Understanding the BMW P1473 Fault Code
The OBD2 diagnostic trouble code P1473 is a manufacturer-specific code for BMW vehicles, indicating a fault in the Secondary Air Injection (SAI) System Pump Relay Control Circuit. This system, often called the Secondary Air Pump (SAP) system, is a critical component of your BMW’s emissions control strategy during a cold start. Its primary function is to reduce hydrocarbon (HC) and carbon monoxide (CO) emissions by injecting fresh air into the exhaust manifold immediately after starting the engine. This extra oxygen helps the catalytic converter “light off” and reach its optimal operating temperature much faster. The P1473 code specifically points to an electrical problem with the circuit that controls the relay for the SAP pump, preventing the system from activating.
Role of the Secondary Air Injection System
During the first 30-90 seconds after a cold start, engine emissions are at their highest because the catalytic converter is not yet warm enough to be efficient. The SAP system addresses this by using an electric air pump to force ambient air into the exhaust ports or manifold. This injected air mixes with the hot exhaust gases, promoting further combustion of unburned fuel, which rapidly heats up the catalytic converter. A failure signaled by P1473 means this process is disabled, leading to increased emissions and potentially causing the vehicle to fail an emissions test.
The SAP Pump Relay: The Heart of the Circuit
The Secondary Air Pump is a high-current device. The relay acts as an electrically operated switch, allowing the low-current signal from the BMW’s Engine Control Module (DME or DDE) to safely turn the high-current pump on and off. Code P1473 is set when the DME detects an irregularity in this control circuit—such as an open, short to ground, or short to power—that doesn’t match its expected parameters. This is a circuit code, not necessarily a component failure code; the pump itself or the relay could be fine, but the wiring or connector is faulty.
Symptoms and Causes of P1473 in Your BMW
While sometimes the P1473 code may illuminate the Check Engine Light (CEL) or Service Engine Soon light with no noticeable drivability issues, there are common symptoms and root causes every BMW owner or technician should recognize.
Common Symptoms of a P1473 Fault
- Illuminated Check Engine Light (MIL): The most immediate and common indicator.
- Failed Emissions Inspection: Elevated HC and CO levels during the cold-start test cycle.
- Rough Idle on Cold Start (Possible): While not always present, some models may exhibit a slightly uneven idle if the emission correction is affected.
- Audible Lack of Pump Operation: On a cold start, a functioning SAP pump produces a distinct whirring or humming sound from the engine bay (typically near the firewall on the passenger side) for about a minute. Its absence can be a clue.
- Other Related Codes: Codes like P0410 (Secondary Air Injection System Malfunction) or P0411 (Incorrect Flow Detected) may appear alongside P1473.
Primary Causes of the P1473 Diagnostic Code
- Failed Secondary Air Pump Relay: The relay itself is a common point of failure. It can become internally stuck open or closed, or its contacts can corrode.
- Faulty Secondary Air Pump Motor: A seized or shorted pump motor can draw excessive current or create a short circuit, triggering the code.
- Damaged Wiring or Connectors: Corrosion, chafing, or rodent damage to the wiring harness between the DME, relay, and pump. Focus on connectors at the pump, relay, and DME.
- Blown Fuse: The SAP circuit is protected by a fuse (often 30A or 40A, located in the glove compartment or trunk electronics box). A blown fuse is a simple but critical check.
- Faulty Engine Control Module (DME): Rare, but a failure in the DME’s internal driver circuit for the relay can cause this code.
Step-by-Step Diagnostic and Repair Guide for P1473
Diagnosing P1473 requires a methodical approach, starting with the simplest checks before moving to more complex components. You will need a basic multimeter and possibly a BMW-specific diagnostic scanner for live data.
Step 1: Preliminary Checks and Visual Inspection
Begin by locating the Secondary Air Pump (usually on the passenger side firewall) and its relay (commonly in the main power distribution box in the engine bay or the trunk). Perform a thorough visual inspection.
- Check the relevant fuse. Replace if blown, but investigate why it blew (e.g., a shorted pump).
- Inspect all wiring connectors for the pump, relay, and associated grounds for corrosion, bent pins, or moisture.
- Listen for the pump on a cold start. No sound points to a power or control issue.
Step 2: Testing the Secondary Air Pump and Relay
Testing the Pump: Disconnect the electrical connector from the pump. Using a multimeter, check the resistance between the pump’s terminals. You should read a low resistance (typically 0.5 – 3.0 ohms). A reading of infinite resistance (OL) indicates an open circuit in the pump motor. A reading near zero ohms may indicate an internal short. You can also apply direct 12V power (with appropriate fusing) to the pump terminals; it should run smoothly.
Testing the Relay: Locate the relay and identify its control (coil) pins (usually 85 & 86) and switched power pins (30 & 87). Test it by applying 12V across the control pins; you should hear/feel a distinct click, and there should be continuity between pins 30 and 87. No click or no continuity means a bad relay.
Step 3: Circuit and Voltage Diagnostics
With the battery disconnected, use the multimeter in continuity mode to check for opens or shorts in the wiring between components:
- Check for continuity on the power feed from the fuse to the relay (pin 30).
- Check for continuity on the power output from the relay (pin 87) to the pump.
- Check the control wire from the DME to the relay coil (pin 86). It should not be shorted to ground or power.
- Verify the ground path for both the relay coil (pin 85) and the pump.
Reconnect the battery and, with the ignition on (engine off), use the multimeter in DC voltage mode to check for commanded activation. Have an assistant start the cold engine; the control pin (86) at the relay should show battery voltage (approx. 12V) for the duration of the SAP operating cycle.
Step 4: Final Repairs and Code Clearing
Based on your findings, proceed with the necessary repair:
- Replace the Fuse: If blown, replace with the correct amperage rating.
- Replace the Relay: Use an OEM or high-quality equivalent.
- Replace the Secondary Air Pump: A common repair on higher-mileage BMWs. Ensure the new pump includes a new non-return valve if integrated.
- Repair Wiring: Solder and seal any damaged wires. Clean corroded connectors with electrical contact cleaner.
After repairs, clear the P1473 code with your OBD2 scanner. Perform a cold-start test drive cycle to ensure the code does not return and that the SAP system operates audibly. Monitor readiness monitors to confirm the emission system is fully functional.
Conclusion: Ensuring Your BMW’s Emissions Compliance
While a P1473 code may not immediately cripple your BMW’s performance, it disables a vital emissions system designed for environmental protection. Ignoring it will lead to a failed emissions test and contributes to higher pollutant output. By understanding the system’s function—the pump, relay, and control circuit—you can effectively diagnose this fault. Starting with simple fuse and visual checks before moving to electrical tests on the pump, relay, and wiring is the most efficient path to a repair. Addressing a P1473 code promptly restores your vehicle’s emission integrity and ensures it runs as engineered.