What is the test for substitute counsel in a personal injury case?

"The governing legal principles [derived from Marsden, supra] are well settled. `Under the Sixth Amendment right to assistance of counsel, "`"[a] defendant is entitled to [substitute another appointed attorney] if the record clearly shows that the first appointed attorney is not providing adequate representation [citation] or that defendant and counsel have become embroiled in such an irreconcilable conflict that ineffective representation is likely to result."'" [Citation.] Furthermore, "`"When a defendant seeks to discharge appointed counsel and substitute another attorney, and asserts inadequate representation, the trial court must permit the defendant to explain the basis of his contention and to relate specific instances of the attorney's inadequate performance."'"' [Citation.] `"[S]ubstitution is a matter of judicial discretion. Denial of the motion is not an abuse of discretion unless the defendant has shown that a failure to replace the appointed attorney would `substantially impair' the defendant's right to assistance of counsel." [Citations.]'" (People v. Valdez (2004) 32 Cal.4th 73, 95.)