The NOx sensors are how your engine computer measures whether the SCR catalyst is working. When their readings drift or a sensor fails, the system can set the P20EE code even if the catalyst is fine. This guide explains how the sensors work and how to test one before you replace it.

Upstream and downstream NOx sensors

Most SCR systems use two NOx sensors: one upstream of the catalyst and one downstream. The computer compares the two. If the catalyst is doing its job, the downstream sensor should read much lower NOx than the upstream sensor. When that difference is smaller than expected, the system reports “efficiency below threshold” and stores P20EE.

Symptoms of a failing NOx sensor

• Check engine light with P20EE, P229F, or P2200 stored.
• Reduced power or a limp mode on some vehicles as the emissions strategy protects itself.
• Slightly worse fuel economy and higher DEF consumption.

Testing before you replace

Use a scan tool that shows live data. Watch both NOx sensor values at operating temperature. A sensor that stays flat, reads implausible numbers, or fails its self-test is suspect. Check the connector and wiring for corrosion or chafing, and look for an exhaust leak near the sensor, because a leak lets in outside air and skews the reading. Replacing a good sensor will not clear a code caused by a leak or bad DEF.

Replacement and cost

NOx sensors are threaded into the exhaust and unplug at a connector. Budget roughly $300 to $650 per sensor including labor, and use an OEM-quality part, since cheap sensors often fail early. After replacement, clear the codes and drive a full monitor cycle. For the wider diagnostic picture, start with our main P20EE code guide and rule out DEF quality first.