Scanner vs scope diagnostics
Newer vehicles, such as 1995+ in North America and 2001+ in Europe, come equipped with OBD-II systems. Inside the powertrain control module (PCM) there are two computer ”rooms” – the OEM VIN-specific room and the OBD-II room. Each has a separate processor with its own specification window for each PID, the minimum/maximum for each parameter identification (PID). One room may have a wider or narrower window than the other room.
Have you ever had a car come in with a check engine light on but no diagnostic trouble code (DTC)? This could be the result of having different specs in the OBD-II and OEM rooms. The OBD-ll room is where something went out of spec, set a code and turned on the light. But that PID did not go out of spec in the OEM room because the OEM specification window is wider. Checking both rooms is vital to ensure you don’t waste time, replace the wrong part or have a comeback on your hands.
How much time could be wasted if you don’t check both rooms? Could you replace the wrong part, face a comeback or worse, give up and send the vehicle elsewhere without getting paid?
This is why checking both rooms should be a standard part of your workflow. If it’s not, you’re setting yourself up for problems. Say you find a DTC in the OBD-II room, fix the issue and return the vehicle. But, if there’s a separate issue stored in the OEM room, the light will come back on and the customer will assume you didn’t fix it right the first time. The only worse thing than a comeback is a NEVER comeback .
We’ve talked about the two rooms and specification windows, so now let’s talk about “doors.”
Think of the PCM as having a front and two back doors. The back doors send processed data to the scan tool, one back door to the OEM room, accessed through the scanner icon, and one from the OBD-II room via the OBD direct icon. I call it the back door because raw data comes in the front door, gets processed and exits through the back door, giving us direction on where the problem might be. The scanner is like a compass – it points you in the right direction, even though clean (good) data needs to be verified at the component level.
The front door is where the raw data enters. You’ve heard the term “garbage in – garbage out,” right? If the input is bad, the output is bad. That’s why it’s crucial to catch garbage coming through the front door – misleading raw data can result in incorrect diagnoses. What we need is a lie detector at the front door. That’s where the lab scope or meter, also known as the lie detector, comes in. A scanner is wonderful, pointing you in the right direction. Even with good data, we’d need to pinpoint the component or circuit.
It could be said that the fundamental difference between scanners and scopes comes down to refresh rate. By the time scanner data is processed and displayed, it’s already in the past. It’s history. Scanner refresh rates depend on the PCM baud rates, which are much slower than the lab scope. A high-quality scope will refresh at 1.5 million samples per second. That speed means a scope can catch what a scanner may completely miss, especially with intermittent problems.
One of the biggest time-saving features of a Snap-on scope is showing voltage over time. You can set how long it takes voltage to move across the screen. When diagnosing intermittent wiring issues, connect the scope, move along the circuit and monitor the voltage. No change? Keep moving. See a drop? You’ve found the issue.
If you’re not familiar with using a scope, Snap-on guided component tests can help. They walk you through how to hook up, where to hook up and what a good waveform looks like. There are over 7 million VIN-specific tests available, with images to help locate components.
My wife and I enjoy crime shows, and in 60 years of watching, I’ve noticed something – when the suspect is arrested in their home, one police officer covers the front door and their partner covers the back door. If they don’t, the suspect would escape. This is the same with the PCM. If you don’t cover both doors, the problem could escape you.
Wireless connectivity on modern scan tools help make this even easier. The wireless scan module can stay in the vehicle, retrieving back door data while you’re under the hood, with a meter or a scope checking front door data for lies.
Some big news: on 2010 and newer vehicles, Snap-on scan tools check both rooms simultaneously during the pre-scan. That’s a huge advantage. We’ve also added permanent codes (mode 10) to the pre-scan. Ever seen a vehicle where someone cleared the codes before you got to it? Permanent codes can’t be cleared by a scan tool. They remain for 400 days or until the issue doesn’t return for two drive cycles. What a great feature in OBD II, that prevents misdiagnosis.
There are differences between a scanner and a scope, but they are great partners when it comes to catching the bad guys.
