EN

Severity: Moderate Code type: Generic (all makes)

P229F Code: NOx Sensor Circuit Range/Performance Bank 1 Sensor 2

The NOx sensor after the SCR catalyst is sending believable-looking but wrong or sluggish readings.

At a glance

Severity: 3/5 – Moderate3/5

Severity: Moderate

Meaning
NOx Sensor Circuit Range/Performance Bank 1 Sensor 2
System
Emission Controls · Powertrain
Safe to drive?
With caution
Typical repair cost
$80–$900
DIY difficulty
Moderate

NOx sensor 2 sits downstream of the SCR catalyst, usually well back under the truck, and tells the ECM how much NOx is left after DEF dosing. Like the upstream sensor it has its own small control module on the harness that heats the ceramic element to several hundred degrees and sends a digital NOx and oxygen value over a CAN link. P229F is not a broken wire: the ECM has decided the values themselves don't make sense, for example sensor 2 not matching sensor 1 while dosing is off, an oxygen reading that doesn't reach fresh-air level during a fuel cut, or a signal that reacts too slowly. One quirk makes this sensor tricky: it can't tell ammonia from NOx, so DEF overdosing and ammonia slip make it read high even when the sensor is fine. Because the SCR efficiency check depends entirely on this sensor, P20EE and a derate countdown often follow.

Can I keep driving? The engine runs normally, but most trucks start the emissions countdown to a speed limit once the SCR monitor can't run. Plan the repair before the displayed distance runs out, and avoid long idling, which makes sensor diagnosis harder.

How to read P229F

How to read P229FP122239F4
  1. P Powertrain
  2. 2 Generic (SAE)
  3. 2 Fuel & air (injector circuit)
  4. 9F Specific fault

Symptoms

  • Check engine light with an emissions or DEF system warning
  • Countdown to reduced speed shown on the dash
  • P20EE (SCR efficiency) stored at the same time or shortly after
  • No change in how the engine runs
  • Possibly an ammonia smell at the tailpipe if DEF is being overdosed

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. NOx sensor 2 aged or contaminated, reading drifted or responding slowly 35%
  2. Exhaust leak between the SCR catalyst and sensor 2 diluting the gas with outside air 15%
  3. Ammonia slip from DEF overdosing or a leaking dosing injector, read as NOx 15%
  4. Water or road salt in the sensor module connector or damaged harness near the rear of the exhaust 12%
  5. Sensor 1 actually at fault, so the cross-check wrongly blames sensor 2 10%
  6. Degraded SCR catalyst causing values the ECM finds implausible 8%
  7. ECM software needing an update 5%

Diagnosis step by step

Diagnostic flow: P229F1Check freeze frame and related codes2Cross-check sensors with dosing off3Run the fuel-cut oxygen test4Look for leaks and harness damage atthe rear5Rule out ammonia slip6Replace and run the SCR self-test

  1. Check freeze frame and related codes

    Look for heater or circuit codes for sensor 2 (such as P2209), DEF dosing codes and P20EE. An electrical code alongside means fix the wiring or module power first; P229F alone means the readings are the problem.

    Tools: OBD-II scanner

  2. Cross-check sensors with dosing off

    With the engine fully warm and DEF dosing inhibited or not yet active, NOx sensor 1 and sensor 2 should read within a few ppm of each other. If sensor 2 reads clearly higher or lower while sensor 1 tracks engine load sensibly, sensor 2 is the prime suspect.

    Tools: OBD-II scanner

  3. Run the fuel-cut oxygen test

    Coast down from about 50 mph in gear so the injectors shut off. The oxygen value from sensor 2 should rise to roughly 20–21 %; a sensor that stalls at a much lower value or climbs very slowly is worn.

    Tools: OBD-II scanner

  4. Look for leaks and harness damage at the rear

    Inspect every clamp and joint between the SCR catalyst and the sensor, and check the sensor module mounting, connector seals and harness for corrosion or chafing. Rear-mounted modules take a lot of road spray.

    Tools: Flashlight

  5. Rule out ammonia slip

    If sensor 2 reads high mainly while dosing is active, check the DEF injector for dripping and compare actual DEF consumption with normal. Overdosing sends unused ammonia to sensor 2, which the sensor reports as NOx.

    Tools: OBD-II scanner, Flashlight

  6. Replace and run the SCR self-test

    If the sensor fails the tests, replace sensor and module as the assembly they come as, then clear codes and run the OEM SCR or NOx sensor verification test. Many trucks also need a steady highway drive before the countdown resets.

    Tools: OBD-II scanner

Where the parts are

Charging system and control modules – P229F1234567
Charging system and control modules
  1. 1Battery
  2. 2Alternator
  3. 3Engine computer (ECM)
  4. 4CAN bus
  5. 5Transmission
  6. 6Wheel speed sensor
  7. 7High-voltage battery

Repair options & costs

RepairDIY?Estimated cost
Replace NOx sensor 2 with module Yes $400–$900
Repair exhaust leak or clamp after SCR catalyst Yes $80–$300
Repair sensor module connector or harness No $100–$350
Replace leaking DEF dosing injector No $350–$900
ECM software update No $100–$250

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Ram

NOx sensors on the 6.7L Cummins are known as a frequent failure item, and the downstream sensor is affected as well as the upstream one. Confirm with the sensor cross-check before ordering, as the parts are expensive.

Emissions / smog test impact

Depends

Where diesels are OBD-tested, P229F with the check engine light on fails, and the SCR monitor will usually show not ready. Many states exempt diesels; either way, NOx sensor 'emulators' are illegal tampering devices.

Frequently asked questions

Which sensor is sensor 2?

It's the NOx sensor after the SCR catalyst, closest to the tailpipe. Sensor 1 is in front of the DEF injection point, usually near the turbo or DPF.

Why does sensor 2 read high with a good catalyst?

NOx sensors also respond to ammonia. If too much DEF is injected, the unused ammonia reaches sensor 2 and shows up as NOx, so an overdosing injector can look like a bad sensor or a bad catalyst.

Can I swap sensor 1 and sensor 2 to test?

Physically they often look alike, but they are usually coded differently and the ECM expects a specific module address. Swapping can create new faults; use the live data cross-check instead.

Sources