Why a code read is not a diagnosis

Most AdBlue jobs that arrive with us have already been “diagnosed” somewhere. A code was pulled, a part was named, and the fault came back. That is not bad luck and it is not usually incompetence. It is what happens when a stored code is treated as an answer rather than as a starting point.

An SCR fault code describes a symptom the ECU observed: not enough NOx conversion, pressure out of range, a circuit that did not respond. Several different components can produce the same observation. A catalyst efficiency code, for example, is logged when the downstream sensor sees more NOx than it should — which happens if the catalyst has degraded, but equally if the injector is half blocked, if the pump cannot build pressure, or if the fluid in the tank is not what it claims to be. Four causes, one code.

The only way to separate them is to watch the system operate. Below are the five readings that do that, in the order we take them. If you want the code definitions themselves, our AdBlue fault code master list covers what each one means; this guide is about what to do once you have one.

Reading 1: pressure during a pump prime

The first thing we do is command the pump to prime and watch the pressure trace while it runs. Three things can happen, and each points somewhere different.

If pressure rises to target promptly and holds when the pump stops, the supply side is healthy. Whatever is wrong is downstream of the pump, and the injector becomes the prime suspect.

If pressure rises slowly, or never quite reaches target, the pump is struggling. That is wear, a partially restricted line, or a filter that has been in service far too long. Short-journey vans suffer most here, because the system rarely gets a long hot cycle.

If pressure builds normally and then bleeds away the moment the pump stops, there is a leak — either externally at a union, or internally past the injector seat. That distinction matters, because one is a cheap fitting and the other is a new injector. We cover the overlap between these two failure patterns in more detail in our comparison of a pressure sensor fault against a genuine pump fault, which is the single most common misread on this system.

Reading 2: commanded dose versus actual dose

With the engine at operating temperature, the ECU calculates how much AdBlue the exhaust needs and commands the injector to deliver it. Most systems expose both the commanded figure and a measured or inferred actual figure. Watching the two together is the most informative thirty seconds of the whole job.

When commanded and actual track each other closely, dosing is working and the fault lies with measurement — a sensor — rather than delivery. When actual consistently falls short of commanded, delivery is restricted, which in practice means a crystallised injector far more often than anything else. When the ECU commands nothing at all despite the engine being hot and loaded, the problem is upstream of dosing entirely: a disabled system, a fluid quality flag, or a stored fault that has already put the SCR system into a fallback state.

That third case is worth dwelling on, because it is the one most often misread as “the pump is dead”. The pump is fine. The ECU has simply decided not to use it.

Reading 3: the two NOx sensors, compared

A Euro 6 SCR system has a sensor before the catalyst and a sensor after it. The before figure tells the ECU how much NOx the engine is producing; the after figure tells it how much survived. The gap between them is the conversion efficiency, and it is the number the whole system is judged on.

Read as a pair, they are diagnostic. If the upstream reading looks plausible for the load and the downstream reading is far too close to it, conversion genuinely is not happening — and reading 2 will already have told you whether that is a dosing problem or a catalyst problem. If the two sensors report near-identical figures at idle and under load, one of them is almost certainly reporting nonsense rather than the catalyst being perfectly transparent. And if a sensor reads a flat, unchanging value regardless of what the engine is doing, it has failed outright.

The trap here is that NOx sensors are expensive consumables that live in exhaust heat, so they do fail — often enough that fitting one is a tempting guess. Confirm it first. Our guide to the symptoms of a failing NOx sensor lists the behaviours that genuinely point at the sensor rather than at the dosing side.

Reading 4: exhaust temperature at the injector

SCR only works when the exhaust is hot enough for the injected urea to convert to ammonia. Below that threshold the ECU will not dose at all, and a system that is not dosing looks, to a hurried diagnosis, exactly like a system that cannot dose.

This is why a van that only ever does short urban runs can throw efficiency faults with no failed component anywhere in it. The exhaust never gets hot enough for long enough, the catalyst never reaches its working window, and the ECU eventually logs a complaint about a system that was never given the conditions to work. We check the temperature trace before condemning anything, and on a genuinely short-cycle vehicle the honest answer is sometimes a change in how the van is used rather than a part.

Temperature also explains where crystallisation forms. Dried urea deposits build up where fluid meets heat, which is at the injector tip, and that is why the injector is the component most often found partially blocked. The mechanism is covered in our article on how AdBlue crystallisation damages injectors and pumps.

Reading 5: heater circuit current draw

AdBlue freezes at low temperatures, so the tank, the supply line and often the pump carry heating elements. A failed heater or a corroded connector produces a fault that appears on cold mornings and disappears once the engine bay warms up — the classic intermittent that survives three visits to a garage.

Current draw when the heater is commanded on is the giveaway. No draw means an open circuit: a failed element, a broken feed, or a connector that has corroded in road salt. A draw well below expectation usually means resistance in the connector rather than a failed element, which is a cheaper repair if it is found rather than assumed. Any AdBlue fault with an obviously seasonal pattern should have this checked before anything is replaced.

The check that comes before all five

Before any of the above is worth doing, the fluid itself has to be ruled out. AdBlue is a specified product, not a generic liquid: it is a 32.5% aqueous urea solution whose quality characteristics for SCR use are set out in ISO 22241-1, the international standard defining the quality requirements for the NOx reduction agent AUS 32. Fluid outside that specification will not convert NOx correctly no matter how healthy every component is.

Just as importantly, fluid that left the factory in specification can be spoiled afterwards. A companion part of the same standard, ISO 22241-3, deals specifically with handling, transportation and storage, which tells you how easy it is to ruin. In the workshop the culprits are always mundane: a funnel that has held diesel or screenwash, a drum left open in a van for a year, or a container stored in full sun.

Putting it in order: a worked sequence

The five readings are not a menu; the order matters, because each one narrows what the next has to explain.

Worked through in that order, most AdBlue faults resolve to a single component within an hour, and — just as valuable — the components that are working get eliminated rather than replaced on suspicion. That is the difference between a repair that holds and one that comes back in six weeks with a different part number on the invoice.

It is also why we quote for diagnosis separately from repair. Being told which part has failed, with the live data to back it up, has value on its own, whoever ends up fitting the part. If you want the practical side of how that works at the roadside or in a yard, our page on mobile AdBlue diagnostics explains what we bring and what we can complete on site.