P2891 Cylinder 1 Ion Current Sense Circuit Range/Performance

Here the ion signal of cylinder 1 is present but implausible: its level or waveform does not fit engine speed and load, or it differs clearly from the other cylinders. Fouled or worn spark plugs are a frequent reason, since soot or oil deposits form a leakage path that distorts the small measuring current; a cracked insulator or water in the plug well has a similar effect. In Saab's later processing module (9-3 from 2009), cylinder 1 enters on its own ion input and is combined with cylinder 3 in a combustion signal used for synchronisation and misfire detection, so a distorted cylinder 1 signal affects that pair. No manufacturer documents P2891 publicly. Electrical opens and shorts are covered by P2890, P2892 and P2893.

Likely causes

Symptoms

• MIL, sometimes together with misfire codes
• Rough idle or hesitation
• Reduced power due to cautious knock control

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: law.cornell.edu, saabwisonline.com, en.wikipedia.org.