P225D: NOx Sensor Performance - Signal Biased/Stuck Low (Bank 1 Sensor 1)

P225D is the opposite case to P225C: the upstream NOx sensor of bank 1 reads too little NOx or refuses to rise when the engine should be producing it. Volvo and Mack list P225D among NOx codes where the sensor is very unlikely to be at fault, because the probe is simply measuring fresh air; their guide says not to replace it. FCA uses P225D on the 3.0 EcoDiesel, from 2014 Ram 1500 and Grand Cherokee flash bulletins to 2020 Ram 1500 updates, and its 2021 case file for Wrangler, Gladiator and Ram 1500 reports mostly fault-free returned sensors; it names charge air or EGR leaks, fuel contamination that soots up the sensor ports and skewed MAF or coolant inputs. A low-reading sensor hides real NOx, so SCR control and the catalyst monitor lose accuracy.

Medium severity. Can I keep driving?You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

MIL on
Code persists after a new NOx sensor
Possible related intake, EGR or MAF codes

Most likely causes

Charge air, intake or exhaust leak that lets the sensor see diluted gas
Sensor ports clogged with soot, for example after fuel contamination
Low-pressure EGR valve partly stuck open
Outdated engine control software that a published flash corrects

What to check, in order

Start with the simplest. You can tick off each step.

Read the codes and the NOx data before and after the SCR catalyst
With the connector unplugged, check whether the signal wire is making contact with ground.
Check the sensor's connector and wiring
Look for exhaust leaks ahead of the sensor
Confirm that the AdBlue/DEF is of good quality
Replace the sensor only after ruling out power, ground, and leaks

Good to know

Repair difficulty: hard
It affects emissions.

What each character of P225D means

P: Powertrain: engine and transmission
2: Generic code: same on every make
2: Fuel injection
5D: Specific fault in that system

Frequently asked questions

Can I keep driving with code P225D?

You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.

What causes code P225D?

The most common causes are: charge air, intake or exhaust leak that lets the sensor see diluted gas; sensor ports clogged with soot, for example after fuel contamination; low-pressure EGR valve partly stuck open.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.