Diagnose and fix a P2096 Subaru trouble code
A P2096 Subaru trouble code is defined as P2096 POST CATALYST FUEL TRIM SYSTEM TOO LEAN (BANK 1). This code refers to the oxygen sensor located AFTER the catalytic converter for the side of the engine that houses cylinder #1. Here’s how the system works:
The pre-cat air/fuel ratio sensor detects the amount of oxygen in the exhaust stream and reports that to the ECM to adjust fuel trims. Fuel trims can be either long term or short term. Fuel trim is exactly what it appears to be; the computer is either adding or subtracting fuel to “trim up” the initial commanded air/fuel ratio.
What is an air/fuel ratio sensor?
Most people call an air/fuel ratio sensor an oxygen sensor because it measures the amount of oxygen in the exhaust stream. But an A/F sensor works differently than a typical oxygen sensor, where the sensor works like a battery to generate voltage based on the amount of oxygen it sees. An A/F sensor, on the other hand is provided a voltage and the amount of oxygen forces that voltage higher or lower than a baseline reading. What does that have to do with the POST catalytic converter trouble code? Read on grasshopper.
What symptoms does a P2096 Subaru code cause?
Since the code is telling about a lean condition, you can expect to see a rough engine that has low power. It may stall and have poor acceleration. It may even stall at lower engine RPMs. You’ll also see a decrease in gas mileage.
What does a post catalytic converter sensor do?
It’s important to think of the catalytic converter as an incinerator. Its job is to burn off excess fuel, oil, or coolant (oil or coolant that leaks into the exhaust). The pre-cat oxygen sensor is constantly shifting between rich and lean as the ECM tries to find the perfect A/F ratio for every driver input. When the A/F mixture is lean, excess oxygen is in the exhaust (the fuel burned but left extra oxygen) and the catalytic converter stores that oxygen to use later. When the mixture is too rich, the exhaust contains excess fuel and the catalytic converter pulls the oxygen out of storage to help burn off that extra fuel.
If the catalytic converter is doing its job, the post cat sensor shouldn’t see wide variations between rich and lean; ie, it should provide a steady reading between rich and lean. In this case, the post cat sensor is seeing a constantly lean condition AFTER the exhaust has gone through the cat converter.
What causes a P0296 Subaru code?
• A fault in the PRE-cat A/F ration sensor or the wiring harness. Check the sensor voltage on the pre-cat A/F sensor to see if it’s showing a low voltage. If so, add propane to the intake and see if it rises. If it does, suspect a bad A/F sensor that has a rich bias. In other words, it thinks a normal exhaust is rich, so the computer keeps lowering fuel trims, which actually causes a LEAN mixture. That lean mixture sends way too much oxygen into the cat converter. With lots of oxygen and no fuel to burn, the post cat sensor will always see a lean exhaust and the ECM will set the P2096 Subaru code.
• A vacuum leak that’s introducing UNMETERED air into the engine. When the computer sees a continual lean condition from the pre-cat A/F sensor, it adds fuel to compensate.
So your first job is to scan the computer to see if it’s adding fuel. Normal fuel trims should stay around ± 10% at idle and cruise. If your readings are above 10%, suspect a vacuum leak or an exhaust leak that’s letting in too much air.
Next, reset the fuel trims and drive the vehicle with the A/F sensor UNPLUGGED. The ECM will run the engine off factory default values. If the engine performance gets BETTER, then the problem isn’t related to a vacuum or exhaust leak and is limited to the either the A/F ratio sensor itself or the wiring harness going to the A/F sensor.
What fixes the P2096 Subaru code in most cases?
It’s usually a bad pre-cat A/F fuel ratio sensor. If you don’t
want to do any diagnostics and are willing to gamble $150 or $200 (depending on whether you install Denso or genuine Subaru), install a new A/F ratio sensor. Clear the trouble code and clear the fuel trims. Then drive the vehicle. If the P2096 Subaru code returns, check the wiring harness and check for a vacuum and exhaust leak.
©, 2017 Rick Muscoplat
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