P0173
Fuel Trim Malfunction (Bank 2)
Encountering the engine check light code P0173 signifies an explicit mechanical or electrical operational breakdown categorized as "Fuel Trim Malfunction (Bank 2)". Professional scanner tools usually flag this subsystem loop early on.
Driver's Summary
Code P0173 means your vehicle detected a problem with the fuel trim malfunction (bank 2) system. Typical symptoms include decreased fuel economy, check engine light, rough idle. This condition is classified as high severity. Prompt diagnosis is essential to prevent cascading damage to related components.
Symptoms
Decreased fuel economy, check engine light, rough idle
Common Causes
- Vacuum leak specific to Bank 2 intake manifold
- Faulty fuel injectors on Bank 2
- Failed Bank 2 O2 sensor
- Exhaust leak before the O2 sensor
How to Fix
- 1 Inspect Bank 2 intake gaskets
- 2 Clean or replace Bank 2 fuel injectors
- 3 Replace upstream O2 sensor
- 4 Repair exhaust manifold leak
Technical Explanation
Code P0173 is confirmed when the ECM's diagnostic algorithm detects a parameter deviation that persists across a defined number of consecutive drive cycles. The diagnostic runs during closed-loop operation only, ensuring the engine is at full operating temperature and the PCM's fuel trim feedback loop is active before confirming any out-of-range condition. The fault remains stored in memory even after the MIL is cleared; it becomes a confirmed DTC after failing two consecutive drive cycles, and the PCM logs a freeze frame record of the engine's exact operating state at the moment of detection.
Is It Safe to Drive?
This fault carries real mechanical risk. The root causes — including vacuum leak specific to bank 2 intake manifold — can trigger a chain reaction of component failures if the vehicle continues to be driven. Have it towed or drive directly to a shop without delay.
Mechanic's Pro Tip
The most common mistake with P0173 is replacing the sensor without verifying the reference voltage and ground integrity first. Use a scan tool to monitor the sensor's live output; a truly failed sensor shows a stuck, flatlined reading — a sensor that fluctuates but reads slightly off usually indicates a wiring or vacuum issue, not a dead sensor. Always spray electrical contact cleaner on the connector pins before condemning the sensor.
Intake gaskets: 300 - 600; Injectors: 200 - 500