The Smart Roadster secondary air injection system is one of the least understood emission control components on the 452, yet it generates some of the most persistent fault codes owners encounter — particularly P205E. If your engine warning light is on and a scan reveals a secondary air injection fault, it is tempting to clear the code and hope for the best. That rarely works. This guide explains exactly what the secondary air system does on the 698cc three-cylinder engine, why it fails, how to diagnose it properly, and what your repair options actually are — including whether bypassing or deleting the system is viable on a car that is primarily used on road.
What Is the Secondary Air Injection System on the Smart Roadster?
Secondary air injection (SAI) is an emissions control technology designed to reduce hydrocarbon and carbon monoxide output during cold start, before the catalytic converter reaches its operating temperature. The Bosch MEG 1.1 ECU commands a secondary air pump to push fresh air into the exhaust manifold immediately after the engine fires from cold. This extra oxygen accelerates the oxidation of unburnt hydrocarbons in the exhaust stream, heating the catalyst faster and dramatically cutting cold-start emissions.
On the Smart Roadster’s mid-rear mounted 698cc unit, the SAI system comprises three main components: an electric air pump (usually located in the engine bay near the battery), a solenoid-controlled switching valve that opens the air path on ECU command, and a one-way check valve that prevents hot exhaust gases from flowing back into the pump and destroying it. All three must function correctly for the system to pass its self-test. The ECU monitors pump current draw and downstream oxygen sensor response to verify the system is working — which is precisely how faults like P205E are detected.
For broader context on the engine fault codes generated by the MEG 1.1 ECU, the full P2xxx engine error code reference for the Smart Roadster covers the complete family of engine management faults you may encounter alongside SAI codes.
Understanding P205E and Related Secondary Air Fault Codes
P205E is the most commonly reported secondary air injection code on the Smart Roadster 452. It translates as Secondary Air Injection System — Switching Valve Stuck Open (Bank 1). Because the 698cc three-cylinder is a single-bank engine, Bank 1 is the only bank, so this code effectively means the switching valve is not closing when commanded to, or the ECU cannot confirm it has closed based on sensor feedback.
Full Code List
- P205E — Secondary Air Injection Switching Valve Stuck Open (Bank 1): the most frequent; often caused by a failed or seized solenoid valve.
- P0410 — Secondary Air Injection System Malfunction: a general SAI failure flag, often set alongside P205E when the pump itself is at fault.
- P0411 — Secondary Air Injection System Incorrect Flow Detected: the pump runs but the oxygen sensor does not see the expected lean spike, pointing to a blocked or failed check valve.
- P0412 — Secondary Air Injection Switching Valve A Circuit Malfunction: an electrical fault in the solenoid valve circuit — check wiring, connector and relay before replacing the valve.
- P0418 — Secondary Air Injection Relay A Circuit: relay failure or open circuit to the air pump relay; confirm relay operation before condemning the pump.
It is common to see two or three of these codes stored simultaneously. Always address them in order: electrical faults first (P0412, P0418), then mechanical failures (P205E, P0410, P0411). Replacing parts at random without following this sequence wastes money.
Why Does the Smart Roadster Secondary Air System Fail?
The 452’s secondary air injection system has a finite service life and several predictable failure modes, most of which are age and mileage related rather than design defects.
Electric Air Pump Failure
The pump motor brushes wear over time. Symptoms include the pump running noisily, running intermittently, or not running at all. A failed pump typically triggers P0410 or P0418. You can test pump operation by back-probing the relay socket and applying 12V directly — if the pump does not spin, it needs replacement. Used pumps from breakers are plentiful and cheap; new aftermarket units are readily available.
Switching Valve Seizure
The solenoid switching valve is the most common culprit behind P205E specifically. The valve body corrodes internally, particularly in cars that have lived in wet climates or seen infrequent use. A seized valve may stick open (causing P205E) or stick closed (causing P0411 — no flow detected despite the pump running). Applying a solenoid ground signal with the ignition on should produce an audible click from the valve; silence confirms failure. Replacement valves are inexpensive and the job takes under an hour.
Check Valve Blockage or Failure
Carbon and moisture can block the one-way check valve, or the valve can fail mechanically and allow exhaust gas backflow. A blocked check valve will trigger P0411. With the system cold, remove the check valve and blow through it — it should pass air freely in one direction only. Any restriction or free flow in both directions means replacement is required.
Wiring and Relay Issues
Rodent damage, chafing against the engine casing, and corrosion at the multi-pin connector block are common on cars over fifteen years old. Always inspect the harness visually before replacing components. Check the SAI relay in the engine bay fuse box — swapping it with an identical relay of the same rating is a free diagnostic step.
Electrical faults in the engine bay are not unique to the SAI system. The MAP sensor fault and testing guide covers similar wiring inspection techniques that apply equally well when chasing SAI electrical codes.
Diagnosing Secondary Air Injection Faults Step by Step
A methodical approach saves time and money. Work through these steps before ordering parts.
- Scan and record all codes. Use an OBD-II scanner capable of reading manufacturer-specific codes, not just generic OBD-II PIDs. Note every stored and pending code, not just P205E.
- Check the relay. Locate the SAI relay in the engine compartment fuse box. Swap it with an identical spare and clear codes. If the fault does not return after several cold starts, the relay was the issue.
- Inspect the wiring harness. Trace the pump and solenoid valve wiring from the relay to each component. Look for chafing, melted insulation and corroded terminals. Repair any damage found.
- Test the pump directly. With the relay socket exposed, bridge the supply and earth terminals and listen for the pump. A healthy pump runs smoothly and quietly. A worn pump will groan or fail to start.
- Test the switching valve solenoid. Apply 12V earth to the solenoid connector (with ignition on to provide power on the supply pin). You should hear a distinct click. No click indicates a failed solenoid coil or stuck valve body.
- Inspect the check valve. Remove, clean and blow-test it as described above.
- Clear codes and run cold-start cycles. The ECU runs its SAI self-test only during cold starts below approximately 35°C coolant temperature. You must allow a genuine cold soak before retesting.
Repair Options: Fix, Delete or Bypass?
Once you have identified the failed component, repair is straightforward and inexpensive. A replacement switching valve costs under £30, a new pump under £60, and a check valve under £20. For a road-registered car subject to MOT emissions testing, repairing the system is always the correct choice. Secondary air injection faults will not in themselves cause an MOT failure on the emissions plate test — the catalyst does most of the heavy lifting once warm — but a stored fault code may illuminate the MIL (malfunction indicator lamp), which will fail a UK MOT from 2018 onwards.
SAI delete, where the pump and plumbing are removed and the ECU map is modified to disable the SAI routine, is technically possible on a remapped car. However, it is only appropriate for track-only or non-road-registered vehicles. If your car is road registered and driven on public roads, deletion creates a permanent emissions compliance issue and will fail future MOT inspections. Fix the system — the parts are cheap and the job is manageable for anyone with basic mechanical ability.
If your Roadster is already running a performance map and you are considering how the SAI system interacts with other engine management changes, it is worth understanding how boost is managed via the wastegate actuator, which operates on a separate pneumatic circuit but is monitored by the same ECU.
Does the SAI System Affect Performance?
In normal operation, the secondary air injection pump only runs for the first 30 to 120 seconds after a cold start. Once the engine and catalyst are up to temperature, the ECU cuts the pump and closes the switching valve. The system has no measurable effect on power output or fuel economy during normal driving. A failed SAI pump that runs continuously (rare, but possible with a shorted relay) can add a small parasitic electrical load, but this is not a performance concern on the 452’s 12V system.
Where SAI faults do matter is reliability signalling: a P205E stored alongside other engine management codes can mask more serious faults or make it harder to interpret live data cleanly. Keeping the system functional and fault-free allows the ECU to run clean diagnostics. This matters particularly if you are pushing the engine harder than standard — the oil cooling arrangement on the 60kW variant is another system that warrants understanding if you are running the engine at higher loads or considering a power upgrade.
For owners considering a remap alongside these repairs, our performance mapping packages are designed specifically for the Smart Roadster 452 and account for all emission system configurations.
Secondary Air Injection Fault Codes: Quick Reference
Use this table as a fast-reference guide when scanning your car.
- P205E — Switching valve stuck open: replace solenoid valve.
- P0410 — General SAI malfunction: check pump, relay and wiring first.
- P0411 — Incorrect flow: inspect and replace check valve; verify pump output.
- P0412 — Switching valve circuit fault: inspect wiring, connector and solenoid coil resistance (typically 20–30 ohms).
- P0418 — Relay circuit fault: replace relay; check fuse and supply voltage to relay socket.
Secondary air injection faults rarely indicate catastrophic engine damage, but they should not be ignored on a road-registered car. The SAM unit fault code guide is useful if you find B1xxx codes stored alongside your engine management faults, as SAM-related wiring issues can occasionally affect power supply to engine bay relays including the SAI relay circuit.
The Smart Roadster secondary air injection system is simple, inexpensive to repair and well worth maintaining on any road-going 452. P205E almost always points to a failed or seized switching valve — a £25 part and an hour’s work. Trace the fault methodically, fix the specific component that has failed, and keep the system intact if your car is MOT-registered. Clear the codes, run several genuine cold-start cycles to allow the ECU self-test to complete, and your engine management light should stay off. If multiple SAI codes return persistently after replacing the identified component, revisit the wiring harness — age-related corrosion in the engine compartment connectors is the hidden cause in the majority of stubborn cases.
