests a short circuit within the motor.
Step 3 — TPS Sensor Voltage Check
With ignition on and the connector plugged in, use a back-probe to check the two TPS signal wires simultaneously. TPS1 should read approximately 0.5V at idle closed position and rise towards 4.5V at full open. TPS2 should do the inverse — reading approximately 4.5V at closed and falling towards 0.5V at full open. If both sensors read the same direction, there is a wiring fault. If one is erratic, the internal sensor is failing.
Step 4 — Live Data with a Diagnostic Tool
A diagnostic tool capable of reading Smart Roadster engine live data (WIS/DAS, Autophix SM800, or iCarsoft MB V3 all work) will display desired versus actual throttle position in real time. Command a throttle sweep via the actuator test function if available. A healthy actuator tracks the commanded value within 1–2%. A faulty one will lag, overshoot, or fail to respond entirely.
Smart Roadster Throttle Actuator P2042: Your Repair Options
Option A — Cleaning (Carbon Build-Up Only)
If the fault is solely caused by carbon deposits, a thorough clean with throttle body cleaner spray, a soft brush, and careful manual cycling of the plate will often cure the fault without any parts cost. Do not spray cleaner directly onto the TPS sensors — apply it to a cloth and wipe. After cleaning, the ECU will need to relearn the throttle closed position; on the Bosch MEG 1.1 this happens automatically within a few key cycles, but you can accelerate it by performing a key-on/key-off cycle three times without starting the engine.
Option B — Remanufactured or Second-Hand Throttle Body
A replacement throttle body for the Smart Roadster 452 is the most definitive fix for a mechanically failed actuator motor. Remanufactured units are available from specialist suppliers and are far preferable to unknown-mileage used units from breakers. When fitting a new or remanufactured unit, the ECU throttle adaptation must be reset — this is done via diagnostic software by clearing the throttle position learned values, then performing the key-cycle relearn procedure described above. Skipping this step will result in poor idle quality and possible immediate recurrence of fault codes.
Option C — Wiring Repair
If testing reveals the fault is in the harness rather than the throttle body itself, repair or replacement of the relevant section of loom is the correct fix. This is worth doing carefully: a botched wiring repair that introduces resistance into the TPS signal circuit will cause intermittent faults for years.
It is also worth noting that the throttle actuator does not operate in isolation from the rest of the engine management system. Fault codes in other subsystems — for example, the secondary air injection circuit — can occasionally mask or accompany throttle-related faults, particularly after the ECU has entered a protected mode. Always clear all codes after a repair and check for re-emergence before declaring the job complete.
Limp Mode, Safety Implications, and the Smart Roadster
When the Bosch MEG 1.1 detects a P2042 fault, it activates a throttle-control fail-safe strategy. In mild cases, the ECU simply limits throttle opening to a fixed percentage — typically around 20–25% — which translates to roughly 30–35 hp available. This is enough to move the car at low speed but makes motorway driving impossible and can be dangerous if the fault occurs during an overtake.
In severe cases, particularly when companion codes P2111 or P2112 are present, the ECU may command idle-only operation. The engine will not respond to accelerator input at all. This is a genuine safety event if it occurs on a moving car.
The Smart Roadster’s mid-rear engine layout means the throttle body is accessible from the engine service hatch, but working space is tight. A common mistake is to disturb the intake pressure sensor or its hose during throttle body removal — our article on the intake manifold pressure sensor fault explains how to diagnose a fault code that can appear if that sensor is disturbed.
Because the Smart Roadster uses an automated manual gearbox (Softouch) rather than a true automatic, the throttle position signal also influences gear shift timing and torque management in the gearbox controller. A limp-mode throttle signal can cause the Softouch to behave erratically during gear changes, which is sometimes misdiagnosed as a gearbox fault in its own right. If you have simultaneous throttle and gearbox symptoms, investigate the throttle actuator first.
Parts, Costs, and What to Expect
A remanufactured throttle body for the Smart Roadster 452 typically costs between £80 and £180 depending on supplier and warranty terms. Original Smart/Mitsubishi units (the Roadster shares its throttle body with certain Mitsubishi Colt and Smart City-Coupé applications) are increasingly scarce, but the aftermarket supply is reasonable. Allow approximately 1.5 hours of labour if the job is being done professionally, though a competent DIY mechanic familiar with the car can do it in under an hour once the engine cover is removed.
Diagnostic time should not be skipped. A throttle body replacement on a car with a wiring fault will not cure anything, and the new unit may be damaged if there is an underlying short circuit. Test first, replace second.
For completeness: the 45kW Lite variant uses the same throttle body assembly, but given the Lite’s lack of oil cooler and its general unsuitability for hard use, a thorough inspection of the whole engine bay is advisable on any Lite presenting with P2042 — the fault may be symptomatic of a wider maintenance deficit.
The Smart Roadster throttle actuator P2042 code is ultimately a manageable fault if approached methodically. Identify whether the cause is mechanical (carbon, motor wear), electrical (wiring, connectors), or systemic (voltage supply) before buying any parts. Most owners who work through the diagnostic steps above resolve the fault with either a cleaning session or a sub-£150 throttle body replacement, and the car returns to full performance immediately thereafter.
Frequently Asked Questions
Can I drive my Smart Roadster with a P2042 fault code active?
In limp mode the car is driveable at low speed but not safe at motorway speeds or during overtaking manoeuvres. Power is typically limited to around 20–25% throttle opening. Do not ignore the fault: if the throttle plate sticks closed at speed, the car will abruptly lose drive. Address it promptly.
Will clearing the P2042 code fix the problem?
Clearing the code removes it from memory but does not fix the underlying fault. On a worn or dirty throttle body the code will return within minutes of driving. Clearing is useful as a diagnostic step — to check if the fault is intermittent — but is not a repair. You must identify and correct the root cause.
Is the Smart Roadster throttle body the same as on other cars?
Yes. The 452 Roadster shares its throttle body assembly with certain Mitsubishi Colt applications (the 698cc engine is Mitsubishi-derived) and the Smart City-Coupé 450 with the same engine variant. This means aftermarket supply is reasonable, though always verify the connector pinout matches before purchasing a used unit from a non-Roadster donor car.
Does the Bosch MEG 1.1 ECU need reprogramming after a throttle body replacement?
No reprogramming is required, but the throttle position adaptation values must be reset and relearned. This is done by clearing the adaptation data via diagnostic software and then performing the key-cycle relearn procedure. Skipping this step causes rough idle and may re-trigger fault codes within the first few drive cycles after replacement.
