DTC Codes

[Rich]

Hello all. I'm hoping I can get some clarification on the difference between "History" codes and "Current" codes displayed on the DIC. My understanding is that if the code is a history code designated with a "H" after the trouble code, then the problem causing the code to appear has cleared, or is no longer occurring. If the code is a current code designated with a "C" after the trouble code, then the issue causing the code to be set is still occurring and the problem needs to be addressed. Following this logic, if the codes that are appearing are history codes, they will clear from the DIC after a certain time period, or after a certain number of starts without the problem that set the code occurring.

Is this close to accurate? Can anyone elaborate?

[adallak]

A current code is one set during the current ignition cycle because the computer (ECM, BCM) noticed a problem. If during the next ignition cycle the computer does not see the problem, the current code becomes a history one and will stay that way during consequent 50 ignition cycles. After that it will disappear.

[Cadillac Jim]

In checking which codes are Current, run the codes after driving the car but before turning the key off. If you turn the key off before you check the codes, everything will go to History.

[Ranger]

No, that's not correct. Once a fault is detected, a current DTC is set. IF it is intermittent or does not fail again, the code will go to history after 3 ignition cycles with no fault detected. There are some DTC's that will only show current while they are failed. I have seen that once on a body code for the A/C high side temp sensor, but I think that is the exception.

[adallak]

No, that's not correct. Once a fault is detected, a current DTC is set. IF it is intermittent or does not fail again, the code will go to history after 3 ignition cycles with no fault detected. There are some DTC's that will only show current while they are failed. I have seen that once on a body code for the A/C high side temp sensor, but I think that is the exception.

Never heard of 3 ignition cycles. It does not make much sense to me and adds uncertainty.

[Cadillac Jim]

Ranger is right. It depends on the fault that the DTC detects and the implications of that fault. If the fault interferes with the closed-loop DFI control or other major driveability issues and clearing it with the fault in place reduces driveability, the code will be more "sticky" and some cause "default" modes to be in effect as History codes. If the MFI light comes on, it's more likely to need a couple of driving cycles to go to History. All in all, it really depends on the fault. Each fault has its own logic that is based on safety, driveability, and protection of the car from damage as well as emissions, fuel economy, etc.

[adallak]

Ranger is right. It depends on the fault that the DTC detects and the implications of that fault. If the fault interferes with the closed-loop DFI control or other major driveability issues and clearing it with the fault in place reduces driveability, the code will be more "sticky" and some cause "default" modes to be in effect as History codes. If the MFI light comes on, it's more likely to need a couple of driving cycles to go to History. All in all, it really depends on the fault. Each fault has its own logic that is based on safety, driveability, and protection of the car from damage as well as emissions, fuel economy, etc.

I did not say Ranger was wrong. Could they change the procedure for OBDII (3 cycles instead of one)?

[Ranger]

Adallac, I know I got that figure out of the FSM. Now was it an OBDI or an OBDII manual? I can't recall (and I'm too lazy to go out in the garage and search).

[Cadillac Jim]

I believe that OBD II is an evolving standard that defines the codes, the connector for the code reader, and the signals on the connector as one of five protocols (USA requirements for 2008 and newer cars all use one of them). The exact actions regarding codes is defined only in emissions laws regarding OBD alerts. The rest are implemented as the manufacturer designs them. The list is extensible, and so there are additional manufacture-dependent codes. Wikipedia article:

http://en.wikipedia.org/wiki/On-board_diagnostics#Signal_protocols

The OBD III concept under consideration by California, the EPA, and the ASA isn't a reconfiguration or redesign of OBD II but an additional external signal that transmits your VIN and any P codes to roadside or inspection station detectors when you have an emissions problem (e.g., your MIL is on). The idea is to simplify enforcement, like red-light cameras and radar speed cameras do for traffic laws. It's been under testing in California since 1994. Link:

http://www.asashop.org/autoinc/may/obd_iii_new.cfm

What I said, that codes revert to History when the key is turned off, applies to most informational codes. Those that reflect a fault of a sensor that requires a change in the vehicle mode, such as a MAP sensor failure requiring a change in the DFI mixture control, or a suspension sensor or actuator that requires that the governor speed be reduced to 90 mph, are programmed differently in how they are reported as Current or History.

[adallak]

I cannot find my 1991 Seville/Eldorado FSM, but the FSM for 1995 Fleetwood indicates that at least HVAC DTCs will stay history for 199 ignition cycles and there is nothing about three cycles for the current code before it becomes history. It looks like the rules of turning a current code to a history one vary depending on the car, model year and function the code is related to.

[Rich]

Thanks to all for your input. Didn't mean to open a can of worms here, just wanted to be sure my Eldorado wasn't developing some sort or driveability issue. I thought I felt the engine stumble a bit after a sudden deceleration from 75 mph to about 10mph. I checked to codes and found TCS U1056H and B0533H, among a few other seemingly unrelated codes, none of which appear to relate to an engine stumble. The Eldorado seems to be performing fine with no apparent issues. I'll just wait fo 50 or so driving cycles and run for codes again then. Maybe they'll all be gone.

Again, thanks to all for all the input.

[OynxSTS]

I did not say Ranger was wrong. Could they change the procedure for OBDII (3 cycles instead of one)?

Depends on the code.. some codes will take three tests to fail before the code is set... other codes only one...

It is even more complicated then that! Some "current" codes will never display "current"... Take the code for a steering sensor failure for example... it will almost never be current... For this one, when the car detects a "current" code it turns off the Stabilitrac system (since it has no clue where the steering wheel is at) and the code immediately goes into history.

At the next ignition cycle, the Stabiltrac "reboots" and the test happens again... the error happens, the BCM turns off the Stabilitrac and the error is "History" again. And you get the "Once per drive, almost always at the same light I get the service stability system message" Ah steering input sensor.

There is NO rule of thumb for how, when or why code are set... for every code you need to consult the FSM (or online tool) to consider when, how, and why the code is set... or "historied"

[adallak]

Thanks to all for your input. Didn't mean to open a can of worms here, just wanted to be sure my Eldorado wasn't developing some sort or driveability issue. I thought I felt the engine stumble a bit after a sudden deceleration from 75 mph to about 10mph. I checked to codes and found TCS U1056H and B0533H, among a few other seemingly unrelated codes, none of which appear to relate to an engine stumble. The Eldorado seems to be performing fine with no apparent issues. I'll just wait fo 50 or so driving cycles and run for codes again then. Maybe they'll all be gone.

Again, thanks to all for all the input.

You do not have to wait - you can clear all the codes. How to clear? That depends on the car. Probably someone with your model year will chime in with instructions.

[Cadillac Jim]

Press the A/C OFF and the Warmer rocker for passenger temp for several seconds until all the dash lights come on except the MIL. You know this drill because you ran the codes. You can skip doing that again by pressing the fan DOWN rocker and go right to the PCM? prompt. Press DOWN four more times and this will bring you to the TCS? prompt. Press UP, then DOWN until you see "CLEAR CODES?" then press UP. Press "Clear Info" to exit the diagnostic mode (or just turn the key off) and you are done.