Section 1042.110 - Recording reductant use and other diagnostic functions(a) Engines equipped with SCR systems using a reductant other than the engine's fuel must meet the following requirements:(1) The diagnostic system must monitor reductant supply and alert operators to the need to restore the reductant supply, or to replace the reductant if it does not meet your concentration specifications. Unless we approve other alerts, use a warning lamp and an audible alarm. You do not need to separately monitor reductant quality if your system uses input from an exhaust NOX sensor (or other sensor) to alert operators when reductant quality is inadequate. However, tank level or DEF flow must be monitored in all cases.(2) The onboard computer log must record in nonvolatile computer memory all incidents of engine operation with inadequate reductant injection or reductant quality. Use good engineering judgment to ensure that the operator can readily access the information to submit the report required by § 1042.660 . For example, you may meet this requirement by documenting the incident in a text file that can be downloaded or printed by the operator.(3) SCR systems must also conform to the provisions of paragraph (d) of this section if they are equipped with on-off controls as allowed under § 1042.115(g) .(c) You may equip your engine with other diagnostic features. If you do, they must be designed to allow us to read and interpret the codes. Note that §§ 1042.115 and 1042.205 require that you provide us any information needed to read, record, and interpret all the information broadcast by an engine's onboard computers and electronic control units.(d) For Category 3 engines equipped with on-off NOX controls (as allowed by § 1042.115(g) ), you must also equip your engine to continuously monitor NOX concentrations in the exhaust. See § 1042.650 to determine if this requirement applies for a given Category 1 or Category 2 engine. For measurement technologies involving discrete sampling events, measurements are considered continuous if they repeat at least once every 60 seconds; we may approve a longer sampling period if it is necessary or appropriate for sufficiently accurate measurements. Describe your system for onboard NOX measurements in your application for certification. Use good engineering judgment to alert operators if measured NOX concentrations indicate malfunctioning emission controls. Record any such operation in nonvolatile computer memory. You are not required to monitor NOX concentrations during operation for which the emission controls may be disabled under § 1042.115(g) . For the purpose of this paragraph (d), "malfunctioning emission controls" means any condition in which the measured NOX concentration exceeds the highest value expected when the engine is in compliance with the installed engine standard of § 1042.104(g) . Use good engineering judgment to determine these expected values during production-line testing of the engine using linear interpolation between test points and accounting for the degree to which the cycle-weighted emissions of the engine are below the standard. You may also use additional intermediate test points measured during the production-line test. Note that the provisions of paragraph (a) of this section also apply for SCR systems covered by this paragraph (d). For engines subject to both the provisions of paragraph (a) of this section and this paragraph (d), use good engineering judgment to integrate diagnostic features to comply with both paragraphs. For example, engines may use on-off NOX controls to disable certain emission control functions only if the diagnostic system indicates that the monitoring described in this paragraph (d) is active. 73 FR 37243 , June 30, 2008, as amended at 75 FR 22998 , Apr. 30, 2010; 81 FR 74145 , Oct. 25, 2016; 88 FR 4660 , Jan. 24, 2023 81 FR 74145 , 12/27/2016; 88 FR 4660 , 3/27/2023