OBD-II trouble code
P2216: NOx Sensor Circuit High (Bank 2)
The bank 2 nitrogen oxide reading is above its plausible ceiling. The awkward part is that this sensor cannot tell nitrogen oxide from the ammonia used to destroy it, so a high reading at the downstream position can mean too little treatment or too much of it — opposite faults with the same number.
Quick facts
- System
- Powertrain
- Category
- Auxiliary Emissions Controls
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $120 – $3,200
- DIY difficulty
- Intermediate DIY
Browse every code in P2200–P229F, or start from the full code library.
What does P2216 mean?
This code carries a piece of chemistry that changes how the number should be read, and it is the single most useful thing to know about the downstream nitrogen oxide sensor.
The measuring element is not perfectly selective. It responds to nitrogen oxide, and it also responds to ammonia — and ammonia is exactly what the reductant system injects to destroy nitrogen oxide in the first place. Under normal operation almost all of it is consumed inside the catalyst, so the sensor after the catalyst sees very little of either. When dosing runs richer than the catalyst can use, or when the catalyst is cold or saturated and cannot store what it is being given, unreacted ammonia passes through. The sensor reports it, honestly, as a number it cannot distinguish from nitrogen oxide.
So a high reading at this position has two opposite explanations. Either the system is not removing enough nitrogen oxide, or it is putting in more reductant than it can use. Both produce a number above the ceiling, and treating the second as though it were the first — replacing catalysts and chasing exhaust gas recirculation faults on a vehicle that is actually over-dosing — is the expensive mistake this code invites.
The pattern that separates them is behavioural rather than numerical. Over-dosing tends to show itself shortly after the catalyst reaches temperature, or during and just after a regeneration event, and often comes with a sharp acrid smell at the tailpipe and reductant consumption noticeably above the expected rate. Under-conversion tends to follow load: the reading climbs when the engine is working hard and settles when it is not, and reductant consumption is normal or low. Logging the value against catalyst temperature and dosing command for a single drive will usually make the distinction obvious.
There is a second discriminator available on this code that costs nothing and that the bank 1 page cannot offer, and it depends on which sensor holds which address. Where the two sensors sit on either side of the catalyst, comparing them locates the fault immediately. If the upstream reading is normal and only this one is high, the engine is producing the nitrogen oxide it should and the aftertreatment is not dealing with it — the problem is downstream of the engine. If both are high together, the engine is genuinely making more than it should, and the search belongs upstream with exhaust gas recirculation flow, injection timing and combustion temperature. Reading one sensor in isolation discards half the information the vehicle is giving you.
The electrical possibility should not be dismissed, though it is the minority case on a device of this design. The controller receives a calculated value over a communication link rather than a voltage, so a high verdict is a value above the plausible ceiling, not a wire pulled up to battery voltage. Sensors whose internal control unit has failed do report fixed implausible values, and a reading that sits at a constant maximum regardless of engine speed, load or temperature has the signature of a device reporting a default rather than a measurement. A number that moves with conditions is being measured; a number that never moves is not.
One inspection belongs here and nowhere else in the family: look for an exhaust leak between the catalyst and this sensor. A leak in that short section lets the gas sample be drawn from the wrong place, and unlike a leak upstream — which dilutes and reads low — it can present as a reading that behaves erratically at the high end.
The vehicle drives normally. The consequence is emissions and, on diesels, the usual escalation if the fault is left standing.
Common causes
- Reductant over-dosing letting unreacted ammonia pass the catalyst and register as nitrogen oxide
- Selective catalytic reduction catalyst degraded, cracked or converting poorly
- Catalyst below light-off temperature on short journeys, so nothing is being converted
- Exhaust gas recirculation valve stuck closed or passages blocked, raising real nitrogen oxide output
- Injection timing advanced by a remap or a fuelling fault, increasing combustion temperature
- Engine running hot or lean for an unrelated reason
- Failed sensor internal control unit reporting a fixed maximum value
- Exhaust leak between the catalyst and the sensor disturbing the sample
- Reductant injector leaking or dosing continuously
- Aged or contaminated element reading high across the board
- Incorrect replacement sensor with a different calibration fitted
Symptoms
- Check engine light with an emissions or reductant system message
- Sharp, acrid smell at the tailpipe, particularly after the engine is hot
- Reductant consumption well above or well below the expected rate
- Reading that climbs steadily with engine load, or one that never moves at all
- Catalyst efficiency or reductant codes stored alongside
- Reduced power inducement on diesels if unresolved
- Visible exhaust haze under hard acceleration
- Emissions monitor that will not complete
- Failed emissions test with no drivability complaint
Diagnostic steps
- 1.Compare this sensor against the one on the other side of the catalyst before anything else. Only this one high means the aftertreatment is not converting; both high means the engine is genuinely producing more nitrogen oxide and the search belongs upstream.
- 2.Log the value against catalyst temperature and the dosing command over a single drive. A rise that follows dosing and catalyst warm-up suggests ammonia passing through; a rise that follows engine load suggests genuine under-conversion.
- 3.Check reductant consumption against the expected rate for the vehicle, and ask the owner how often the tank needs topping up. Consumption well above normal supports over-dosing.
- 4.Smell the tailpipe after the engine has reached full operating temperature. A sharp acrid note is a useful, if unscientific, pointer toward unreacted reductant.
- 5.Determine whether the reading responds to conditions at all. A value locked at maximum regardless of speed, load or temperature is a default being reported, not a measurement being taken.
- 6.Inspect the section of exhaust between the catalyst and the sensor for leaks at joints, clamps and welds.
- 7.Verify exhaust gas recirculation flow and check the valve and passages for carbon, since suppressed recirculation raises real nitrogen oxide output.
- 8.Check the reductant injector for leaking or continuous dosing, and confirm it shuts off when commanded to.
- 9.Review injection timing and look for evidence of remapping or a fuelling fault raising combustion temperature.
- 10.Inspect the catalyst for physical damage, rattle or thermal distress before condemning it, and confirm its temperature actually reaches the light-off range on this vehicle's typical journeys.
- 11.Replace the bank 2 sensor only once the dosing rate, catalyst behaviour and engine-out figures have been accounted for, then re-verify across a full drive cycle.
Repair cost
$120 – $3,200
Diagnosis is $120 to $300 on this code because the useful work is data logging rather than component testing, and it is money well spent given what sits at the top of the range. A reductant injector is $250 to $900 fitted. Exhaust gas recirculation valve replacement is $300 to $900. An exhaust leak repair between catalyst and sensor is $120 to $450. A nitrogen oxide sensor is $250 to $900 for the part plus $100 to $350 labour. A selective catalytic reduction catalyst is $1,500 to $3,200 fitted, and no one should reach that figure without having first ruled out over-dosing.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with nox sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.