What vehicle health data can OBD-II give a fleet manager?

Which vehicles have OBD-II?

OBD-II is the emissions diagnostic system in every car and light truck sold in the US from model year 1996. Its 16-pin port is under the dashboard, usually near the steering column. It exists to find emissions faults, not to run a fleet, so what it reports is set by emissions law. The detailed rule is California's 13 CCR § 1968.2, which EPA adopts, in its 2013 version, for 2017–2026 vehicles in 40 CFR § 86.1806-17, and in its 2022 version from model year 2027 in 40 CFR § 86.1806-27.

That rule covers passenger cars, light-duty trucks and medium-duty vehicles, which reach up to 14,000 lb (6,350 kg) gross vehicle weight rating (GVWR). Most small-fleet cars, SUVs, pickups and vans fall inside it. Heavier trucks are covered by a separate rule (see below).

What can OBD-II tell a fleet manager?

Trouble codes and the check engine light

Our guide to pending, stored and permanent codes explains each type. For a fleet, two items stand out (our view). Pending codes let you book a repair before a breakdown. Distance with the light on shows whether a warning was reported promptly.

Readiness

Readiness monitors report whether each emissions self-test has run since codes were last cleared. That matters before an emissions inspection; see why a car says "not ready".

Live engine data

The regulation's data-stream list (section (g)(4.2)) requires these on demand on every vehicle:

• calculated load, coolant temperature, engine speed, vehicle speed and throttle position (if equipped);
• the number of stored codes and MIL status.

Where equipped, it also requires fuel system status, fuel trims, intake air temperature, manifold pressure, mass air flow and oxygen sensor readings. On 2005 and newer vehicles using the CAN protocol, the list adds engine control module system voltage, time since engine start and barometric pressure. Fuel level is on the list only where it's used to enable other diagnostics.

Newer additions. For 30% of 2019, 60% of 2020 and 100% of 2021 and newer vehicles certified to the California rule (section (g)(4.2.2)(A)(ii)), the port must also report:

• odometer reading, which scan tools commonly request as mode 01 PID A6 (the PID number comes from the SAE J1979 standard, not the regulation; we didn't check the SAE document itself);
• engine fuel rate and vehicle fuel rate;
• engine torque values.

So a true odometer through the port is required only on some 2019–2020 and all 2021 and newer vehicles certified to California's rule. Vehicles certified only to EPA are different:

• 2017–2026 model years: EPA's rule adopts the California text as approved on July 31, 2013 (40 CFR § 86.1806-17). The odometer and fuel-rate requirement came later: CARB staff first proposed it in 2015 (CARB staff report), and it took effect in 2016. So EPA does not require it for these years, although a maker may choose to meet a later California version, and many vehicles are certified to both.
• 2027 and newer: EPA's rule adopts the California text operative November 30, 2022, which includes it (40 CFR § 86.1806-27). Makers may delay this for vehicles over 6,000 lb (2,722 kg) GVWR not yet subject to all the Tier 4 standards, and small-volume makers until model year 2030.

Our reading: for an EPA-only 2019–2026 vehicle, check whether it reports an odometer before relying on it. On older vehicles, and any that don't report it, a tool has to estimate distance from speed or use the driver's entries (our note). Where it's reported, fuel rate lets idling be measured rather than estimated (our inference); see how much idling costs a fleet.

VIN

The rule requires every 2005 and newer vehicle to report its vehicle identification number (VIN) through the port, from one control unit. That lets a tool confirm which vehicle it's plugged into, and check recalls by VIN.

Battery voltage at the port

The port also carries battery power. A common ELM327-based adapter can report the voltage it receives. The ELM327 datasheet describes the AT RV command, which reads the chip's input voltage with an uncalibrated accuracy of "typically about 2%." At 12.6 V, 2% is about ±0.25 V, roughly the gap between a full and a three-quarters-charged flooded battery (our arithmetic), so a single reading can't grade a battery. It's more useful as a trend read the same way each time, such as resting voltage before the morning start (our inference; clone adapters may differ from the genuine chip). It isn't a battery load test; see when to replace fleet batteries.

What can't OBD-II tell you?

Tire tread and pressure, brake pad wear, oil level and oil life, location, heavy-duty J1939 data and most maker-specific data, as the table shows. These come from what's missing from the required data list, plus our own notes:

A change coming to newer vehicles. The California rule lets manufacturers use a newer diagnostic standard, SAE J1979-2, on 2023–2026 vehicles, and requires it from model year 2027. A tool or dongle built only for the older SAE J1979 may not read those vehicles fully (our inference). Check support before you buy hardware for new vehicles.

How should a small fleet use OBD-II data?

Our suggestions:

1. Check codes on a routine, such as weekly, and act on pending codes before the light comes on.
2. Log the VIN with each reading, so data can't get attached to the wrong vehicle.
3. Watch trends, not single readings: resting voltage, a pending code that keeps returning, fuel trims drifting over weeks.
4. Keep the human checks. OBD-II doesn't replace tire, brake and light inspections.

Safety. Drivers shouldn't look at a phone app while driving. If a code comes with a flashing check engine light, overheating, a brake or oil-pressure warning, or a smell of fuel, stop where it's safe. See can I keep driving with the check engine light on? A plug-in device also draws a little power while the vehicle sits (our note); see parasitic battery drain.

Does fleet vehicle data raise driver privacy issues?

Yes, once it's linked to a driver. Engine data looks impersonal, but it becomes personal once it's linked to who was driving and when: speed, trip times and idle time. Location from a tracker makes it more so.

• Canada. In PIPEDA Case Summary #2006-351, the Office of the Privacy Commissioner found that GPS data from company vehicles is personal information, because the drivers are identifiable. It accepted purposes such as dispatch, safety and locating stolen vehicles. It warned that a technology's purposes "should be precisely specified" and the technology "restricted to its intended purposes." It also said employees should be told before the system is used.
• Ontario. Employers with "25 or more employees on January 1 of any year" must have a written policy on electronic monitoring. The province gives GPS tracking of delivery vehicles as an example of monitoring the policy must address.
• Elsewhere. US states and other provinces have their own rules. We checked only the two above; this isn't legal advice.

Good practice, whatever the law (our suggestions):

• tell drivers what you collect and why;
• collect only what you use;
• decide who can see it and how long you keep it;
• don't quietly turn a maintenance tool into a performance tracker.