3 Common Bad Downstream O2 Sensor Symptoms

There are affiliate links on this article. If you make a purchase through any of the links, I may earn a small commission at no extra cost to you.

So, what are the bad downstream O2 sensor symptoms that show the sensor is failing? Generally, a failing O2 sensor (whether upstream or downstream) would trigger the check engine light to illuminate on the dashboard.

But that’s not all, there are other symptoms that depict your car’s downstream O2 sensor is failing or already damaged, including drop in engine performance.

The downstream sensor detects the level of contamination in the emissions released from the CAT to the muffler, which would go down the tailpipe, and then to the atmosphere. Hence, downstream O2 sensors are quite important and should be replaced on time when they get damaged or start failing.

Bad Downstream O2 Sensor Symptoms

A car with a failing downstream oxygen sensor would typically exhibit the following symptoms. However, when you see these signs, it is important to run a check – troubleshoot your vehicle – before concluding that it could be a faulty downstream O2 sensor that’s causing the anomaly.

1. Your Vehicle Won’t Pass Emissions Tests

If you go for an emissions test and your car fails to pass the test, one of the reasons for that is a bad or failing oxygen sensor. Yes, O2 sensors are part of the emissions system in every modern car; thus, when they go bad, it is most likely that your car won’t pass emissions tests.

Notwithstanding, a bad catalytic converter, muffler, or exhaust manifold can also prevent your vehicle from passing an emissions test; hence, you need to troubleshoot your emissions system to be sure of the component that really caused the test failure.

2. Check Engine Light (CEL) Is On

While there are several issues that could cause the check engine light to illuminate, a bad or failing downstream sensor can also do that. How does an O2 sensor make the check engine light come on?

Simple, O2 sensors send information to the ECU; thus, when wrong signals are sent to the ECU, it’d read it as an error message, and therefore cause the CEL to illuminate on your dashboard.

3. Drop In Engine Performance

All components that make up a vehicle are interconnected; thus, when one of them fails, it directly, or indirectly affects other components, and as such, you may notice a drop in the performance of your engine. That said, a bad O2 sensor is likely to affect the ECU, which in turn affects the engine.

What More?

Mainly, the downstream oxygen sensors work differently, while the upstream sensors work differently. If the downstream sensors go bad, that would not affect the upstream sensors, but the ECU would act on the wrong signals sent by the failing or bad downstream sensors, and then adjust engine performance or display the CEL.

When you notice these downstream O2 sensor symptoms, firstly, it is advisable that you troubleshoot your vehicle to be sure that the symptoms were actually triggered by a bad downstream sensor.

Once that is done, you should endeavor to have the bad sensor(s) changed. It’s quite inexpensive to replace O2 sensors. Hence, some drivers wouldn’t mind changing the entire sensors at the same time.