P239E: NOx sensor oxygen reading too high during diesel deceleration

P239E is cataloged as an oxygen-sensor out-of-range code during deceleration. Mazda's SKYACTIV-D application uses the oxygen-concentration information supplied by its NOx sensor.

Identify the measured quantity and the device

The PCM compares detected oxygen concentration with a calculated value during fuel cut. The fault condition is a reading above that calculation for two continuous seconds. This is not a direct NOx-concentration limit, and the generic bank/sensor wording should not be used to select a conventional oxygen sensor without checking Mazda's component identification.

The published prerequisites exclude active DENOx/DESOx operation and require DPF regeneration to have been inactive for two minutes. They also require normal NOx signal receipt and very low EGR command. These conditions establish a controlled exhaust state for the comparison.

What can corrupt the comparison

Mazda lists sensor deterioration, heating malfunction, clogging and connector trouble, plus PCM-related faults. It considers P220A and U029D before the NOx sensor connection and component inspection. The documented monitor uses two drive cycles and inhibits DENOx/DESOx after detection.

The confirmation includes professional aftertreatment preparation and a controlled deceleration event. It should not be improvised on public roads or initiated without establishing that regeneration is appropriate. The useful repair conclusion identifies why the NOx sensor's oxygen channel disagreed under fuel-cut conditions, rather than replacing a part solely from the generic title.

Documented application

Mazda3 workshop manual 01/2019 onward, d9n1-1a-22d_ver10; PCM (SKYACTIV-D). Verify exact vehicle specification; this is a Mazda application example. Mazda factory diagnostic page, mirrored by CHARM.