Check engine light and Clean Truck Check: will you pass?

Short answer: a check engine light that is on because of an emissions fault is a common reason trucks fail the Clean Truck Check OBD test. Readiness monitors that are not set are another. Fix the fault, drive the truck normally so the monitors complete, and do not clear codes right before the test.

How the CARB OBD test reads your truck

The CARB-verified OBD test applies to 2013 and newer diesel engines and 2018 and newer alternative-fuel engines. Instead of measuring smoke, the tester connects to the truck's diagnostic port and reads what the engine computer reports about its emissions systems.

Two things in that data cause most problems:

• The malfunction indicator lamp (MIL), better known as the check engine light
• Readiness monitors, the self-checks the truck runs on its emissions systems

CARB sets the exact pass and fail criteria. This post explains the ideas so you know what to watch for. For the official rules, check CARB's Clean Truck Check page.

Will a check engine light fail Clean Truck Check?

A check engine light on because of an emissions fault is one of the most common reasons a truck fails. When the computer commands the MIL on, it is telling the tester that an emissions system has a problem it has confirmed.

Common sources on heavy-duty diesels include NOx sensors, the DEF and SCR system, EGR faults and DPF faults. On a semi truck, a check engine light before an emissions test is your cue to get it diagnosed now, not the day before.

What to do if the light is on

1. Have a qualified repair shop read the codes and find the cause.
2. Fix the cause, not just the light.
3. Drive the truck normally afterward so the monitors complete.
4. Then book the test.

What readiness monitors are

Readiness monitors are the truck's own tests of its emissions systems. Each monitor runs when the right driving conditions happen and then reports as ready (complete) or not ready (incomplete).

When readiness monitors are not ready, the truck has not yet finished checking itself. That is a common reason for an OBD failure even when nothing is broken. You may see it described as an "OBD not ready" result.

How many monitors can be not ready?

CARB decides how many readiness monitors may be incomplete, and the answer can depend on the vehicle. We do not give a number here because it is CARB's rule to set. Check CARB, or ask us before the test and we will explain what applies to your truck.

How to set readiness monitors on a diesel truck

Monitors set themselves when the truck is driven through the conditions each one needs, often called a drive cycle. The exact diesel truck drive cycle varies by engine maker and model.

• Drive the truck normally for a while after any repair, including highway and steady-speed driving where possible.
• Do not keep restarting a short idle-and-stop routine and expect monitors to set.
• Ask your dealer or repair shop for the drive cycle for your engine. Freightliner Cascadia, Peterbilt and other platforms each follow the engine maker's procedure.
• Make sure no faults return. If the light comes back on, the repair is not finished.

Should you clear codes before the test?

No. Clearing codes turns the light off but does not fix anything. It also resets readiness monitors to not ready, so a truck with cleared codes is often not ready for the OBD test. Disconnecting the battery on a semi truck can have the same effect.

If a shop cleared codes as part of a repair, give the truck time and miles to set its monitors before you test.

Pending and permanent codes

The OBD system stores different kinds of codes:

• Pending codes point to a fault the computer has seen but not yet confirmed.
• Permanent codes stay in memory even after codes are cleared, until the truck's own monitoring confirms the repair worked.

How CARB treats each type in the Clean Truck Check pass criteria is set by CARB, so check CARB for your vehicle. Practically, a pending code is an early warning to get looked at, and a permanent code means the system still needs to verify a repair through normal driving.

Readiness and the check engine light for fleets

For a fleet, one truck with a light on can throw off a whole test day. Ask drivers to report check engine lights as soon as they appear, and get those trucks to the shop between test windows. Keep a note by VIN of any recent repairs or cleared codes, so you know which trucks may need more driving before they are ready.

The same ideas apply to 2018 and newer alternative-fuel engines, which also take the OBD test.

A pre-test checklist

• No check engine light on
• No known emissions faults (DEF, SCR, NOx, EGR, DPF)
• No codes cleared or battery disconnected recently
• Normal driving since the last repair
• Diagnostic port and connector in good shape
• Correct VIN on record in CTC-VIS

If the truck fails anyway

You will know exactly what to fix. See what to do after a failed Clean Truck Check. For older non-OBD diesels, the test is different; see how to pass the CARB smoke test.

How Diesel Dynamix helps

We run the CARB-verified OBD test on site at your yard, jobsite, terminal, port or home across Southern California. We explain results in plain English, including which monitors were not ready or which fault caused a failure, so your shop knows where to start. We do not do repairs; we test and explain, then retest when you are ready.

• CARB OBD testing
• How testing works
• Fleet testing

Book a test, or call, text or WhatsApp (858) 342-8328.