Truck Won't Communicate for the CARB OBD Test?

Short answer: if your truck won't communicate during the CARB OBD test, the tester cannot read the engine computer, so the test cannot be completed. The cause is usually a damaged OBD port, a wiring or power problem, a blown fuse, or a module fault. DEF, SCR and other emissions faults are a different issue: the truck talks, but stored faults or a check engine light can fail the test. In both cases the truck needs repair by a qualified heavy-duty repair shop, then a retest.

What does "no communication" mean on an OBD test?

The CARB OBD test for 2013 and newer diesel and 2018 and newer alternative-fuel engines works by plugging CARB-verified equipment into the truck's diagnostic port and reading the onboard computer. If the equipment cannot connect, there is no data to check and nothing to submit. See OBD testing for what the test normally reads.

No communication is not the same as a failed reading, but the result is similar: the truck is not compliant until it can be tested and pass.

Why won't my truck communicate with the OBD test?

Common causes include:

• Damaged or corroded OBD port. Bent pins, dirt or corrosion stop a clean connection.
• No power at the connector. A blown fuse or broken wire can leave the port dead.
• Wiring or network problems. Damaged harnesses, chafed wires or aftermarket add-ons can disrupt the data network.
• Engine computer faults. A failing or reprogrammed module may not respond.
• Low battery voltage. Weak batteries can cause dropped connections.
• Tampering. Modified or deleted emissions software can affect communication. Tampered systems fail and are illegal.

We cannot diagnose the root cause in a blog post. A qualified heavy-duty repair shop can trace it.

What should I check before the OBD test?

A few owner checks can save a trip:

1. Look at the port. Make sure it is clean, dry and not loose or cracked.
2. Check fuses for the diagnostic and engine circuits, using your owner's manual.
3. Check the batteries and connections.
4. Note any warning lights on the dash before the tester arrives.
5. Remove aftermarket devices plugged into the port, if they are not needed.

For connector location and brand quirks, see OBD test by truck brand.

Will a DEF system fault fail the emissions test?

It can. DEF (diesel exhaust fluid) and SCR (selective catalytic reduction) systems control NOx on modern diesels. When the engine computer detects a problem, it usually turns on the check engine light (MIL) and stores a fault code. An active MIL or emissions fault can cause the OBD test to fail.

Common DEF and SCR problems include:

• Bad or contaminated DEF
• Failed DEF pump, heater, injector or lines
• NOx sensor faults
• SCR catalyst efficiency faults

What other emissions faults cause OBD failures?

• DPF faults, such as a clogged filter or failed pressure sensor
• EGR faults, such as a stuck valve or cooler problem
• Readiness monitors not set, often after codes were cleared or a battery was disconnected
• Check engine light on for any emissions-related reason
• Tampered or deleted equipment, which fails and is illegal

Our readiness monitors and check engine light guide explains monitors, and DPF deletes and Clean Truck Check covers deleted trucks.

Should I clear the codes before the test?

Clearing codes does not fix the problem, and it resets readiness monitors, which can cause a fail on its own. Have the fault repaired, then drive the truck normally so the monitors can set before testing.

What to do if your truck has a communication or emissions fault

1. Write down what the tester saw. We tell you in plain English whether the truck would not connect, had a MIL on, had monitors not ready or had stored faults.
2. Take the truck to a qualified heavy-duty repair shop that works on diesel emissions systems. Share the test notes.
3. Get the repair done and confirmed. Ask the shop to verify the fault is gone and the port communicates.
4. Let the monitors set by driving normally after the repair.
5. Check your deadline in CTC-VIS and book the retest inside your window.

If the deadline is close or has passed, read missed Clean Truck Check deadline and failed Clean Truck Check: what to do.

Does Diesel Dynamix repair emissions problems?

No. We test and explain. We do not repair trucks, and we do not help with deletes. We recommend taking the truck to a qualified heavy-duty repair shop for diagnosis and repair, then calling us for the retest. Keeping testing and repair separate means we give you a straight reading of what the truck shows.

How Diesel Dynamix helps

We bring CARB-verified OBD testing and smoke opacity testing to your yard, jobsite, terminal, port or home across Southern California, including Los Angeles, Long Beach, the Inland Empire, Orange County and San Diego.

• Plain-English explanation of no-communication, MIL and fault results
• On-site retest once repairs are done, so the truck does not need another trip
• Next due date, reminders and help with CTC-VIS questions
• Per-truck pricing for fleets. See fleet testing

See how testing works, then book a test or retest or call or text (858) 342-8328.